Glucose intolerance of variable severity with onset or first recognition during pregnancy, resulting from placental hormones that induce maternal insulin resistance.

Exam domain	One-glance essentials
Definition / diagnosis	*GDM is hyperglycaemia first recognised during pregnancy that does not meet criteria for overt pre-existing diabetes* ^[1]^[2] It reflects pregnancy-induced insulin resistance exceeding pancreatic beta-cell compensation ^[1]
Epidemiology / risks	Risk factors include previous GDM or macrosomia, obesity, family history, high-risk ethnicity, PCOS, and increasing maternal age ^[1]^[2] Risk-factor screening alone misses cases, so follow the local universal or targeted screening pathway ^[1]
Core mechanism	Placental hormones → progressive insulin resistance in later pregnancy → maternal hyperglycaemia → fetal hyperinsulinaemia and overgrowth ^[1]^[2] Maternal glucose crosses the placenta; insulin does not ^[2]
Clinical picture	*GDM is usually asymptomatic and detected by screening* ^[1] Severe hyperglycaemia may cause polyuria or polydipsia; accelerated fetal growth or polyhydramnios may be clues ^[1]^[2]
Investigations	Use the local oral glucose tolerance test schedule and diagnostic thresholds; assess whether results meet overt-diabetes criteria ^[1]^[2]^[3] After diagnosis, monitor capillary glucose and fetal growth; assess blood pressure and other pregnancy risks ^[1]
Management	Start with nutrition therapy, safe exercise, glucose monitoring, and weight-gain guidance; add insulin or locally recommended medication if targets are not met ^[1]^[2]^[3] Plan birth according to glycaemic control, fetal growth, complications, and obstetric factors rather than GDM label alone ^[1]
Complications	Maternal risks include pre-eclampsia, operative birth, shoulder dystocia, and future type 2 diabetes ^[1]^[2] Fetal or neonatal risks include macrosomia, birth trauma, neonatal hypoglycaemia, respiratory morbidity, and later metabolic disease ^[1]
Prevention / follow-up	Optimise pre-pregnancy weight and lifestyle; after birth, arrange postpartum diabetes testing and lifelong periodic screening ^[1]^[2]^[3] Encourage breastfeeding and address cardiovascular risk factors ^[1]
Exam traps	*Urine glucose is not an adequate screening or monitoring test for GDM* ^[1] Treatment reduces risk but does not eliminate macrosomia or shoulder dystocia; avoid assuming Caesarean delivery is automatically required ^[1]^[2]

References

^[1] Lecture slides: GC 115. I am pregnant medical problems complicating pregnancy.pdf (p. 18 — GDM definition and management) ^[2] Lecture slides: Block C - I am pregnant_ medical problems complicating pregnancy.pdf (pp. 10–13 — maternal diabetes and fetal effects) ^[3] NICE guideline NG3: Diabetes in pregnancy: management from preconception to the postnatal period

Gestational Diabetes Mellitus (GDM)

Gestational diabetes mellitus (GDM) is defined as glucose intolerance of variable severity with onset or first recognition during pregnancy. ^[1]^[2]

Let's break that down:

"Gestational" → Latin gestare = to carry (i.e., during pregnancy)
"Diabetes" → Greek diabainein = to pass through (referring to polyuria)
"Mellitus" → Latin mellitus = honey-sweet (referring to sweet urine from glycosuria)

The key conceptual distinction is between pre-existing diabetes mellitus (PGDM) that happens to be present when a woman becomes pregnant, versus GDM which is a condition caused by or first detected during pregnancy. This mirrors exactly the same conceptual framework as hypertension in pregnancy — it can either be pre-existing or caused by pregnancy. ^[1]

Critical Distinction

GDM must be distinguished from overt (pre-gestational) diabetes first detected in pregnancy. If a woman presents in the first trimester with fasting glucose ≥ 7.0 mmol/L or HbA1c ≥ 6.5%, she likely has pre-existing undiagnosed DM rather than true GDM. The 2024 ADA Standards of Care and IADPSG recommend classifying this as "diabetes mellitus in pregnancy" rather than GDM. To confidently diagnose GDM (rather than pre-existing DM), do an OGTT after the baby is born — if glucose intolerance persists, it was pre-existing DM all along. ^[1]

A woman who is not diabetic may become diabetic during pregnancy, or diabetes may be first detected during pregnancy — this is gestational diabetes. She and her baby face similar problems to pre-existing DM but usually to a milder degree. ^[2]

Epidemiology

GDM affects approximately 14% of pregnancies worldwide (IDF 2021), though prevalence varies enormously by population, diagnostic criteria used, and screening strategy.
Prevalence is rising in parallel with the global obesity epidemic and increasing maternal age.

Hong Kong prevalence: approximately 15–20% of pregnancies, which is higher than Western populations, reflecting the higher baseline prevalence of insulin resistance and type 2 DM in East Asian populations.
The Chinese population has a relatively higher prevalence of GDM despite lower average BMI compared to Caucasians — this is partly because East Asians have proportionally more visceral adiposity and lower β-cell reserve relative to insulin demand.
DM affects > 10% of HK adults ^[3], and with increasing maternal age (mean age at first birth in HK now ~32 years) and rising obesity rates, GDM incidence is climbing.

Recurrence rate: 30–84% in subsequent pregnancies (depending on the population and interval weight gain).
Up to 50% of women with GDM will develop overt type 2 DM within 5–10 years postpartum — this is one of the strongest predictors of future T2DM.
GDM is more common in multiparous women and those with advancing maternal age.

Understanding risk factors requires understanding the pathophysiology (below), but in summary:

Risk Factor	Mechanism / Explanation
Obesity (BMI ≥ 25 or ≥ 23 for Asians)	↑ Visceral adiposity → ↑ FFA and adipokine release → ↑ insulin resistance
Advanced maternal age ( ≥ 35 years)	Age-related decline in β-cell compensatory capacity
Family history of T2DM	Shared genetic susceptibility for β-cell dysfunction and insulin resistance
Previous GDM	Demonstrates pre-existing marginal β-cell reserve
Previous macrosomic baby ( ≥ 4 kg)	Suggests prior undiagnosed or subclinical glucose intolerance
Previous unexplained stillbirth / congenital anomaly	May have been due to undiagnosed hyperglycaemia
Ethnicity (East Asian, South Asian, Hispanic, African)	Higher baseline insulin resistance and/or lower β-cell reserve
PCOS	Insulin resistance is central to PCOS pathophysiology
Pre-diabetes (IGT/IFG) before pregnancy	Already on the continuum toward DM
Metabolic syndrome: HTN, hyperlipidaemia, PCOS, NAFLD ^[3]^[4]	All manifestations of underlying insulin resistance
Glucosuria in current pregnancy	May be physiological (↓ renal threshold in pregnancy) but should prompt screening
Multiple pregnancy	Greater placental mass → more placental hormones → more insulin resistance
Corticosteroid use	Exogenous glucocorticoids → ↑ hepatic gluconeogenesis + ↑ insulin resistance

High Yield: Asian-Specific Risk

In Hong Kong, use BMI ≥ 23 kg/m² as the obesity threshold for screening (Asian cut-off), not the Western cut-off of 25 kg/m². East Asians develop metabolic complications at lower BMI due to proportionally greater visceral fat and lower muscle mass.

Anatomy and Physiology: The Placenta as an Endocrine Organ

To understand GDM, you need to understand why pregnancy is inherently diabetogenic. The key player is the placenta.

Normal Carbohydrate Metabolism in Pregnancy

Pregnancy alters carbohydrate metabolism. The major changes are the decreased sensitivity to insulin with increasing gestation due to an increase in factors antagonizing insulin such as cortisol, oestrogens, progesterone and human placental lactogen (hPL), together with the degradation of insulin by the placenta. ^[1]^[5]

Let me walk through this in detail:

Early pregnancy is actually characterised by enhanced insulin sensitivity.
Oestrogen and progesterone stimulate β-cell hyperplasia → ↑ insulin secretion.
The net effect is ↑ peripheral glucose uptake and ↑ glycogen storage — the mother is "building reserves" for later.
This can sometimes result in fasting hypoglycaemia ^[5] — the mother's glucose is being actively shunted to early fetal development and stored as glycogen.

As the placenta grows, it produces increasing amounts of counter-regulatory (anti-insulin) hormones:

Placental Hormone	Mechanism of Insulin Resistance
Human placental lactogen (hPL)	Most important! Structurally similar to GH. Causes lipolysis → ↑ FFA → ↑ insulin resistance. Also directly antagonizes insulin signalling at the receptor level.
Cortisol (↑ from placental CRH)	↑ Hepatic gluconeogenesis, ↑ insulin resistance in peripheral tissues
Oestrogen	Impairs insulin receptor signalling
Progesterone	Impairs insulin receptor signalling
Placental growth hormone (PGH)	Similar to pituitary GH; stimulates gluconeogenesis and lipolysis
TNF-α from placenta	Pro-inflammatory cytokine that impairs insulin receptor substrate (IRS-1) phosphorylation
Placental insulinase ^[1]^[5]	Degrades circulating insulin, further reducing effective insulin levels

The result: insulin resistance increases progressively from ~20 weeks, peaking at 32–36 weeks — this is when GDM is most likely to manifest.
In a normal pregnancy, the mother's pancreatic β-cells compensate by increasing insulin secretion 2–3 fold. This is why most women remain euglycaemic.

GDM = failure of β-cell compensation in the face of pregnancy-induced insulin resistance.
Women who develop GDM have pre-existing subclinical β-cell dysfunction — their β-cells simply cannot ramp up insulin production enough to overcome the rising insulin resistance.
Think of it as unmasking a latent deficiency: the "stress test" of pregnancy reveals the pancreas's inadequacy.
This is conceptually identical to how T2DM develops (insulin resistance → β-cell failure), just accelerated by the physiological insulin resistance of pregnancy.

Key Concept: The Placenta Drives Everything

The placenta produces hormones that have an antagonising effect on insulin ^[5]. After delivery, when the placenta is removed, insulin resistance drops dramatically — this is why GDM typically resolves immediately after delivery. If hyperglycaemia persists postpartum, it was pre-existing DM all along.

Glucose crosses the placenta via facilitated diffusion (GLUT-1 and GLUT-3 transporters) — it moves down a concentration gradient.
Insulin does NOT cross the placenta (it is a large peptide hormone, ~5.8 kDa).
Therefore, maternal hyperglycaemia → fetal hyperglycaemia → fetal pancreatic β-cell stimulation → fetal hyperinsulinaemia.
This is the Pedersen hypothesis — the core mechanism explaining all fetal complications of GDM (macrosomia, neonatal hypoglycaemia, etc.).

Aetiology and Pathophysiology

GDM is a multifactorial condition arising from the interplay of:

Genetic susceptibility: Many of the same genetic loci implicated in T2DM (e.g., TCF7L2, KCNJ11, GCK, MTNR1B) also confer risk for GDM. In the Chinese population, specific polymorphisms in CDKAL1 and IGF2BP2 have been associated with GDM risk.
Pre-existing insulin resistance: Often subclinical before pregnancy. The most common substrate is central obesity (even at "normal" BMI in Asians) and the metabolic syndrome.
β-cell dysfunction: The fundamental defect. Women destined to develop GDM have demonstrably lower first-phase insulin secretion even before pregnancy. Their β-cells cannot increase output 2–3 fold as needed.
Placental factors: As described above — the progressive increase in counter-regulatory hormones and placental insulinase.
Inflammatory/adipokine imbalance: ↑ TNF-α, ↑ IL-6, ↑ leptin, ↓ adiponectin — all promote insulin resistance. Adiponectin (an insulin-sensitizing adipokine) is characteristically low in GDM.
Gut microbiome: Emerging evidence suggests altered gut microbiome composition in GDM, affecting bile acid metabolism and incretin signalling.

Step 1: Pre-pregnancy state

Woman has subclinical β-cell dysfunction ± mild insulin resistance (may have IFG, IGT, or completely normal glucose tolerance).
Often has risk factors: family history of T2DM, PCOS, obesity, previous GDM.

Step 2: Early pregnancy (1st trimester)

Enhanced insulin sensitivity (anabolic phase) → glucose tolerance may actually improve.
β-cell hyperplasia begins under the influence of oestrogen and progesterone.

Step 3: Progressive insulin resistance (2nd–3rd trimester)

Placental hormones (hPL, cortisol, oestrogens, progesterone) + placental insulinase → progressive ↑ in insulin resistance ^[1]^[5].
Normally, β-cells compensate with a 2–3× increase in insulin secretion.
In GDM: β-cells fail to compensate → relative insulin deficiency → hyperglycaemia.

Step 4: Maternal hyperglycaemia → Fetal consequences (Pedersen Hypothesis)

Excess glucose crosses the placenta (facilitated diffusion).
Fetal pancreas responds with hyperinsulinaemia.
Fetal insulin acts as a growth factor → macrosomia, organomegaly, ↑ fat deposition.
After birth, maternal glucose supply is cut off but the hypertrophied fetal pancreas continues secreting insulin → neonatal hypoglycaemia.

Step 5: Postpartum

Delivery of placenta → rapid ↓ in counter-regulatory hormones → insulin resistance resolves.
If hyperglycaemia resolves → confirmed GDM.
If hyperglycaemia persists → was pre-existing DM (or the woman now has new T2DM that pregnancy accelerated).

Classification

Category	Description
Pre-gestational (pre-existing) DM	T1DM or T2DM diagnosed before pregnancy
Overt DM first detected in pregnancy	Meets standard DM diagnostic criteria in the 1st trimester (likely pre-existing but undiagnosed)
Gestational DM (GDM)	Glucose intolerance with onset or first recognition during pregnancy (typically 24–28 weeks), not meeting criteria for overt DM

The White Classification was originally designed for pre-gestational DM but helps conceptualise severity:

Class	Description
A1	GDM controlled by diet alone
A2	GDM requiring pharmacotherapy (insulin or metformin)

Approach	When	Who
Universal screening	24–28 weeks	All pregnant women (used in HK)
Selective/risk-based screening	Early pregnancy + 24–28 weeks	Only those with risk factors
In HK: OGTT between 26–30 weeks as screening for diabetes in low-risk patients; earlier one for high-risk patients ^[1]

Clinical Features

GDM is often asymptomatic — this is why screening is essential. Most women with GDM are detected through routine screening OGTT rather than through symptoms. When symptoms are present, they are identical to other forms of diabetes but typically milder.

She and her baby face similar problems [to pre-existing DM] but usually to a milder degree. ^[2]

Symptoms (with pathophysiological basis)

Symptom	Pathophysiological Basis
Polyuria	Hyperglycaemia → plasma glucose exceeds renal threshold for reabsorption (~10 mmol/L, though this is lower in pregnancy due to ↑ GFR) → glucose remains in tubular fluid → osmotic diuresis → ↑ urine volume. Note: polyuria can be masked in pregnancy because urinary frequency is already common due to uterine compression of the bladder.
Polydipsia	Osmotic diuresis → ↓ intravascular volume → ↑ plasma osmolality → stimulation of hypothalamic thirst centre → excessive thirst. Again, can be masked by the normal ↑ thirst of pregnancy.
Fatigue / malaise	Relative insulin deficiency → glucose cannot enter cells efficiently (especially muscle) → cells are "starving" despite ambient hyperglycaemia → fatigue. Also contributed by osmotic diuresis → dehydration.
Recurrent vulvovaginal candidiasis	Glycosuria + hyperglycaemia in vaginal secretions → provides excellent growth medium for Candida albicans. Urogenital infections are a recognized complication of DM. ^[3]
Recurrent UTIs	Glycosuria → bacterial growth in urinary tract. Also, pregnancy itself predisposes to UTI (progesterone → ureteral smooth muscle relaxation → urine stasis).
Blurred vision	Hyperglycaemia → osmotic swelling of the lens → refractive changes. This is typically reversible with glycaemic control.
Weight gain (excessive)	Hyperinsulinaemia (early compensatory phase) promotes lipogenesis. Also, fetal macrosomia contributes to overall weight.
Nocturia	Osmotic diuresis + supine position redistributes fluid → ↑ renal blood flow at night → ↑ urine production. Difficult to distinguish from normal pregnancy nocturia.

Clinical Pearl

Most women with GDM are completely asymptomatic. The classic osmotic symptoms (polyuria, polydipsia) are often subtle and easily attributed to normal pregnancy. This is precisely why we screen with OGTT — we cannot rely on symptoms.

Symptom	Significance
Headache, visual disturbances, epigastric pain, oedema	May indicate pre-eclampsia (which is more common in GDM)
Reduced fetal movements	May indicate fetal compromise / stillbirth (more common in poorly controlled diabetes)
Excessive weight gain / polyhydramnios symptoms (abdominal distension, breathlessness)	Polyhydramnios due to fetal polyuria from fetal hyperglycaemia

Signs (with pathophysiological basis)

Sign	Pathophysiological Basis
Obesity / Central adiposity	Most women with GDM are overweight/obese — visceral fat is the driver of insulin resistance through FFA and adipokine release
Acanthosis nigricans	Velvety, hyperpigmented plaques in skin folds (neck, axillae, groin). Caused by hyperinsulinaemia → stimulation of keratinocyte and fibroblast IGF-1 receptors → epidermal hyperplasia and hyperpigmentation. A cutaneous marker of insulin resistance. Features of metabolic syndrome and acanthosis nigricans should be looked for. ^[3]
Skin tags (acrochordons)	Also associated with insulin resistance and hyperinsulinaemia (same IGF-1 mechanism).

Sign	Pathophysiological Basis
Macrosomia (large-for-gestational-age fetus) ^[5]	Pedersen hypothesis: maternal hyperglycaemia → fetal hyperglycaemia → fetal hyperinsulinaemia → insulin acts as anabolic growth factor → ↑ fat deposition, ↑ glycogen storage, ↑ protein synthesis → large baby. Symphysio-fundal height may be large for dates.
Polyhydramnios ^[5]	Fetal hyperglycaemia → fetal osmotic diuresis → fetal polyuria → excess amniotic fluid. May present as uterus large for dates, tense uterus, difficulty palpating fetal parts.
Fetal malpresentation	Secondary to polyhydramnios — excess amniotic fluid allows the fetus more freedom to adopt non-cephalic positions.

Sign	Significance
Hypertension	GDM ↑ risk of pre-eclampsia (both share insulin resistance as a common pathogenic pathway). Also, metabolic syndrome co-exists.
Proteinuria	May indicate pre-eclampsia or pre-existing diabetic nephropathy.
Peripheral oedema (beyond normal pregnancy oedema)	Pre-eclampsia or nephrotic-range proteinuria from diabetic nephropathy.
Fundoscopic changes (if pre-existing DM)	Diabetic retinopathy — though this would suggest the DM predates the pregnancy. Pre-existing DM → due to worsened control of DM during pregnancy → increased risk of complications involving cardiovascular, renal and optic systems. ^[5]

Sugar is teratogenic. ^[5] This is one of the most important teaching points. Let me explain every fetal complication and its mechanism:

Fetal/Neonatal Feature	Pathophysiological Basis
Congenital malformations ^[5]	Hyperglycaemia during organogenesis (weeks 3–8) → ↑ oxidative stress → ↑ apoptosis in embryonic cells → structural defects. Most common: cardiac (VSD, TGA), neural tube defects (anencephaly, spina bifida), caudal regression syndrome (pathognomonic of diabetic embryopathy), renal agenesis, GI atresias. Primarily a risk in pre-existing DM (because organogenesis occurs before GDM is typically detected), but can occur in early-onset GDM if hyperglycaemia is present in the first trimester.
Spontaneous miscarriage ^[1]^[5]	High sugar level is associated with higher risk of miscarriage ^[5]. Hyperglycaemia-induced oxidative stress → impaired implantation and early embryonic development → miscarriage. Risk correlates with HbA1c level at conception.
Macrosomia ^[1]^[5]	Pedersen hypothesis (as above). Defined as birth weight > 4000 g or > 90th centile for gestational age (LGA). Specific pattern: truncal obesity with ↑ shoulder and abdominal circumference relative to head (unlike constitutionally large babies who are proportionally large).
Shoulder dystocia	Consequence of macrosomia — disproportionately large shoulders → impaction behind pubic symphysis during delivery → obstetric emergency. Can cause brachial plexus injury (Erb's palsy).
Stillbirth ^[1]^[5]	Fetal hyperinsulinaemia → ↑ fetal oxygen consumption → chronic fetal hypoxia. Also, hyperglycaemia → placental vasculopathy → impaired oxygen transfer. Risk is highest in the third trimester, especially > 36 weeks.
Respiratory distress syndrome (RDS) ^[1]^[5]	Fetal hyperinsulinaemia inhibits surfactant production by type II pneumocytes (insulin antagonizes the stimulatory effect of cortisol on surfactant synthesis). → ↓ surfactant → atelectasis and RDS at birth. This occurs even at relatively later gestational ages where RDS would not normally be expected.
Neonatal hypoglycaemia	At birth, the umbilical cord is clamped → maternal glucose supply ceases abruptly. But the neonate's hypertrophied pancreatic β-cells continue secreting insulin → hyperinsulinaemia without glucose supply → profound hypoglycaemia. This is the single most common immediate neonatal complication.
Neonatal hypocalcaemia	Mechanism not fully understood. Likely related to functional hypoparathyroidism in the neonate (hyperinsulinaemia may suppress PTH) and/or maternal hypomagnesaemia → fetal hypomagnesaemia → impaired PTH secretion.
Neonatal polycythaemia and hyperbilirubinaemia	Chronic fetal hypoxia (from ↑ oxygen consumption) → ↑ erythropoietin → polycythaemia. After birth, the excess RBCs are broken down → ↑ unconjugated bilirubin → neonatal jaundice.
Hypertrophic cardiomyopathy	Fetal hyperinsulinaemia → glycogen deposition in the interventricular septum → septal hypertrophy → outflow tract obstruction. Usually transient and resolves within weeks.
Fetal programming / Long-term metabolic risk ^[5]	Evidence suggests the concept of fetal programming — the baby may be more predisposed to other metabolic and cardiovascular diseases later in life ^[5]. Intrauterine hyperglycaemic environment → epigenetic modifications → offspring have ↑ risk of obesity, insulin resistance, T2DM, and CVD in adulthood. This is the "Barker hypothesis" / developmental origins of adult disease.

Pedersen Hypothesis — The Unifying Concept

Almost every fetal complication can be traced back to one mechanism: maternal hyperglycaemia → fetal hyperglycaemia → fetal hyperinsulinaemia. Insulin doesn't cross the placenta, but glucose does. The fetus responds to excess glucose by making more insulin, and insulin acts as a growth factor causing macrosomia, drives oxygen consumption causing hypoxia, inhibits surfactant causing RDS, and persists after cord clamping causing neonatal hypoglycaemia.

Pre-existing DM effects on pregnancy, MATERNAL SIDE → increased complication risk (pre-eclampsia, UTI, preterm labour) + increased incidence of cesarean section and instrumental delivery. ^[5]

Maternal Complication	Pathophysiology
Pre-eclampsia ^[1]^[5]	Shared pathogenic pathway with GDM: insulin resistance → endothelial dysfunction → defective trophoblast invasion → impaired spiral artery remodelling → placental ischaemia → release of anti-angiogenic factors (sFlt-1, sEng) → systemic endothelial dysfunction → hypertension + proteinuria. GDM increases pre-eclampsia risk ~2–4 fold.
UTI / pyelonephritis ^[5]	Glycosuria (provides growth medium for bacteria) + pregnancy-related ureteral stasis (progesterone effect) → ↑ risk of ascending UTI.
Preterm labour ^[5]	Polyhydramnios → uterine overdistension → premature contractions. Also, UTI/infections can trigger preterm labour. Pre-eclampsia may also necessitate iatrogenic preterm delivery.
Caesarean section / instrumental delivery ^[5]	Macrosomia → cephalopelvic disproportion / failed progress in labour. Also, shoulder dystocia risk necessitates lower threshold for C-section.
Polyhydramnios	Fetal polyuria (osmotic diuresis from hyperglycaemia) → excess amniotic fluid.
Birth canal trauma	Delivery of macrosomic baby → ↑ risk of perineal tears, cervical lacerations.
Postpartum haemorrhage	Uterine overdistension (from macrosomia/polyhydramnios) → uterine atony after delivery.
Worsening of pre-existing diabetic complications ^[5]	Due to worsened control of DM during pregnancy → increased risk of complications involving cardiovascular, renal and optic systems. ^[5] Pregnancy-related haemodynamic changes (↑ cardiac output, ↑ GFR) can accelerate retinopathy and nephropathy.

Feature	GDM	Pre-Existing DM in Pregnancy
Timing of detection	Usually 24–28 weeks	Known DM before pregnancy OR detected in 1st trimester with overt DM criteria
Congenital malformations	Low risk (organogenesis usually occurs before hyperglycaemia develops)	Higher risk because sugar is teratogenic during organogenesis ^[5]
Severity	Usually milder	Often more severe
Postpartum resolution	Typically resolves	Persists
Need for insulin	~20–30%	Usually requires insulin from the start
Retinopathy/nephropathy	Absent (unless pre-existing DM was undiagnosed)	May be present and worsen

There are two important issues when a woman having a pre-existing medical disorder gets pregnant: (1) the effects of her pregnancy on the pre-existing medical disorder, and (2) the effects of the pre-existing medical disorder on her pregnancy. ^[1]

For GDM specifically:

Effect of pregnancy ON diabetes: pregnancy makes glycaemic control more difficult due to placental counter-regulatory hormones + placental insulinase ^[1]^[5]
Effect of diabetes ON pregnancy: poor control increases the incidence of maternal/fetal complications including spontaneous miscarriage, congenital malformation, stillbirth, RDS, macrosomia, pre-eclampsia and polyhydramnios ^[1]^[5]

Don't treat the condition only during pregnancy — good antenatal care including pre-pregnancy diabetic control, leading into good intra-pregnancy diabetic control ^[5] is the cornerstone. For GDM specifically (where pre-pregnancy control is not possible since the condition arises during pregnancy), the emphasis shifts to early detection and tight glycaemic control from diagnosis onwards.

Key Exam Points to Remember:

GDM = β-cell failure to compensate for pregnancy-induced insulin resistance

hPL is the most important diabetogenic hormone of pregnancy

The Pedersen hypothesis explains all fetal complications: maternal hyperglycaemia → fetal hyperglycaemia → fetal hyperinsulinaemia

Sugar is teratogenic — congenital malformations correlate with peri-conceptional glycaemic control (mainly pre-existing DM)

GDM typically resolves after delivery (placenta removed → insulin resistance drops)

Up to 50% of GDM women develop T2DM within 5–10 years

In HK, universal screening with OGTT at 26–30 weeks; earlier for high-risk

High Yield Summary

Definition: Glucose intolerance first recognized during pregnancy (distinct from pre-existing DM).

Epidemiology: ~15–20% in HK; rising with ↑ maternal age and obesity. Up to 50% develop T2DM within 5–10 years postpartum.

Pathophysiology: Pregnancy is inherently diabetogenic — placental hormones (hPL, cortisol, oestrogen, progesterone) + placental insulinase cause progressive insulin resistance peaking at 32–36 weeks. GDM occurs when β-cells cannot compensate (2–3× increase needed). Pre-existing subclinical β-cell dysfunction is unmasked.

Risk factors: Obesity, age ≥ 35, FHx T2DM, previous GDM/macrosomia, PCOS, ethnicity (Chinese/Asian), metabolic syndrome.

Clinical features: Usually asymptomatic (detected by screening). When symptomatic: polyuria, polydipsia, fatigue, recurrent candidiasis/UTI. Signs: obesity, acanthosis nigricans, LGA fetus, polyhydramnios.

Fetal complications (Pedersen hypothesis): Macrosomia, shoulder dystocia, RDS, neonatal hypoglycaemia, hypocalcaemia, polycythaemia/jaundice, hypertrophic cardiomyopathy, stillbirth. Long-term: fetal programming → future metabolic disease.

Maternal complications: Pre-eclampsia, UTI, preterm labour, ↑ C-section rate, polyhydramnios, PPH. Worsening of pre-existing diabetic microvascular complications.

Key principle: Good glycaemic control is the cornerstone of management — by diet ± insulin.

Active Recall - Gestational Diabetes Mellitus (Definition, Epidemiology, Pathophysiology, Clinical Features)

^[1] Lecture slides: Block C - O&G Theme Case 1.docx.pdf (GDM definition, effects of pregnancy on DM and DM on pregnancy) ^[2] Lecture slides: GC 115. I am pregnant medical problems complicating pregnancy.pdf (p18, GDM definition and management principles) ^[3] Senior notes: Ryan Ho Endocrine.pdf (p75–80, DM overview, T2DM risk factors, metabolic syndrome) ^[4] Senior notes: Maksim Medicine Notes.pdf (p80, DM overview, metabolic syndrome associations) ^[5] Lecture slides: Block C - I am pregnant_ medical problems complicating pregnancy.pdf (p10–17, DM effects on pregnancy maternal and fetal, management, GDM screening)

Differential Diagnosis of Gestational Diabetes Mellitus

When a pregnant woman presents with hyperglycaemia — whether detected through routine screening OGTT, incidental blood tests, or classic osmotic symptoms — the knee-jerk reaction might be to label it "GDM" and move on. But this is a trap. The differential diagnosis is crucial because:

The aetiology determines the management trajectory: A woman with undiagnosed pre-existing T2DM needs lifelong follow-up and has already been exposed to hyperglycaemia during organogenesis (↑ congenital malformation risk). A woman with true GDM will likely resolve postpartum.
Some causes require specific treatments: e.g., T1DM requires insulin from the start; Cushing's syndrome requires addressing the cortisol excess; thyrotoxicosis needs antithyroid drugs.
To confidently diagnose GDM (rather than pre-existing DM), do an OGTT after the baby is born — if glucose intolerance persists, it was pre-existing DM all along. ^[1]

The differential diagnosis framework revolves around a single question: Why is this pregnant woman's blood glucose elevated?

The causes of hyperglycaemia in pregnancy can be categorised based on the mechanism of glucose dysregulation:

Detailed Differential Diagnosis

This is the most important differential and the most commonly missed. Why?

T2DM affects > 10% of HK adults ^[3], and many are undiagnosed.
T2DM is often asymptomatic in its early stages — usually asymptomatic at diagnosis with insulin-deficiency symptoms occurring late ^[3] — so a woman may enter pregnancy not knowing she has DM.
The first blood glucose check may happen during pregnancy screening, leading to misclassification as "GDM."

How to distinguish from GDM:

Feature	GDM	Pre-existing T2DM (undiagnosed)
Timing of hyperglycaemia detection	Usually 24–28 weeks	Often detected at first antenatal visit (1st trimester) or even before pregnancy
HbA1c at booking	Usually < 6.5%	≥ 6.5% suggests pre-existing DM ^[6] (though HbA1c has caveats in pregnancy — see below)
Fasting glucose at booking	Usually < 7.0 mmol/L	Fasting glucose ≥ 7.0 mmol/L suggests overt DM ^[6]
Risk factors	Standard GDM risk factors	Features of metabolic syndrome: obesity, HTN, hyperlipidaemia, PCOS, NAFLD, acanthosis nigricans, Hx of gestational DM ^[3]^[4]
Congenital malformations	Low risk (hyperglycaemia develops after organogenesis)	Higher risk — sugar is teratogenic during weeks 3–8 ^[5]
Postpartum OGTT	Normalises	Remains abnormal — confirms it was pre-existing ^[1]
Severity	Usually milder	Often more severe; may need insulin earlier

Exam Trap

Gestational diabetes uses a different set of diagnostic criteria from standard DM. ^[6] If a woman in the first trimester meets standard DM criteria (fasting glucose ≥ 7.0 mmol/L, random glucose ≥ 11.1 mmol/L with symptoms, or HbA1c ≥ 6.5%), she should be classified as having overt DM in pregnancy, NOT GDM. This distinction matters because she has been hyperglycaemic during organogenesis and needs different counselling and surveillance.

Though rare to present de novo during pregnancy, it can happen. T1DM accounts for ~5% of all DM in Chinese ^[3].

Why consider it:

A young, lean pregnant woman with abrupt-onset hyperglycaemia, marked weight loss, and ketonuria should raise suspicion.
LADA clinically presents as type 2 DM (insulin resistance), but circulating antibodies are positive (e.g., anti-islet cell, anti-GAD 65) ^[4].
LADA is particularly important because these women will progress to absolute insulin deficiency more rapidly than T2DM and are at higher risk of DKA.

How to distinguish:

Feature	GDM	T1DM / LADA
Body habitus	Usually overweight/obese	Usually non-obese, young, usually thin ^[4]
Onset	Gradual, detected on screening	Abrupt (weeks), severe hyperglycaemic symptoms ^[3]
Ketones	Usually absent	May have ketonuria/ketonaemia; risk of DKA is high ^[4]
Autoantibodies	Negative	> 85% positive: anti-islet cell Ab (e.g., anti-GAD), anti-insulin Ab ^[3]^[4]
C-peptide	Normal to high (insulin resistance pattern)	↓ C-peptide at presentation ^[3]
Insulin requirement	~20–30% eventually need insulin	Requires insulin from the outset
Associations	Metabolic syndrome features	Graves' disease, myasthenia gravis, Addison's disease, pernicious anaemia ^[4]

Clinical Pearl

If a lean pregnant woman develops hyperglycaemia with significant ketonuria, do NOT simply label this as "GDM with poor control." Check anti-GAD antibodies and C-peptide — she may have T1DM or LADA, and needs insulin immediately. Oral hypoglycaemics alone will be insufficient and she is at risk of DKA.

MODY is a monogenic autosomal dominant form of DM, the commonest monogenic form (2–5% of all DM) ^[3]^[4]. It should be considered in the differential of any young woman with hyperglycaemia in pregnancy.

"MODY" → "Maturity-Onset" = presents like T2DM (non-insulin-dependent), "of the Young" = onset < 25 years.
Various mutations interfering with pancreatic β-cell's ability to secrete insulin ^[3].
Often misdiagnosed as T1 or T2DM — alert when early onset (< 25y) non-obese NIDDM patients with negative islet cell autoantibodies ^[3].

Key MODY types relevant to GDM differential:

MODY Type	Gene	Clinical Relevance in Pregnancy
MODY 2 (GCK-MODY)	Glucokinase ^[4]	Most commonly confused with GDM. Glucokinase is the "glucose sensor" of β-cells; mutation raises the glucose set-point → mild, stable fasting hyperglycaemia (5.5–8 mmol/L) that is present lifelong but may only be detected during pregnancy screening. No treatment needed (long-term outcomes similar to healthy individuals), EXCEPT: if fetus inherits mutation (50% chance), treatment may cause SGA baby; if fetus does NOT inherit, maternal hyperglycaemia drives fetal overgrowth.
MODY 1 (HNF4α)	Hepatocyte nuclear factor 4α ^[4]	Progressive β-cell dysfunction. May first present in pregnancy. Responds well to sulphonylureas ^[3]^[4] (but these are generally avoided in pregnancy).
MODY 3 (HNF1α)	Hepatocyte nuclear factor 1α ^[4]	Most common MODY type. Progressive hyperglycaemia. Also responds to sulphonylureas ^[3]^[4].

How to distinguish from GDM:

Feature	GDM	MODY
Age	Any age (usually > 25)	Young (< 25 years) ^[4]
Family history	May have FHx of T2DM	Multigenerational (> 2 generations) ^[4] — autosomal dominant pattern
BMI	Usually overweight	Usually non-obese
Autoantibodies	Negative	Negative ^[4]
Fasting glucose pattern	Variable; worsens with gestation	MODY 2: mild, stable fasting hyperglycaemia (doesn't worsen much with gestation)
Postpartum	Resolves	Persists (lifelong)
Diagnosis	OGTT criteria	Genetic testing ^[3]

4. Secondary Causes of Diabetes in Pregnancy

Secondary causes of diabetes include pancreatic diseases, overproduction of counter-regulatory hormones, drug-induced, and genetic syndromes. ^[3]^[4]

The most important in the obstetric setting:

Drug	Mechanism	Context in Pregnancy
Glucocorticoids (steroids) ^[4]	↑ Hepatic gluconeogenesis + ↑ insulin resistance + ↓ peripheral glucose uptake	Given for fetal lung maturity (betamethasone/dexamethasone at 24–34 weeks) or for maternal conditions (e.g., asthma, autoimmune disease). Commonly causes transient hyperglycaemia that can be misinterpreted as GDM. Drug-induced DM: steroids ^[4].
Progesterone (high-dose, e.g., for luteal support in IVF)	Impairs insulin receptor signalling	IVF pregnancies have higher rates of GDM partly due to exogenous progesterone
Thiazide diuretics ^[4]	↑ Insulin resistance + ↓ insulin secretion (K+ depletion → impaired β-cell function)	Rarely used in pregnancy (generally avoided), but may be continued from pre-pregnancy
Phenytoin ^[4]	Directly impairs insulin secretion from β-cells	Used for epilepsy management in pregnancy
β-agonists (e.g., ritodrine, salbutamol for tocolysis)	Stimulate hepatic glycogenolysis and gluconeogenesis via β₂-receptor activation	Used for preterm labour management; can cause significant transient hyperglycaemia

Important Clinical Scenario

Stress hyperglycaemia can be unmasked by infections, pregnancy, steroid therapy or stroke. ^[3] When antenatal steroids are given for fetal lung maturation (e.g., betamethasone 12 mg × 2 doses), blood glucose can spike dramatically for 48–72 hours. This is not GDM — it is steroid-induced hyperglycaemia. However, it does need monitoring and may need short-term insulin. If the hyperglycaemia persists beyond the steroid effect window, re-evaluate for true GDM or pre-existing DM.

Condition	Mechanism	How to Distinguish
Cushing's syndrome	↑ Counter-regulatory hormone (cortisol) ^[4] → ↑ gluconeogenesis + ↑ insulin resistance	Look for Cushingoid features: moon face, buffalo hump, striae, proximal myopathy, central obesity. Can be caused by exogenous steroids or (rarely) adrenal/pituitary tumour. Note: cortisol is physiologically elevated in pregnancy, making biochemical diagnosis more difficult.
Thyrotoxicosis	Excess thyroid hormones → ↑ hepatic glucose output + ↑ intestinal glucose absorption + ↑ glycogenolysis	Look for tachycardia, tremor, weight loss, goitre, exophthalmos. Check TSH and FT4. Thyroid hormones are listed as drug-induced causes of DM ^[4]. Gestational thyrotoxicosis (hCG-mediated) is transient and usually mild.
Phaeochromocytoma ^[4]	Catecholamine excess → ↑ glycogenolysis + ↑ gluconeogenesis + ↑ lipolysis + direct β-cell inhibition (α₂ effect)	Paroxysmal hypertension, headache, sweating, palpitations. Rare but dangerous in pregnancy (hypertensive crises).
Acromegaly ^[4]	GH excess → ↑ hepatic gluconeogenesis + ↑ insulin resistance	Coarsened facial features, large hands/feet, macroglossia. Very rare in pregnancy context.

Condition	Mechanism	Clues
Chronic pancreatitis ^[4]	Pancreatic exocrine and endocrine insufficiency — secondary diabetes when β-cell mass is lost ^[7]. Progressive destruction of pancreatic islets → insulin deficiency. Also lose α-cells (glucagon) → ↑ hypoglycaemia risk.	History of alcohol use, recurrent abdominal pain, steatorrhoea. IGT occurs early but overt DM usually late (30%) ^[7].
CA pancreas ^[4]	Infiltration/destruction of pancreatic islets	New-onset DM in an older patient, weight loss, jaundice, back pain — less relevant in typical GDM age group but should be considered in older mothers.
Haemochromatosis ^[4]	Iron accumulation in pancreatic islets → β-cell damage ^[8] → "bronze diabetes."	Skin hyperpigmentation, hepatomegaly, arthropathy, cardiomyopathy. 50% develop DM due to progressive iron accumulation in pancreatic islets. ^[8] Rare in Chinese compared to Caucasians, but not impossible. Check ferritin and transferrin saturation.

Genetic syndromes associated with DM include Down's, Klinefelter's, Turner's, and DIDMOAD (Wolfram's syndrome: DI, DM, optic atrophy, nerve deafness). ^[4]

These are rare but should be considered if there are dysmorphic features or other syndromic associations.
Maternally inherited diabetes with deafness (MIDD): mitochondrial mutation ^[4] — suspect if the woman has sensorineural hearing loss + DM with maternal inheritance pattern.

Condition	Explanation	How to Distinguish
Physiological glycosuria of pregnancy	Normal pregnancy: ↑ GFR (50% increase) → ↑ filtered glucose load → renal tubular reabsorption threshold exceeded → glycosuria even with normal blood glucose.	Glycosuria ≠ GDM. Always confirm with blood glucose / OGTT. Glycosuria on urine dipstick is NOT diagnostic.
Stress hyperglycaemia	Unmasked by infections, pregnancy, steroid therapy or stroke ^[3]. Acute illness triggers counter-regulatory hormones (cortisol, catecholamines, glucagon) → transient hyperglycaemia.	Resolves when acute illness resolves. Should re-check OGTT after recovery. Does NOT persist throughout pregnancy.
Normal pregnancy carbohydrate changes	As described in pathophysiology section — early pregnancy can cause fasting hypoglycaemia ^[5] and late pregnancy causes physiological insulin resistance. Some women may have borderline glucose values that don't quite meet GDM criteria.	These women have "pre-diabetes of pregnancy" — they are at the lower end of the GDM spectrum and should still be monitored.

Condition	Key Features
Permanent neonatal diabetes mellitus (PNDM) ^[4]	Persistent activation of KATP channel (sulphonylurea receptor) ^[4]. Extremely rare to present in a pregnant woman (would have been diagnosed in infancy), but her offspring may be affected.
Cystic fibrosis-related diabetes (CFRD)	Cystic fibrosis is a secondary cause of diabetes (pancreatic disease) ^[4]. CF patients are surviving longer and becoming pregnant. CFRD is managed with insulin.
Post-transplant diabetes	Immunosuppressants (tacrolimus, ciclosporin, steroids) → drug-induced DM. Relevant in transplant recipients who become pregnant.

When confronted with hyperglycaemia in a pregnant woman, work through this systematic framework:

Step 1: Timing

First trimester → think pre-existing DM (T2DM most likely, or T1DM/LADA, or MODY)
24–28 weeks (routine screening) → most likely true GDM, but still consider pre-existing DM that was missed

Step 2: Clinical Profile

Young, lean, acute onset, ketonuria → T1DM / LADA → check autoantibodies + C-peptide
Young, lean, strong multigenerational FHx, stable mild hyperglycaemia → MODY → genetic testing
Overweight/obese, metabolic syndrome features, acanthosis nigricans → T2DM or GDM
History of steroid use, tocolysis, or acute illness → drug-induced / stress hyperglycaemia
Features of endocrinopathy (Cushingoid, thyroid eye disease, paroxysmal HTN) → secondary DM

Step 3: Investigations

HbA1c at booking visit: ≥ 6.5% → overt DM. 5.7–6.4% → high-risk for GDM (but HbA1c is unreliable in pregnancy due to ↑ RBC turnover — falsely low due to rapid RBC turnover in pregnancy ^[4]).
Autoantibodies (anti-GAD, anti-islet cell, anti-insulin, anti-IA-2, anti-ZnT8): positive → autoimmune DM
C-peptide: ↓ = absolute insulin deficiency (T1DM); normal/↑ = insulin resistance pattern (T2DM/GDM)
Postpartum OGTT (6–12 weeks): This is the definitive test — if glucose tolerance normalises, it was GDM; if it persists, it was pre-existing DM ^[1]

HbA1c Caveat in Pregnancy

HbA1c is falsely low in pregnancy due to physiological haemodilution, increased RBC turnover, and iron deficiency. A "normal" HbA1c in a pregnant woman does NOT exclude DM. Conversely, HbA1c can be falsely high in conditions with rapid RBC turnover such as haemolytic anaemia or thalassaemia major ^[4]. Always interpret HbA1c cautiously in pregnancy and rely on glucose-based testing (OGTT) for GDM diagnosis.

Condition	Onset	BMI	FHx Pattern	Autoantibodies	C-peptide	Postpartum	Key Distinguishing Feature
GDM	24–28 wks	Usually ↑	T2DM	Negative	Normal/↑	Resolves	Detected on screening OGTT
Pre-existing T2DM	1st trimester / pre-pregnancy	Usually ↑	T2DM	Negative	Normal/↑	Persists	HbA1c ≥ 6.5% or FG ≥ 7.0 at booking
T1DM / LADA	Any	Lean	Autoimmune	Positive (anti-GAD, anti-islet cell) ^[4]	↓	Persists	Acute onset, ketonuria, lean habitus
MODY	Any (often < 25y)	Lean	Multigenerational AD ^[4]	Negative	Variable	Persists	Genetic testing diagnostic
Drug-induced	After drug exposure	Variable	Variable	Negative	Variable	Resolves after drug stopped	Temporal relationship to steroids / β-agonists
Endocrinopathy	Variable	Variable	Usually absent	Negative	Variable	Persists until cause treated	Specific clinical features (Cushingoid, etc.)
Stress hyperglycaemia	During acute illness	Variable	Variable	Negative	Variable	Resolves with illness	Transient; re-check after recovery
Physiological glycosuria	Any trimester	Any	N/A	N/A	N/A	N/A	Normal blood glucose; urine glucose ≠ DM

High Yield DDx Points:

The most important differential of GDM is undiagnosed pre-existing T2DM — confirmed by postpartum OGTT.

Always check timing of detection: 1st trimester hyperglycaemia = likely pre-existing DM, not GDM.

Lean + acute + ketonuria = think T1DM/LADA → autoantibodies + C-peptide.

Young + lean + strong multigenerational FHx = think MODY → genetic testing.

Steroids for fetal lung maturity commonly cause transient hyperglycaemia → not GDM.

HbA1c is unreliable in pregnancy (physiologically falsely low).

Postpartum OGTT at 6–12 weeks is the definitive test for confirming GDM vs. pre-existing DM.

High Yield Summary

Most important differential: Undiagnosed pre-existing T2DM — distinguished by timing (1st trimester detection), HbA1c ≥ 6.5% at booking, and persistent hyperglycaemia postpartum.

Key differentials framework: (1) Pre-existing DM: T2DM (most common), T1DM/LADA (lean, autoantibodies +ve, ↓ C-peptide), MODY (young, AD FHx, genetic testing); (2) Secondary DM: drug-induced (steroids most important in obstetrics), endocrinopathies (Cushing's, thyrotoxicosis, phaeochromocytoma), pancreatic disease; (3) Non-pathological: physiological glycosuria of pregnancy, stress hyperglycaemia.

Critical investigations for DDx: Autoantibodies (anti-GAD, anti-islet cell), C-peptide, postpartum OGTT (6–12 weeks), genetic testing (for MODY), endocrine workup if clinical suspicion.

Postpartum OGTT is the definitive test: If normal → was GDM. If abnormal → was pre-existing DM all along.

Active Recall - Differential Diagnosis of GDM

References

^[1] Lecture slides: Block C - I am pregnant_ medical problems complicating pregnancy.pdf (p17, GDM definition vs pre-existing DM, postpartum OGTT) ^[3] Senior notes: Ryan Ho Endocrine.pdf (p75–80, DM overview, T1DM and T2DM features, secondary causes, stress hyperglycaemia, diagnostic criteria) ^[4] Senior notes: Maksim Medicine Notes.pdf (p80–81, DM overview table, MODY, secondary causes, HbA1c caveats) ^[5] Lecture slides: Block C - I am pregnant_ medical problems complicating pregnancy.pdf (p10–12, effects of pregnancy on DM and DM on pregnancy) ^[6] Senior notes: Ryan Ho Chemical Path.pdf (p35, diagnostic criteria for DM, GDM uses different criteria) ^[7] Senior notes: Ryan Ho GI.pdf (p348, chronic pancreatitis and secondary DM) ^[8] Senior notes: Ryan Ho GI.pdf (p294, haemochromatosis and iron deposition in pancreatic islets)

Diagnostic Criteria, Algorithm, and Investigations for Gestational Diabetes Mellitus

This is a fundamental concept that students often miss. Gestational diabetes uses a different set of criteria from standard DM. ^[6]

Why? Because the standard DM diagnostic thresholds (fasting ≥ 7.0 mmol/L, 2h ≥ 11.1 mmol/L) were derived from the glucose level at which microvascular complications (retinopathy, nephropathy) begin to increase in the general population. But in pregnancy, the concern is different — we are primarily worried about adverse pregnancy outcomes (macrosomia, pre-eclampsia, neonatal hypoglycaemia, shoulder dystocia). These complications begin to increase at glucose levels well below the standard DM thresholds. Therefore, GDM uses lower cut-offs that are calibrated to the risk of adverse pregnancy outcomes.

Think of it this way: a fasting glucose of 5.5 mmol/L would be perfectly fine for a non-pregnant adult (not even pre-diabetes by ADA criteria). But in pregnancy, this level already begins to increase fetal macrosomia risk. The GDM criteria exist to capture this "grey zone" that is dangerous in pregnancy but not in non-pregnant individuals.

Diagnostic Criteria

There are multiple sets of criteria in use globally. The most important ones for HKUMed exams are:

These criteria were derived from the HAPO Study (Hyperglycaemia and Adverse Pregnancy Outcomes, 2008), a landmark multinational study of ~25,000 pregnancies that established a continuous relationship between maternal glucose levels and adverse outcomes. The thresholds were set at the glucose level associated with 1.75× the odds of macrosomia, primary C-section, and neonatal hypoglycaemia.

75g OGTT at 24–28 weeks — GDM is diagnosed if ANY ONE of the following is met:

Time Point	Venous Plasma Glucose Threshold
Fasting	≥ 5.1 mmol/L
1-hour	≥ 10.0 mmol/L
2-hour	≥ 8.5 mmol/L

Key points about IADPSG criteria:

This is a one-step approach: a single 75g OGTT, with THREE glucose values measured (fasting, 1h, 2h).
Only ONE abnormal value is needed for diagnosis — this is a lower threshold than older criteria which required two abnormal values.
This is the approach used in most major centres in Hong Kong under the Hospital Authority.

The ADA endorses either the IADPSG one-step approach (above) or the traditional two-step approach:

Two-step approach:

Step 1: 50g non-fasting glucose challenge test (GCT) — a screening test.
- If 1h glucose ≥ 7.8 mmol/L (140 mg/dL) → proceed to Step 2.
- Some centres use ≥ 7.2 mmol/L (130 mg/dL) for higher sensitivity.
Step 2: 100g OGTT (fasting) — the diagnostic test.
- GDM diagnosed if ≥ 2 of 4 values are met (Carpenter-Coustan criteria):

Time Point	Threshold
Fasting	≥ 5.3 mmol/L
1-hour	≥ 10.0 mmol/L
2-hour	≥ 8.6 mmol/L
3-hour	≥ 7.8 mmol/L

One-Step vs Two-Step: What Does HK Use?

In HK, use the OGTT between 26–30 weeks as screening for diabetes in low-risk patients, and an earlier one for high-risk. ^[1] Hong Kong predominantly uses the one-step IADPSG/WHO 75g OGTT approach. The two-step approach (with 50g GCT screening) is more commonly used in North America. For HKUMed exams, know both but emphasise the IADPSG criteria.

If a woman presents early in pregnancy (typically first antenatal visit in the 1st trimester) and meets standard DM criteria, she has overt DM in pregnancy, NOT GDM:

Parameter	Overt DM in Pregnancy	GDM
Fasting glucose	≥ 7.0 mmol/L ^[4]^[6]	≥ 5.1 but < 7.0 mmol/L
Random glucose	≥ 11.1 mmol/L with symptoms ^[4]^[6]	N/A (diagnosed by OGTT)
HbA1c	≥ 6.5% ^[4]^[6]	Not used for GDM diagnosis
2h OGTT	≥ 11.1 mmol/L	≥ 8.5 mmol/L but < 11.1 mmol/L

Critical Exam Point

If a woman at her booking visit has a fasting glucose of 7.2 mmol/L, she does NOT have GDM. She has overt DM in pregnancy (most likely pre-existing T2DM that was undiagnosed). This distinction matters enormously: she has been hyperglycaemic during organogenesis → sugar is teratogenic ^[5] → higher risk of congenital malformations. She needs different counselling, more intensive surveillance, and lifelong follow-up.

Pre-procedure: 3 days of normal diet and activity, fast overnight. Procedure: at 9am, drink 75g anhydrous glucose in 300mL water in 10 minutes (free water intake in between). Measurement: check plasma glucose at baseline (fasting), 60 min, and 120 min. ^[4]

Step	Detail	Rationale
3 days normal diet	Unrestricted carbohydrate intake (≥ 150g/day) for 3 days before	Carbohydrate restriction can artificially improve glucose tolerance → false negative
Normal physical activity	No unusual exercise	Exercise improves insulin sensitivity → false negative
Overnight fast ≥ 8 hours	Fasting defined as no caloric intake for ≥ 8h ^[6]	Standardises baseline glucose
Fasting blood sample	Venous plasma glucose	Baseline value (threshold ≥ 5.1 mmol/L for GDM)
75g glucose load	75g anhydrous glucose dissolved in 300mL water, drunk within 10 minutes	Standardised glucose challenge
1h blood sample	Venous plasma glucose at 60 minutes	Threshold ≥ 10.0 mmol/L
2h blood sample	Venous plasma glucose at 120 minutes	Threshold ≥ 8.5 mmol/L
Patient remains seated	No eating, drinking (except water), smoking, or walking during test	Physical activity lowers blood glucose; smoking affects catecholamines

Contraindication to OGTT: If the initial fasting glucose is already very high (e.g., > 11.1 mmol/L), do NOT proceed with the glucose load — contraindicated if initial glucose is very high due to risk of life-threatening hyperglycaemia ^[4]. The diagnosis of overt DM is already established.

Screening Strategy and Diagnostic Algorithm

Category	Timing	Rationale
All pregnant women (universal screening)	26–30 weeks (HK practice) ^[1]	Peak insulin resistance occurs 28–36 weeks; screening at 26–30 weeks catches most cases before major complications develop
High-risk women — earlier screening	First antenatal visit or early in pregnancy ^[1]	Risk factors: previous GDM, obesity (BMI ≥ 25 / ≥ 23 for Asians), age ≥ 35, FHx T2DM, PCOS, previous macrosomic baby, glucosuria. Early screening detects pre-existing undiagnosed DM and early-onset GDM
If early screen negative	Repeat at 24–28 weeks	Because insulin resistance continues to rise — a woman who was normal at 12 weeks may develop GDM by 28 weeks

Investigation Modalities

Once GDM is diagnosed (or suspected), a comprehensive set of investigations is needed. These serve three purposes: (1) confirm the diagnosis, (2) assess severity and glycaemic control, (3) screen for complications and comorbidities.

1. Diagnostic Investigations

This is the gold standard for GDM diagnosis.

Aspect	Detail
What it measures	The body's ability to clear a standardised glucose load — reflects both insulin secretion and insulin sensitivity
Why 75g glucose?	Standardised dose that approximates a moderate carbohydrate meal; reproducible across centres
Why fasting, 1h, AND 2h values?	Fasting glucose reflects hepatic glucose output (how well insulin suppresses overnight gluconeogenesis). 1h and 2h values reflect peripheral glucose disposal (how effectively muscles and fat take up glucose after a challenge) and β-cell secretory capacity. Different women may fail at different time points — some have impaired fasting (hepatic issue), others have impaired disposal (peripheral issue).
Interpretation	See IADPSG criteria above. Any one abnormal value = GDM.

Aspect	Detail
At booking visit	FPG ≥ 7.0 mmol/L → overt DM (not GDM). FPG 5.1–6.9 mmol/L at first visit → some guidelines consider this sufficient for GDM diagnosis without full OGTT. FPG < 5.1 mmol/L → reassuring; proceed to OGTT at 24–28 weeks.
Why fasting glucose reflects hepatic output	Overnight, insulin suppresses hepatic glucose production. If fasting glucose is elevated, it means insulin is insufficient to restrain the liver → indicates significant insulin resistance or β-cell failure.

HbA1c is glycated haemoglobin that persists throughout the life of the RBC → reflects mean blood glucose level over the lifespan of the RBC (~120 days). ^[4]

Aspect	Detail
Role in GDM	NOT used to diagnose GDM (because of physiological changes in pregnancy that affect its reliability). However, it IS used to: (1) diagnose overt DM in pregnancy (≥ 6.5% at booking), (2) assess pre-conceptional glycaemic control in pre-existing DM, (3) monitor glycaemic control during pregnancy.
Why HbA1c is unreliable in pregnancy ^[4]	Falsely low due to: (1) haemodilution (plasma volume expansion → ↑ RBC production → younger RBC population → less time for glycation), (2) ↑ RBC turnover (iron deficiency common in pregnancy → ↑ reticulocyte count), (3) physiological anaemia of pregnancy. Therefore a "normal" HbA1c does NOT exclude GDM. Falsely high if rapid RBC turnover (e.g., haemolytic anaemia, thalassaemia major) ^[4] — and thalassaemia trait is very common in HK (8–9% carrier rate).
Target in pregnancy	ADA 2025: < 6.0% in pregnancy (if achievable without significant hypoglycaemia); < 6.5% is acceptable. HbA1c < 7% is the aim for diabetics; 6.5% is the diagnostic threshold ^[9]. In pregnancy, we aim for tighter control (< 6.0–6.5%) because of the continuous glucose exposure to the fetus.
Alternatives when HbA1c is unreliable ^[4]	Fructosamine, glycated albumin, 1,5-anhydroglucitol, continuous glucose monitoring (CGM) ^[4]. Fructosamine reflects glucose control over 2–3 weeks (albumin half-life) and is not affected by RBC turnover.

HbA1c in Pregnancy — Key Exam Caveat

Do NOT use HbA1c to diagnose GDM. It is physiologically falsely low in pregnancy and also unreliable in thalassaemia carriers (very common in HK). Always use glucose-based testing (OGTT) for GDM diagnosis. HbA1c CAN be used to identify overt DM at the booking visit (≥ 6.5%) and to monitor control, but interpret with caution.

2. Monitoring Investigations (After GDM Diagnosis)

This is the cornerstone of day-to-day management monitoring.

Aspect	Detail
What it involves	Capillary blood glucose measurements using a glucometer ("H'stix")
Timing	Typically 4-point profile: fasting (pre-breakfast) + 1h or 2h post each main meal (post-breakfast, post-lunch, post-dinner). Some centres use 7-point profiles.
Targets (ADA / ACOG 2025)	Fasting: < 5.3 mmol/L (95 mg/dL). 1h post-meal: < 7.8 mmol/L (140 mg/dL). 2h post-meal: < 6.7 mmol/L (120 mg/dL).
Why these targets are tighter than non-pregnant DM	The fetus is continuously exposed to maternal glucose. Even "mild" hyperglycaemia (e.g., fasting 6.0) drives fetal hyperinsulinaemia → macrosomia. The targets are set to minimise the risk of adverse pregnancy outcomes while avoiding maternal hypoglycaemia.
Frequency	Daily (at least 4 readings/day) when first diagnosed; may reduce frequency once stable on diet therapy; increase if starting insulin.

Example from lecture case — Mrs. Au's blood glucose monitoring: ^[9]

Time	Fasting	Post-breakfast	Pre-lunch	Post-lunch	Pre-dinner	Post-dinner
H'stix (mmol/L)	6.2	8.0	7.0	7.8	7.5	9.0

Interpretation: Fasting 6.2 (target < 5.3 → above target). Post-breakfast 8.0 (target 1h < 7.8 or 2h < 6.7 → above target). Post-dinner 9.0 → above target. These values indicate suboptimal glycaemic control even though they don't meet standard DM diagnostic thresholds — in pregnancy, these levels are too high and will drive fetal complications. ^[9]

Aspect	Detail
What it is	A small sensor inserted subcutaneously that measures interstitial glucose every 5 minutes → generates a continuous glucose profile
Advantages over HBSM	Detects glucose excursions (spikes and dips) that point measurements miss; detects nocturnal hypoglycaemia and morning hyperglycaemia ^[3]; calculates "time in range" (% of time glucose is 3.5–7.8 mmol/L in pregnancy)
Indications in GDM	Not routine for all GDM. Mainly used for: (1) insulin-treated GDM with difficult control, (2) suspected nocturnal hypoglycaemia, (3) large discrepancy between HbA1c and HBSM readings, (4) pre-existing DM in pregnancy.
Target	Time in range (3.5–7.8 mmol/L) > 70%; time below range (< 3.5) < 4%; time above range (> 7.8) < 25%.

3. Investigations for Complications and Comorbidities

Investigation	What It Assesses	Key Findings / Interpretation
Blood pressure ^[4]	Pre-eclampsia screening — GDM increases pre-eclampsia risk 2–4×.	Measure BP at every visit. ^[4] HTN in context of GDM → think pre-eclampsia.
Urinalysis (protein + glucose) ^[10]	Protein: pre-eclampsia screening. Glucose: glycosuria (but note physiological glycosuria in pregnancy — glycosuria ≠ GDM).	Proteinuria ≥ 2+ → suspect pre-eclampsia. ^[10] Glycosuria should prompt blood glucose measurement but is not diagnostic.
Renal function (RFT)	Baseline renal function + monitoring for pre-eclampsia (↑ creatinine, ↑ uric acid).	Elevated uric acid is an early marker of pre-eclampsia. In pre-existing DM: assess for diabetic nephropathy.
Urine albumin-to-creatinine ratio (UACR)	Screen for pre-existing diabetic nephropathy (especially if suspecting overt DM rather than GDM).	Microalbuminuria: 30–300 mg/g. Macroalbuminuria: > 300 mg/g.
Lipid profile	Comorbid dyslipidaemia (metabolic syndrome).	Hypertriglyceridaemia, ↓ HDL-C ^[3] suggest metabolic syndrome and underlying insulin resistance. Note lipids are physiologically elevated in pregnancy.
Liver function tests (LFT)	(1) Baseline before starting metformin (if used); (2) HELLP syndrome screening if pre-eclampsia suspected.	↑ Transaminases + ↓ platelets + haemolysis → HELLP syndrome.
Full blood count (FBC)	Baseline; anaemia assessment (iron deficiency very common in pregnancy).	Important because anaemia affects HbA1c reliability.
Thyroid function (TFT)	Screen for concurrent thyroid disease (autoimmune thyroid disease can coexist, especially if suspecting T1DM).	Hypothyroidism → ↑ insulin resistance. Hyperthyroidism → can worsen glycaemic control.
Autoantibodies (if suspecting T1DM/LADA) ^[4]	Anti-GAD (70–80%), anti-islet cell, anti-insulin (60–75%), anti-IA-2 (65–75%), anti-ZnT8 (70–80%) ^[3]	Positive → autoimmune DM, not GDM. Requires insulin from the outset.
C-peptide ^[3]	Endogenous insulin secretion — C-peptide is cleaved from pro-insulin during insulin secretion ^[3].	↓ C-peptide: T1DM (absolute insulin deficiency). Normal/↑ C-peptide: T2DM or GDM (insulin resistance). ^[3]
Fundoscopy ^[5]	Screen for diabetic retinopathy (especially if suspecting pre-existing DM).	Due to worsened control of DM during pregnancy → increased risk of complications involving cardiovascular, renal and optic systems. ^[5] If retinopathy present → this is NOT GDM; it is pre-existing DM. Dilated fundus examination annually. ^[3]

Investigation	What It Assesses	Key Findings / Interpretation
Serial ultrasound for fetal growth	Macrosomia (LGA), growth restriction (SGA in pre-existing DM with vasculopathy)	Abdominal circumference (AC) is the most sensitive early marker of macrosomia ^[11] — AC > 90th centile suggests excessive fetal growth from hyperinsulinaemia. Estimated fetal weight (EFW) calculated from biparietal diameter (BPD), femoral length (FL), and AC.
Anomaly scan (18–20 weeks)	Congenital malformations — mainly relevant for pre-existing DM but also if early-onset GDM or overt DM discovered in pregnancy.	Cardiac defects (VSD, TGA), neural tube defects (anencephaly, spina bifida), caudal regression syndrome. Sugar is teratogenic. ^[5]
Fetal echocardiography	Fetal cardiac anomalies + hypertrophic cardiomyopathy	Interventricular septal hypertrophy from fetal hyperinsulinaemia → glycogen deposition.
Amniotic fluid volume (AFI)	Polyhydramnios — AFI > 25 cm or deepest vertical pocket > 8 cm.	Fetal hyperglycaemia → fetal osmotic diuresis → fetal polyuria → excess amniotic fluid. AFI measured on routine growth scans. Large SFH → consider polyhydramnios, macrosomia. ^[10]
Cardiotocography (CTG) / Non-stress test	Fetal wellbeing — especially in the third trimester	Poorly controlled diabetes → chronic fetal hypoxia → abnormal CTG patterns (reduced variability, late decelerations). Usually commenced from 32–36 weeks in GDM, earlier if poor control.
Umbilical artery Doppler	Placental perfusion — mainly for pre-existing DM with vasculopathy	Raised resistance index or absent/reversed end-diastolic flow → placental insufficiency → fetal compromise. Less commonly abnormal in GDM (which has macrosomia rather than growth restriction).
Fetal movement counting (kick chart)	Simple maternal surveillance of fetal wellbeing	Reduced fetal movements may indicate fetal compromise / stillbirth. Fetal hyperinsulinaemia → ↑ oxygen consumption → chronic hypoxia → ↓ movements.

To confidently diagnose GDM (rather than pre-existing DM), do an OGTT after the baby is born — see if there is glucose intolerance still. ^[1]

Investigation	Timing	Criteria	Interpretation
75g OGTT	6–12 weeks postpartum	Use standard (non-pregnancy) DM criteria: FPG ≥ 7.0 or 2h ≥ 11.1 → DM. FPG 5.6–6.9 → IFG. 2h 7.8–11.0 → IGT.	If normal → confirmed GDM (resolved). If IFG/IGT → pre-diabetes (high risk for future T2DM). If DM → was pre-existing, undiagnosed DM.
HbA1c	Can be checked alongside OGTT	≥ 6.5% → DM. 5.7–6.4% → pre-diabetes. ^[4]^[6]	More reliable postpartum than during pregnancy (no longer haemodiluted).
Ongoing screening	Every 1–3 years lifelong	75g OGTT or FPG or HbA1c	Up to 50% of GDM women develop T2DM within 5–10 years. Lifelong surveillance is essential.

Postpartum OGTT — Don't Forget!

Many women are lost to follow-up after delivery. The postpartum OGTT at 6–12 weeks is essential to: (1) confirm GDM resolution vs. persistent DM, and (2) identify pre-diabetes. Women with previous GDM should be counselled that they have a very high lifetime risk of T2DM and should be screened every 1–3 years with lifestyle modification counselling. Pre-pregnancy counselling before subsequent pregnancies is also critical — good antenatal care including pre-pregnancy diabetic control, leading into good intra-pregnancy diabetic control. ^[5]

Category	Investigation	Timing	Purpose
Diagnosis	75g OGTT	24–28 weeks (earlier if high-risk)	GDM diagnosis (IADPSG criteria)
Exclude overt DM	FPG, HbA1c	First antenatal visit	Detect pre-existing undiagnosed DM
Monitoring	HBSM (H'stix)	Daily after diagnosis	Assess glycaemic control
Monitoring	HbA1c	Monthly or every trimester	Overall glycaemic trend (interpret with caution)
Monitoring	CGM	If insulin-treated / difficult control	Continuous glucose profile
DDx	Autoantibodies, C-peptide	If suspecting T1DM/LADA/MODY	Aetiology clarification
Maternal Cx	BP, urinalysis, RFT, uric acid	Every visit	Pre-eclampsia screening
Maternal Cx	Fundoscopy	At booking + each trimester if pre-existing DM	Diabetic retinopathy
Fetal	USS growth scans	Every 2–4 weeks from 28 weeks	Macrosomia, polyhydramnios
Fetal	Anomaly scan	18–20 weeks	Congenital malformations
Fetal	CTG	From 32–36 weeks	Fetal wellbeing
Postpartum	75g OGTT	6–12 weeks postpartum	Confirm GDM resolution vs. persistent DM
Long-term	OGTT / FPG / HbA1c	Every 1–3 years lifelong	Screen for T2DM development

High Yield Diagnostic Points:

GDM diagnostic criteria are LOWER than standard DM criteria — they are calibrated to adverse pregnancy outcomes, not microvascular complications.

IADPSG/WHO criteria (75g OGTT): FPG ≥ 5.1, 1h ≥ 10.0, 2h ≥ 8.5 — ANY ONE abnormal value = GDM.

If FPG ≥ 7.0 or HbA1c ≥ 6.5% at booking → overt DM in pregnancy, NOT GDM.

HbA1c is falsely low in pregnancy — do NOT use it to diagnose GDM.

In HK: universal screening with OGTT at 26–30 weeks; earlier for high-risk.

Postpartum OGTT at 6–12 weeks is essential to confirm GDM vs. pre-existing DM.

HBSM targets in pregnancy: fasting < 5.3, 1h post-meal < 7.8, 2h post-meal < 6.7 mmol/L.

High Yield Summary

Diagnostic Criteria (IADPSG/WHO — used in HK): 75g OGTT at 24–28 weeks. GDM if ANY ONE: fasting ≥ 5.1, 1h ≥ 10.0, 2h ≥ 8.5 mmol/L. Only one abnormal value needed.

Overt DM in Pregnancy (NOT GDM): FPG ≥ 7.0 or random ≥ 11.1 with symptoms or HbA1c ≥ 6.5% — detected at first antenatal visit.

HbA1c in Pregnancy: Do NOT use for GDM diagnosis (physiologically falsely low). CAN use to detect overt DM at booking (≥ 6.5%) and monitor control (target < 6.0%). Unreliable in thalassaemia carriers (common in HK).

OGTT Procedure: 3 days normal diet → overnight fast ≥ 8h → fasting blood → 75g glucose in 300mL water over 10 min → blood at 1h and 2h. Patient seated, no food/drink/smoking/exercise.

Screening Strategy (HK): Universal screening OGTT at 26–30 weeks. Earlier OGTT for high-risk women. If early screen normal → repeat at 24–28 weeks.

HBSM Targets: Fasting < 5.3, 1h post-meal < 7.8, 2h post-meal < 6.7 mmol/L.

Postpartum: OGTT at 6–12 weeks postpartum to confirm resolution. Then lifelong screening every 1–3 years (up to 50% develop T2DM within 5–10 years).

Key Investigations: OGTT (diagnostic), HBSM (monitoring), USS growth scans (fetal macrosomia), CTG (fetal wellbeing), BP + urinalysis (pre-eclampsia), fundoscopy (retinopathy if pre-existing DM), autoantibodies + C-peptide (DDx).

Active Recall - GDM Diagnostic Criteria, Algorithm, and Investigations

References

^[1] Lecture slides: Block C - I am pregnant_ medical problems complicating pregnancy.pdf (p17, GDM screening timing in HK, postpartum OGTT) ^[3] Senior notes: Ryan Ho Endocrine.pdf (p75–81, DM pathophysiology, insulin secretion, C-peptide, autoantibodies, CGM, annual screening) ^[4] Senior notes: Maksim Medicine Notes.pdf (p80–81, DM diagnostic criteria, HbA1c procedure and caveats, OGTT procedure, physical examination) ^[5] Lecture slides: Block C - I am pregnant_ medical problems complicating pregnancy.pdf (p10–13, effects of DM on pregnancy, sugar is teratogenic, pre-pregnancy control) ^[6] Senior notes: Ryan Ho Chemical Path.pdf (p35, HA diagnostic criteria for DM, GDM uses different criteria, fasting definition) ^[9] Lecture slides: Block C - O&G Theme Case 1.docx.pdf (p9, Mrs. Au case — blood glucose monitoring values, HbA1c interpretation) ^[10] Senior notes: Ryan Ho Fundamentals.pdf (p191, obstetric examination — urinalysis, SFH measurement and interpretation) ^[11] Senior notes: Ryan Ho Radiology.pdf (p35, obstetric ultrasound indications — BPD, FL, AC for fetal growth assessment)

Management of Gestational Diabetes Mellitus

Before diving into specifics, let's establish the management philosophy:

Good glycaemic control is the cornerstone of management — by diet ± insulin. ^[2]

The principles in the management of a medical disorder complicating pregnancy are to avoid or reduce the maternal and/or fetal risks. ^[9]

Don't treat the condition only during pregnancy — good antenatal care including pre-pregnancy diabetic control, leading into good intra-pregnancy diabetic control. ^[5]

The management of GDM is built on a stepwise escalation model. Think of it as a pyramid: the broad base is lifestyle modification (diet + exercise), which works for the majority (~70–80%). If that fails, you escalate to pharmacotherapy (insulin as first-line, metformin as alternative). The apex involves managing the delivery itself and the critical postpartum period.

Why is tight glycaemic control so important? Because every fetal complication traces back to the Pedersen hypothesis: maternal hyperglycaemia → fetal hyperglycaemia → fetal hyperinsulinaemia → macrosomia, RDS, neonatal hypoglycaemia, etc. Poor control of diabetes mellitus increases the incidence of maternal/fetal complications including spontaneous miscarriage, congenital malformation, stillbirth, respiratory distress syndrome, macrosomia, pre-eclampsia and polyhydramnios. ^[9]

GDM management is NOT a one-doctor show. It requires coordinated care from:

Team Member	Role
Obstetrician	Pregnancy monitoring, delivery planning, fetal surveillance
Endocrinologist / Physician	Glycaemic management, insulin titration, comorbidity management
Diabetes Nurse Specialist	HBSM education, insulin injection technique, hypoglycaemia management
Dietitian	Medical nutrition therapy — the single most important intervention ^[4]^[9]
Midwife	Routine antenatal care, patient education, emotional support
Neonatologist	Standby for delivery, neonatal blood glucose monitoring

Patient education is a team approach. ^[3] The woman needs to understand WHY control matters (linking maternal glucose to fetal outcomes) and HOW to achieve it (diet, exercise, HBSM, insulin if needed).

These are tighter than for non-pregnant DM because the fetus is continuously exposed to maternal glucose.

Parameter	Target	Rationale
Fasting glucose	< 5.3 mmol/L	Reflects hepatic glucose output; if elevated, means insulin isn't suppressing overnight gluconeogenesis adequately
1h post-meal glucose	< 7.8 mmol/L	Reflects postprandial glucose disposal
2h post-meal glucose	< 6.7 mmol/L	Alternative to 1h if centre uses 2h monitoring
HbA1c	< 6.0% (ideally) [ADA 2025]; < 6.5% acceptable	< 7% is the aim for diabetics aiming for control ^[9], but in pregnancy we push tighter because of fetal exposure. Trying to push down too low → risk of hypoglycaemia ^[9]
Avoid hypoglycaemia	Glucose should NOT drop < 3.5 mmol/L	Maternal hypoglycaemia is dangerous — can cause loss of consciousness, seizures, falls. Blood glucose becomes more difficult to control in pregnancy, sometimes resulting in fasting hypoglycaemia ^[5]

Why Tighter Targets in Pregnancy?

In non-pregnant adults, HbA1c < 7% is the general target ^[4]. In pregnancy, we aim for < 6.0–6.5%. Why the difference? Because in non-pregnant adults, we are trying to prevent chronic microvascular complications over years. In pregnancy, we are trying to prevent ACUTE fetal harm over weeks to months. The fetus responds to even mild hyperglycaemia with hyperinsulinaemia — there is no "safe" threshold, only a continuous dose-response relationship (HAPO study). Tighter maternal control = less fetal insulin exposure = fewer complications.

Step 3: Medical Nutrition Therapy (MNT) — First-Line Treatment

This is where the vast majority of GDM management happens. ~70–80% of women with GDM can achieve glycaemic targets with diet modification alone (White Class A1).

Start with lifestyle modifications ^[4]: Diet (complex carbohydrates, low fat, small frequent meals) + aerobic exercise ^[4].

Principle	Detail	Rationale
Total caloric intake	Based on pre-pregnancy BMI: Normal BMI → 30 kcal/kg/day. Overweight → 25 kcal/kg/day. Obese → 20 kcal/kg/day (but never < 1500 kcal/day).	Caloric restriction in obese women reduces insulin resistance and glucose excursions, BUT excessive restriction → ketosis (harmful to fetus)
Carbohydrate distribution	40–50% of total calories from carbohydrates, distributed across 3 meals + 2–3 snacks	Spreading carbohydrate intake prevents large glucose spikes. A bedtime snack is important to prevent overnight fasting ketosis.
Complex carbohydrates ^[4]	Prefer low glycaemic index (GI) foods: whole grains, legumes, vegetables. Avoid simple sugars, white rice/bread, sugary drinks.	Low-GI foods are digested and absorbed more slowly → slower, lower glucose rise → less insulin demand → less fetal glucose exposure
Small frequent meals ^[4]	3 meals + 2–3 snacks (e.g., mid-morning, mid-afternoon, bedtime)	Prevents large glucose excursions from big meals; maintains steady glucose delivery to fetus; prevents fasting ketosis
Protein	20–25% of calories	Protein slows carbohydrate absorption and provides satiety without significant glucose rise
Low fat ^[4]	25–30% of calories, predominantly unsaturated	Reduces insulin resistance (↓ FFA-mediated insulin resistance); cardiovascular health
Fibre	≥ 25g/day	Slows carbohydrate absorption → ↓ postprandial glucose spikes

The Bedtime Snack

A bedtime snack containing complex carbohydrates + protein (e.g., a few crackers with cheese, or a glass of milk with whole-grain biscuits) is specifically recommended. Why? In late pregnancy, the long overnight fast combined with placental glucose consumption can lead to accelerated starvation — exaggerated lipolysis → ketone body production → starvation ketosis. Ketone bodies may be harmful to fetal brain development. The bedtime snack prevents this by providing a slow-release glucose source overnight.

Aspect	Detail	Rationale
Type	Moderate-intensity aerobic exercise: brisk walking, swimming, stationary cycling, prenatal yoga	These are safe in pregnancy (low impact, low fall risk)
Duration	≥ 30 minutes/day, most days of the week (≥ 150 min/week)	Exercise ↑ GLUT-4 translocation to muscle cell membranes → ↑ glucose uptake independent of insulin. Also ↑ AMPK activation → ↑ glucose uptake + ↑ FFA metabolism ^[3]. Net effect: ↓ insulin resistance.
Timing	Post-meal walks (15–30 min after each meal) are especially effective	Directly blunts the postprandial glucose spike
Contraindications	Preterm labour, cervical incompetence, placenta praevia, pre-eclampsia, ruptured membranes	Standard obstetric contraindications to exercise

Step 4: Pharmacotherapy — When Diet Fails

If glycaemic targets are not met after 1–2 weeks of MNT + exercise, pharmacotherapy is required. About 20–30% of women with GDM will need pharmacotherapy.

A. Insulin — First-Line Pharmacotherapy

Insulin is the preferred pharmacotherapy for GDM because:

Insulin does NOT cross the placenta (large peptide, ~5.8 kDa) → no direct fetal effects.
Long-established safety profile in pregnancy.
Precise dose titration possible.
Effective for both fasting and postprandial hyperglycaemia.

Indications of insulin treatment include pregnancy ^[3].

For pre-existing DM in pregnancy: might consider upping dose of insulin to adjust for these placental hormones ^[5] — insulin requirements typically increase by 50–100% from early to late pregnancy due to rising placental counter-regulatory hormones.

Insulin Type	Examples	Onset	Peak	Duration	Use in GDM
Rapid-acting analogues ^[3]	Insulin Aspart (NovoRapid), Insulin Lispro (Humalog) ^[3]	10–15 min	1–2 h	3–5 h	Prandial (before meals) — to cover postprandial glucose spikes
Short-acting (regular) ^[3]	Actrapid, Humulin R ^[3]	30 min	2–4 h	6–8 h	Prandial; also used in intrapartum glucose management
Intermediate-acting ^[3]	Insulin NPH (Protaphane) ^[3]	1–2 h	4–8 h	12–18 h	Basal — typically given at bedtime to control fasting glucose
Long-acting analogues ^[3]	Insulin Detemir (Levemir)	1–2 h	Peakless	18–24 h	Basal; approved for pregnancy use. Insulin Glargine (Lantus), Insulin Degludec (Tresiba) ^[3] — increasingly used off-label but formal pregnancy safety data is more limited (commonly used in practice).

Consider fasting H'stix for fasting glycaemic control, A1c for postprandial control. ^[3] In GDM, we tailor the regimen to the pattern of hyperglycaemia:

Pattern	Regimen	Explanation
Fasting hyperglycaemia only	Basal insulin only: NPH 10U at bedtime ^[3]	Fasting glucose reflects overnight hepatic glucose output. NPH given at bedtime peaks at ~4–8h → covers the pre-dawn period when hepatic gluconeogenesis is most active.
Postprandial hyperglycaemia only	Rapid-acting insulin before affected meals	Most women with GDM have predominantly postprandial hyperglycaemia. Give rapid-acting insulin (e.g., NovoRapid 4–6U) before the meal causing the spike.
Both fasting AND postprandial	Basal-bolus regimen ^[3]	NPH at bedtime + rapid-acting before each meal. This provides the most flexible and physiological coverage.
Simplification	Pre-mixed insulin BD	Less precise but simpler for women who struggle with multiple injections. E.g., Novomix 30 before breakfast and dinner.

Principle	Detail
Starting dose	Typically 0.1–0.2 IU/kg/day, divided into basal ± prandial. Conservative start to avoid hypoglycaemia.
Titration	Increase 2 units every 3 days to reach target without hypoglycaemia. ^[3] Adjust based on HBSM pattern. If fasting high → ↑ bedtime NPH. If post-meal high → ↑ pre-meal rapid-acting.
Total daily dose progression	Early 2nd trimester: ~0.7 IU/kg/day. Third trimester: ~0.8–1.0 IU/kg/day (increasing due to rising insulin resistance from placental hormones). May need to increase dose as pregnancy advances. ^[5]
Postpartum	Insulin requirements drop dramatically (often by 50% or more) immediately after delivery because the placenta is removed → counter-regulatory hormones disappear. For GDM: STOP insulin immediately after delivery. For pre-existing DM: return to pre-pregnancy doses and retitrate.

Concern	Explanation
Hypoglycaemia	Most common side effect. Educate on symptoms (tremor, sweating, palpitations, confusion), treatment (15g fast-acting carbohydrate), and when to seek help. Trying to push down too low → risk of hypoglycaemia, deadly in some studies. ^[9]
Weight gain	Insulin promotes lipogenesis. Not a major concern in a short course during pregnancy, but contributes to total gestational weight gain.
Injection technique	Subcutaneous injection into abdomen (safe in pregnancy — avoid periumbilical area), thighs, or upper arms. Rotate sites to prevent lipodystrophy due to insulin injection. ^[4]
Insulin storage	Unopened vials in refrigerator; in-use vials/pens at room temperature for up to 28 days.

Metformin is an oral biguanide ("bi" = two, "guanide" = derived from guanidine, a compound found in French lilac). It is the first-line oral therapy for T2DM ^[3] and is increasingly used in GDM as an alternative to insulin.

Aspect	Detail
Mechanism	(1) ↓ Hepatic gluconeogenesis (primary effect — via AMPK activation → ↓ mitochondrial complex I → ↓ ATP → ↓ gluconeogenesis); (2) ↑ Peripheral insulin sensitivity (via ↑ GLUT-4 translocation); (3) ↓ Intestinal glucose absorption. Does NOT increase insulin secretion → does NOT cause hypoglycaemia.
Indication in GDM	(1) Patient refuses insulin injections; (2) Adjunct to insulin in severely insulin-resistant GDM; (3) Some centres use as first-line pharmacotherapy (varies by institution).
Dose	Start 500 mg OD with food → titrate to 500 mg BD → max 2500 mg/day. Slow titration reduces GI side effects.
Safety in pregnancy	Metformin DOES cross the placenta (small molecule, ~129 Da). Long-term fetal safety data is reassuring but less extensive than insulin. Multiple RCTs (MiG trial, others) show no increase in congenital anomalies or adverse neonatal outcomes. However, some concerns about: (1) long-term metabolic programming in offspring (theoretical); (2) slightly higher BMI in children exposed in utero. ADA 2025 acknowledges metformin as an option but states insulin is preferred when pharmacotherapy is needed.
Advantages over insulin	Oral (no injections → better compliance), lower cost, no hypoglycaemia risk, may ↓ gestational weight gain, no dose titration complexity.
Disadvantages	Crosses placenta (unlike insulin), GI side effects (nausea, diarrhoea, metallic taste), ~30–50% of women started on metformin eventually need supplemental insulin anyway (metformin alone insufficient).
Contraindications	C/I to metformin: lactic acidosis risk ^[4] — significant renal impairment (eGFR < 30), hepatic failure, severe infection/sepsis, tissue hypoxia, heavy alcohol use. In pregnancy, also consider if acute illness requiring hospitalisation.
Side effects	GI (nausea, diarrhoea, bloating — ↓ with slow titration and taking with food), B12 deficiency (long-term use — less relevant in short GDM course), lactic acidosis (rare, mostly with renal impairment).

Metformin vs. Insulin in GDM — What Does the Evidence Say?

The MiG Trial (Metformin in Gestational Diabetes, 2008) showed that metformin is non-inferior to insulin for glycaemic control in GDM, with similar composite neonatal outcomes. However, 46% of women randomised to metformin needed supplemental insulin. Metformin-exposed offspring had slightly higher subcutaneous fat but lower visceral fat at 2 years. Current ADA/NICE/ACOG guidelines accept metformin as an alternative when insulin is refused or not feasible, but insulin remains the first-line pharmacotherapy.

Aspect	Detail
Class	Sulphonylurea — "sulphon" = sulphur-containing, "urea" = urea derivative.
Mechanism	Binds to SUR1 (sulphonylurea receptor 1) on β-cell KATP channels → channel closure → depolarisation → insulin secretion. Directly stimulates insulin release regardless of glucose level → risk of hypoglycaemia.
Previous use	Was widely used in GDM in the 2000s based on the Langer trial (2000).
Current status	Largely abandoned for GDM. Multiple subsequent studies showed: (1) higher rates of neonatal hypoglycaemia vs. insulin, (2) higher macrosomia rates, (3) DOES cross the placenta (contrary to initial claims) → directly stimulates fetal β-cells → fetal hyperinsulinaemia. Sulphonylureas have higher risk of causing hypoglycaemia → ?cross placenta to cause neonatal hypoglycaemia. ^[9]
Current guidelines	ADA 2025 and NICE 2025 do NOT recommend glyburide for GDM. If a patient on gliclazide pre-pregnancy becomes pregnant → switch to insulin. ^[9]

Exam Scenario: Mrs. Au's Gliclazide

Mrs. Au was taking Gliclazide (a sulphonylurea). When she became pregnant, the question was: "Should I continue taking my Gliclazide?" ^[9] The answer is NO: (1) Not sufficient data to support use in pregnancy ^[9]; (2) Sulphonylureas have higher risk of causing hypoglycaemia and may cross the placenta to cause neonatal hypoglycaemia ^[9]; (3) Switch to insulin, which does NOT cross the placenta and has established safety.

Drug Class	Why NOT Used in Pregnancy
SGLT2 inhibitors (e.g., empagliflozin, dapagliflozin)	"SGLT2" = sodium-glucose co-transporter 2. Blocks glucose reabsorption in proximal tubule → glycosuria. Concerns: (1) can cause maternal dehydration + urinary tract infections, (2) risk of euglycaemic DKA ^[12], (3) no safety data in pregnancy, (4) affect fetal renal development. Contraindicated in pregnancy.
DPP-4 inhibitors (e.g., sitagliptin)	"DPP-4" = dipeptidyl peptidase-4, which degrades incretin hormones (GLP-1, GIP). Insufficient human pregnancy safety data. Not recommended.
GLP-1 receptor agonists (e.g., liraglutide, semaglutide)	"GLP-1" = glucagon-like peptide-1. Powerful insulin secretagogue and appetite suppressant. Animal studies show teratogenicity. Contraindicated in pregnancy. Must be stopped ≥ 2 months before conception (semaglutide has very long half-life).
Thiazolidinediones (e.g., pioglitazone)	"Thiazolidine" = chemical structure. Insulin sensitiser (PPARγ agonist). Crosses placenta. No safety data in pregnancy. Not recommended. Also causes weight gain and fluid retention.
Acarbose	α-glucosidase inhibitor — slows carbohydrate digestion in small bowel. Minimal systemic absorption, but no safety data in pregnancy. Not routinely used.

GDM increases the risk of macrosomia, polyhydramnios, and (in poorly controlled cases) stillbirth. Fetal surveillance is therefore integral to management:

Surveillance	Timing	Purpose
Serial growth USS	Every 2–4 weeks from 28 weeks	Monitor for macrosomia (AC > 90th centile), polyhydramnios (AFI > 25 cm), growth restriction (if pre-existing DM with vasculopathy)
Fetal movement counting	Daily from 28 weeks	Maternal awareness of fetal movements — reduced movements may indicate fetal compromise
CTG / Non-stress test	From 32–36 weeks (weekly or biweekly)	Assess fetal heart rate pattern for evidence of chronic hypoxia
Biophysical profile	If CTG non-reassuring	Comprehensive assessment: CTG + USS (fetal breathing, movement, tone, amniotic fluid volume)
Anomaly scan	18–20 weeks	Congenital malformations — sugar is teratogenic ^[5]. Mainly for pre-existing DM but also if early-onset GDM or if overt DM discovered in pregnancy.
Fetal echocardiography	~20–24 weeks (if pre-existing DM or very early GDM)	Cardiac anomalies + hypertrophic cardiomyopathy

Step 6: Delivery Management

Clinical Scenario	Recommended Timing	Rationale
GDM, diet-controlled, well-grown fetus	Await spontaneous labour up to 40+6 weeks; consider induction at 40–41 weeks	Well-controlled GDM on diet has outcomes close to normal pregnancy. Prolonging beyond 41 weeks increases stillbirth risk even in non-diabetic pregnancies.
GDM, insulin-requiring	Induction of labour (IOL) at 38–39 weeks	Insulin requirement indicates more significant glucose dysregulation → higher risk of macrosomia and stillbirth. Evidence from ARRIVE-like principles supports IOL at 39 weeks.
Pre-existing DM in pregnancy	IOL at 37–38+6 weeks	Higher baseline complication risk. Balance between prematurity risk and stillbirth risk.
Macrosomia (EFW > 4–4.5 kg)	Discuss IOL vs. elective C-section	Increased incidence of cesarean section + instrumental delivery ^[5]. EFW > 4.5 kg → offer C-section to avoid shoulder dystocia risk.
Complications (pre-eclampsia, poor control, fetal compromise)	Individualised; may need earlier delivery	Delivery is the definitive treatment for pre-eclampsia and severe fetal compromise.

Consideration	Guidance
Vaginal delivery	Preferred for most women with GDM. Even insulin-treated GDM can attempt vaginal delivery.
Caesarean section	Consider if: EFW > 4.5 kg (high shoulder dystocia risk), previous shoulder dystocia, failed IOL, abnormal CTG, other obstetric indications (e.g., placenta praevia, malpresentation).
Shoulder dystocia preparedness	Must have experienced team available. Macrosomic babies of diabetic mothers have disproportionate truncal obesity → shoulder dystocia risk is higher than for constitutionally large babies of the same weight.

Aspect	Detail	Rationale
Blood glucose monitoring	Hourly capillary glucose during active labour	Maternal hyperglycaemia during labour → neonatal hypoglycaemia (fetal pancreas responds to last-minute glucose exposure)
Target	4.0–7.0 mmol/L during labour	This range minimises both maternal hyperglycaemia-induced fetal hyperinsulinaemia AND maternal hypoglycaemia
Diet-controlled GDM	Usually no specific intervention needed; allow light diet. Monitor glucose.	Likely to maintain targets without medication
Insulin-treated GDM	Sliding scale insulin (variable-rate IV insulin infusion) + dextrose infusion during active labour. DKI = dextrose/potassium/insulin drip (titrate according to sliding scale). ^[12] Alberti regimen: 500mL D10 + 10U short-acting insulin + 10mmol KCl, monitor BG Q2–6h. ^[12]	Ensures precise glucose control during the dynamic metabolic state of labour. Oral intake is restricted → dextrose provides substrate; insulin prevents hyperglycaemia.
C-section	Perform as first case of the day if possible. Omit morning insulin/OHA. Start sliding scale insulin + dextrose. Omit morning dose of insulin. Monitor BG pre- and post-op. ^[12]	Morning scheduling minimises fasting duration and optimises glycaemic control.
After delivery of placenta	STOP all GDM-related insulin/metformin immediately.	Placenta delivery → rapid ↓ in counter-regulatory hormones → insulin resistance drops dramatically → continuing insulin → hypoglycaemia. For pre-existing DM: return to pre-pregnancy regimen.

Action	Timing	Rationale
Early feeding	Within 30–60 minutes of birth	Provides exogenous glucose to counter the neonate's ongoing hyperinsulinaemia
Neonatal blood glucose monitoring	At 1h, 2h, then pre-feeds for first 24–48h	Detect neonatal hypoglycaemia early (most common complication). Neonatal hypoglycaemia threshold: < 2.0–2.6 mmol/L (varies by guideline).
If neonatal hypoglycaemia	IV dextrose (D10%) infusion	Oral feeds insufficient → need IV glucose to maintain safe levels
Observation for other complications	First 24–72h	RDS, polycythaemia, jaundice, hypocalcaemia — all consequences of fetal hyperinsulinaemia (as per Pedersen hypothesis)

Action	Detail	Rationale
Stop all GDM medications	Immediately after delivery	Insulin resistance resolves with placenta delivery
Monitor glucose for 24–48h postpartum	Capillary glucose monitoring	Confirm normalisation. If glucose remains elevated → suspect pre-existing DM.
Breastfeeding	Strongly encourage	Breastfeeding improves maternal glucose tolerance postpartum, promotes weight loss, and may reduce the child's future risk of obesity and DM. Lactation ↑ energy expenditure + ↑ glucose utilisation.
Postpartum OGTT ^[1]	6–12 weeks postpartum	To confidently diagnose GDM vs. pre-existing DM ^[1]. Use standard (non-pregnancy) criteria.
Lifestyle counselling	Ongoing	Weight management, diet, exercise → ↓ risk of future T2DM. Up to 50% of GDM women develop T2DM within 5–10 years.
Contraception counselling	Before discharge	Plan interval between pregnancies; pre-pregnancy counselling essential before next pregnancy.
Lifelong screening	Every 1–3 years: FPG, OGTT, or HbA1c	Early detection of T2DM for timely intervention
Pre-pregnancy planning for future pregnancies	Pre-pregnancy diabetic control ^[5]	Optimise glucose control BEFORE next conception to reduce risk of congenital malformations in the next pregnancy

Special Scenarios

Mrs. Au was a 35-year-old clerk diagnosed to have diabetes mellitus two years ago. She was taking Gliclazide. She planned to have a baby. ^[9]

Pre-pregnancy counselling:

Depends on how good control is — cannot say no, but advise better control, lifestyle, medication, blood parameters. ^[9]
Switch from gliclazide to insulin before conception. Why? Sulphonylureas have higher risk of causing hypoglycaemia and may cross placenta. ^[9] Also not sufficient data to support use ^[9].
Optimise HbA1c to < 6.5% (ideally < 6.0%) before conception → reduces congenital malformation risk. Sugar is teratogenic ^[5] — the risk is highest during organogenesis (weeks 3–8), often before the woman knows she is pregnant.
Start folic acid 5 mg daily (high-dose, not standard 400 μg) at least 3 months before conception → reduces neural tube defect risk (already elevated in DM).
Screen for and stabilise diabetic complications: retinopathy (dilated fundoscopy), nephropathy (UACR, RFT), cardiovascular risk.
Review medications: STOP ACEi/ARB (teratogenic — renal agenesis, pulmonary hypoplasia), switch to labetalol/nifedipine for BP control. STOP statins (teratogenic). Aspirin 75–150 mg from 12 weeks if high pre-eclampsia risk.

Medication	Safe in Pregnancy?	Action Required
Insulin (all types)	✅ Yes	First-line pharmacotherapy. Does NOT cross placenta.
Metformin	⚠️ Generally acceptable	Crosses placenta but no teratogenicity shown. Used as alternative.
Gliclazide / Sulphonylureas ^[9]	❌ Not recommended	Switch to insulin. Risk of neonatal hypoglycaemia. ^[9]
SGLT2 inhibitors	❌ Contraindicated	Risk of euglycaemic DKA ^[12]; no pregnancy safety data; affects fetal renal development
GLP-1 RA (semaglutide, liraglutide)	❌ Contraindicated	Animal teratogenicity. Stop ≥ 2 months before conception (semaglutide).
DPP-4 inhibitors	❌ Not recommended	Insufficient data
Thiazolidinediones (pioglitazone)	❌ Not recommended	Crosses placenta; no safety data
ACEi / ARB	❌ Contraindicated	Teratogenic (renal/pulmonary). Switch to labetalol/nifedipine.
Statins	❌ Contraindicated	Teratogenic. Stop before conception.
Low-dose aspirin	✅ Yes (from 12 weeks)	Pre-eclampsia prevention if high-risk
High-dose folic acid (5 mg)	✅ Yes	NTD prevention (start ≥ 3 months pre-conception)

Lin was a 25-year-old clerk with insulin-dependent diabetes since she was 16. She was not very compliant with treatment and had several episodes of DKA in the past. She was admitted one day because of hypoglycaemia. ^[5]

This case illustrates the worst-case scenario:

Poorly controlled T1DM → very high risk of congenital malformations, miscarriage, stillbirth.
Non-compliance → need for intensive education and support.
Episodes of DKA → ↑ fetal mortality (DKA in pregnancy carries ~10–20% fetal mortality rate due to fetal hypoxia from acidosis).
Hypoglycaemia admission → demonstrates the difficulty of tight control — blood glucose becomes more difficult to control in pregnancy ^[5] with the risk of both hypoglycaemia (early pregnancy) and hyperglycaemia (late pregnancy).
Key management: optimise HbA1c pre-pregnancy, intensive basal-bolus or pump therapy, CGM, frequent specialist review, folic acid 5 mg, screening for complications.

High Yield Management Points:

Good glycaemic control is the cornerstone — by diet ± insulin. ^[2]

First step is ALWAYS medical nutrition therapy + exercise (1–2 week trial).

~70–80% controlled by diet alone; ~20–30% need pharmacotherapy.

Insulin is first-line pharmacotherapy (does NOT cross placenta).

Metformin is an acceptable alternative (DOES cross placenta but no teratogenicity).

Sulphonylureas (gliclazide) are NOT recommended — switch to insulin if pre-existing DM.

SGLT2i, GLP-1 RA, DPP-4i, TZDs, statins, ACEi/ARB — all contraindicated/not recommended.

Fetal surveillance: serial growth USS, CTG from 32–36 weeks, daily fetal movements.

Delivery: diet-controlled → up to 40+6 weeks. Insulin-requiring → IOL 38–39 weeks.

Intrapartum: hourly glucose monitoring, target 4–7 mmol/L, sliding scale if on insulin.

Postpartum: STOP all GDM medications. OGTT at 6–12 weeks. Lifelong screening.

High Yield Summary

Management Framework: Stepwise escalation: (1) MNT + exercise → (2) Insulin ± metformin → (3) Delivery planning → (4) Postpartum follow-up.

Glycaemic Targets: Fasting < 5.3, 1h PPG < 7.8, 2h PPG < 6.7 mmol/L. HbA1c < 6.0% (avoid hypoglycaemia).

MNT: Complex carbohydrates, low fat, small frequent meals (3 meals + 2–3 snacks), bedtime snack to prevent starvation ketosis. Exercise ≥ 30 min/day moderate intensity.

Insulin: First-line pharmacotherapy. Does NOT cross placenta. Tailor regimen: basal (NPH at bedtime) for fasting hyperglycaemia, rapid-acting (NovoRapid/Humalog) for postprandial, basal-bolus for both. Start 0.1–0.2 IU/kg/day, titrate every 3 days. Requirements increase as pregnancy advances.

Metformin: Alternative. DOES cross placenta but no teratogenicity. Start 500 mg OD, max 2500 mg/day. ~30–50% will still need supplemental insulin. C/I: renal impairment, hepatic failure.

Contraindicated drugs: Gliclazide/SUs, SGLT2i, GLP-1 RA, DPP-4i, TZDs, ACEi/ARB, statins.

Pre-existing DM: Pre-pregnancy counselling essential. Switch SU → insulin. HbA1c < 6.5% before conception. Folic acid 5 mg. Stop ACEi/ARB/statins. Screen retinopathy/nephropathy.

Delivery: Diet-controlled → up to 40+6 weeks. Insulin-requiring → IOL 38–39 weeks. EFW > 4.5 kg → consider C-section. Intrapartum: hourly BG, target 4–7, sliding scale insulin + dextrose.

Postpartum: STOP all GDM medications. OGTT at 6–12 weeks. Breastfeeding encouraged. Lifelong screening every 1–3 years. Pre-pregnancy counselling before next pregnancy.

Active Recall - GDM Management

References

^[1] Lecture slides: Block C - I am pregnant_ medical problems complicating pregnancy.pdf (p17, postpartum OGTT, HK screening timing) ^[2] Lecture slides: GC 115. I am pregnant medical problems complicating pregnancy.pdf (p18, good glycaemic control is the cornerstone, diet +/- insulin) ^[3] Senior notes: Ryan Ho Endocrine.pdf (p81–90, glycaemic targets, insulin types and regimens, indications for insulin, HBSM, CGM, HbA1c caveats, management principles) ^[4] Senior notes: Maksim Medicine Notes.pdf (p80–81, lifestyle modifications, diet principles, treatment targets, HbA1c, metformin C/I, lipodystrophy) ^[5] Lecture slides: Block C - I am pregnant_ medical problems complicating pregnancy.pdf (p10–13, effect of pregnancy on DM, fasting hypoglycaemia, placental hormones, pre-pregnancy control, sugar is teratogenic) ^[9] Lecture slides: Block C - O&G Theme Case 1.docx.pdf (p8–9, Mrs. Au case, gliclazide in pregnancy, SU risks, HbA1c targets, pre-pregnancy counselling) ^[12] Senior notes: Maksim Surgery Notes.pdf (p25, peri-operative DM management, sliding scale, Alberti regimen, SGLT2i and euglycaemic DKA risk)

Complications of Gestational Diabetes Mellitus

Every complication of GDM can be traced back to a few core pathophysiological mechanisms. Before we go through each complication in detail, let's establish the conceptual framework:

Poor control of diabetes mellitus increases the incidence of maternal/fetal complications including spontaneous miscarriage, congenital malformation, stillbirth, respiratory distress syndrome, macrosomia, pre-eclampsia and polyhydramnios. ^[9]

The complications can be organised along two axes:

Who is affected: Mother vs. Fetus/Neonate vs. Long-term (both)
When they occur: Antepartum vs. Intrapartum vs. Postpartum vs. Long-term

The unifying mechanism for most fetal complications remains the Pedersen hypothesis: maternal hyperglycaemia → fetal hyperglycaemia (glucose crosses placenta via facilitated diffusion) → fetal hyperinsulinaemia (insulin does NOT cross placenta, so the fetal pancreas responds independently) → downstream consequences.

For maternal complications, the unifying mechanism is insulin resistance → endothelial dysfunction + metabolic derangement.

A. MATERNAL COMPLICATIONS

Increased complication risk — pre-eclampsia. ^[5]

Aspect	Detail
Incidence	GDM increases pre-eclampsia risk 2–4 fold compared to non-diabetic pregnancies
Pathophysiology	GDM and pre-eclampsia share insulin resistance as a common upstream mechanism. Insulin resistance → endothelial dysfunction → impaired nitric oxide (NO) production → vasoconstriction + ↑ vascular permeability. Additionally, the hyperglycaemic and inflammatory milieu impairs trophoblast invasion of spiral arteries in the first trimester → defective placental perfusion → placental ischaemia → release of anti-angiogenic factors (sFlt-1, soluble endoglin) → systemic endothelial dysfunction → hypertension + proteinuria + end-organ damage. Hyperinsulinaemia also promotes sodium retention (via renal tubular Na+ reabsorption) → contributes to hypertension.
Clinical features	Hypertension ( > 140/90 mmHg after 20 weeks), proteinuria (≥ 300 mg/24h or protein:creatinine ratio ≥ 30 mg/mmol), oedema, headache, visual disturbances, epigastric pain, hyperreflexia. Severe features: BP > 160/110, ↑ transaminases (HELLP syndrome), thrombocytopaenia, renal impairment, pulmonary oedema.
Why GDM specifically ↑ risk	(1) Shared metabolic substrate (insulin resistance, central obesity, dyslipidaemia); (2) Hyperglycaemia-induced oxidative stress damages endothelium; (3) Chronic low-grade inflammation (↑ TNF-α, IL-6, CRP) common to both conditions
Management	BP monitoring at every visit, low-dose aspirin 75–150 mg from 12 weeks if high-risk, antihypertensives (labetalol, nifedipine, methyldopa), magnesium sulphate for seizure prophylaxis if severe. Definitive treatment of pre-eclampsia is delivery of fetus. ^[5]

Increased complication risk — UTI. ^[5]

Aspect	Detail
Incidence	~2× higher in diabetic pregnancies vs. non-diabetic
Pathophysiology	Two synergistic mechanisms: (1) Glycosuria — hyperglycaemia exceeds the renal tubular reabsorption threshold (which is LOWER in pregnancy due to ↑ GFR) → glucose spills into urine → provides excellent culture medium for bacteria (especially E. coli); (2) Pregnancy itself — progesterone causes smooth muscle relaxation → ureteral dilatation + ↓ peristalsis → urinary stasis → ↑ risk of ascending infection. Additionally, the enlarging uterus compresses the ureters → further stasis.
Consequences	Asymptomatic bacteriuria → cystitis → pyelonephritis (if untreated). Pyelonephritis in pregnancy is dangerous — can trigger sepsis, preterm labour, and ARDS.
Vulvovaginal candidiasis	Hyperglycaemia in vaginal secretions + glycosuria → Candida albicans overgrowth. Presents with pruritus vulvae, white discharge. Very common in poorly controlled GDM.
Management	Screen for asymptomatic bacteriuria (mid-stream urine M/C/S) at booking. Treat promptly with pregnancy-safe antibiotics (amoxicillin, nitrofurantoin [avoid at term], cefalexin). Topical antifungals (clotrimazole) for candidiasis.

Increased complication risk — preterm labour. ^[5]

Aspect	Detail
Pathophysiology	Multiple mechanisms: (1) Polyhydramnios → uterine overdistension → stretching of myometrium → premature activation of contractility pathways (prostaglandin and oxytocin receptor upregulation); (2) Infections (UTI, chorioamnionitis) — bacteria produce phospholipases and endotoxins → prostaglandin release → uterine contractions; (3) Pre-eclampsia may necessitate iatrogenic preterm delivery to save the mother; (4) Hyperglycaemia itself may promote inflammation → ↑ cytokine production → cervical ripening and membrane weakening.
Consequences	Prematurity → neonatal RDS (worsened by diabetic inhibition of surfactant), NEC, intraventricular haemorrhage, long-term neurodevelopmental issues. Note that in diabetic pregnancies, RDS risk is elevated even at gestational ages where it would not normally be expected, because fetal hyperinsulinaemia inhibits surfactant production.
Management	Treat underlying cause (infections, pre-eclampsia). Tocolysis if appropriate (nifedipine, atosiban). Antenatal corticosteroids for fetal lung maturation (but monitor glucose closely — steroids cause significant hyperglycaemia).

Increased incidence of cesarean section + instrumental delivery. ^[5]

Aspect	Detail
Pathophysiology	(1) Macrosomia → cephalopelvic disproportion → failure to progress in labour → need for C-section. Diabetic macrosomia is characterised by disproportionate truncal/shoulder overgrowth (unlike constitutional macrosomia which is proportional), making vaginal delivery more hazardous. (2) Shoulder dystocia risk → lower threshold for offering elective C-section (see below). (3) Failed IOL — induced labours at earlier gestation (38–39 weeks for insulin-treated GDM) may have unfavourable cervix → higher IOL failure rate.
Rates	C-section rates in GDM pregnancies are approximately 25–35% compared to ~15–20% in non-diabetic pregnancies.
Consequences	All surgical complications: wound infection (higher in diabetes due to impaired wound healing from hyperglycaemia-induced microvascular damage + immunosuppression), haemorrhage, VTE, longer recovery.

Polyhydramnios. ^[9]

Aspect	Detail
Definition	Amniotic fluid index (AFI) > 25 cm or deepest vertical pocket > 8 cm
Pathophysiology	Fetal hyperglycaemia → fetal osmotic diuresis → fetal polyuria → excess amniotic fluid. This is a direct consequence of the Pedersen hypothesis. Additionally, hyperglycaemia may impair fetal swallowing (a normally important mechanism for amniotic fluid reabsorption).
Consequences	(1) Uterine overdistension → preterm labour; (2) Malpresentation (fetus has excessive room to move → breech/transverse lie); (3) Cord prolapse (if membranes rupture with presenting part not engaged); (4) Postpartum haemorrhage (uterine overdistension → poor contraction after delivery → atony)
Detection	Serial USS — AFI measured during growth scans
Management	Optimise glycaemic control (→ ↓ fetal polyuria → ↓ amniotic fluid volume). In severe cases: amnioreduction (therapeutic amniocentesis) to relieve symptoms and reduce preterm labour risk.

Aspect	Detail
Pathophysiology	(1) Uterine atony — overdistended uterus (from macrosomic baby and/or polyhydramnios) cannot contract effectively after delivery; (2) Genital tract trauma — delivery of a large baby → perineal tears, cervical lacerations, vaginal wall trauma; (3) Prolonged labour → myometrial fatigue
Risk factors in GDM	Macrosomia, polyhydramnios, operative delivery, prolonged labour
Management	Active management of third stage (oxytocin after delivery), uterine massage, additional uterotonics (ergometrine, misoprostol, carboprost), intrauterine balloon tamponade, surgical intervention if medical management fails

Blood glucose becomes more difficult to control in pregnancy, sometimes resulting in fasting hypoglycaemia. ^[5]

Aspect	Detail
Pathophysiology	In insulin-treated GDM: insulin dose titrated upward to counteract rising insulin resistance → if carbohydrate intake is insufficient or there is unexpected activity/vomiting → hypoglycaemia. In early pregnancy (1st trimester): enhanced insulin sensitivity + placental glucose consumption → fasting hypoglycaemia ^[5]. In pre-existing T1DM: loss of counter-regulatory hormone responses (hypoglycaemia unawareness) makes this especially dangerous.
Clinical features	Adrenergic symptoms: palpitation, sweating, anxiety, tremor, tachycardia. Neuroglycopenic symptoms: hunger, paraesthesia, seizures, focal weakness, decreased consciousness, coma. ^[3]
Diagnosis	Whipple's triad: symptoms compatible with hypoglycaemia + low blood glucose + resolution with correction. ^[3]
Management	Conscious: 10–15g simple carbohydrates (e.g., soft drink), then carbohydrate meal. Decreased consciousness: D50 40mL IV stat via large vein with saline flush (or D20 100mL), then D10 infusion. Unable to obtain IV access: IM glucagon 1mg. ^[4]
Significance	Maternal hypoglycaemia is dangerous → loss of consciousness can cause falls, road traffic accidents, seizures. Trying to push down too low → risk of hypoglycaemia, deadly in some studies. ^[9]

Aspect	Detail
Context	DKA in pregnancy is a medical emergency with ~10–20% fetal mortality. More relevant in pre-existing T1DM (like Lin's case — she had several episodes of DKA in the past ^[5]) but can rarely occur in GDM with significant β-cell failure or in euglycaemic DKA (e.g., from starvation ketosis or SGLT2i exposure).
Why worse in pregnancy	(1) Respiratory alkalosis of pregnancy (normal ↓ pCO₂) means the body has less buffering capacity → DKA progresses faster; (2) Fetal consequences are severe — acidosis impairs placental oxygen transfer → fetal hypoxia → fetal death; (3) Dehydration → ↓ uterine perfusion → further fetal compromise.
Pathophysiology	Absolute insulin deficiency → ↑ gluconeogenesis + glycogenolysis + ↓ glucose uptake → hyperglycaemia. ↑ Lipolysis releases acetyl-CoA → ↑ ketogenesis → metabolic acidosis. ^[4]
Clinical features	Polyuria, polydipsia, hypovolaemia signs (tachycardia, hypotension, dry mucous membranes), Kussmaul's breathing (deep, fast — to compensate for HAGMA), fruity odour (acetone), abdominal pain, vomiting, decreased consciousness. ^[4]
Management	IV fluids (0.9% NaCl), IV insulin infusion (fixed-rate), potassium replacement (monitor closely — total body K+ depleted but serum K+ may be high due to transcellular shift from acidosis and insulin deficiency), identify and treat the precipitant (infection, non-compliance, steroids). Monitor BG and H'stix Q1h until stable. ^[4] Continuous fetal monitoring. Delivery only after maternal stabilisation.

B. FETAL AND NEONATAL COMPLICATIONS

These are the complications most heavily emphasised in the lecture slides and most directly attributable to the Pedersen hypothesis.

Macrosomia. ^[9]

Aspect	Detail
Definition	Birth weight > 4000g, or > 90th centile for gestational age (LGA — large for gestational age)
Incidence	Occurs in ~15–45% of GDM pregnancies (vs. ~10% in non-diabetic) depending on glycaemic control
Pathophysiology	Pedersen hypothesis: maternal hyperglycaemia → glucose crosses placenta (facilitated diffusion via GLUT-1/3) → fetal hyperglycaemia → fetal pancreatic β-cell hyperplasia → fetal hyperinsulinaemia. Insulin acts as a potent anabolic growth factor in the fetus (unlike in adults where insulin's metabolic role dominates): ↑ hepatic glycogen synthesis, ↑ lipogenesis (fat deposition), ↑ protein synthesis. This produces a characteristic asymmetric growth pattern — disproportionate enlargement of the trunk, abdomen, and shoulders relative to the head (unlike constitutionally large babies whose growth is proportionally uniform).
Key characteristic	Asymmetric macrosomia: trunk/shoulder/abdominal circumference disproportionately large compared to biparietal diameter. This is precisely why shoulder dystocia risk is higher in diabetic macrosomia vs. non-diabetic macrosomia of the same weight.
Consequences	Shoulder dystocia, birth injury (brachial plexus injury, fractured clavicle), operative delivery, birth canal trauma, PPH

Aspect	Detail
Definition	Impaction of the anterior fetal shoulder behind the maternal pubic symphysis after delivery of the head — a true obstetric emergency
Pathophysiology	Diabetic macrosomia → disproportionately large shoulders and trunk → during delivery, the head delivers normally but the shoulders cannot follow because the bisacromial diameter exceeds the pelvic outlet. The risk is higher at any given birth weight in diabetic vs. non-diabetic pregnancies because of the asymmetric fat distribution pattern.
Incidence	~1–2% of all deliveries; ~5–9% in macrosomic infants of diabetic mothers
Consequences	Fetal: Erb's palsy (C5–C6 brachial plexus injury → "waiter's tip" position — adducted shoulder, extended elbow, pronated forearm); Klumpke's palsy (C8–T1 — rarer); clavicular/humeral fracture; hypoxic-ischaemic encephalopathy (if delivery is significantly delayed); death (rare). Maternal: fourth-degree perineal tears, uterine rupture, PPH, psychological trauma.
Prevention	Accurate estimation of fetal weight (USS AC > 90th centile is the key early marker); offer elective C-section if EFW > 4.5 kg; avoid prolonged second stage; experienced accoucheur present for vaginal delivery of macrosomic baby; McRoberts manoeuvre and suprapubic pressure as first-line management if shoulder dystocia occurs.

Congenital malformation. ^[9] Sugar is teratogenic. ^[5]

Aspect	Detail
Relevance to GDM	Congenital malformations are primarily a complication of pre-existing DM (T1DM or T2DM), NOT typical GDM. Why? Because organogenesis occurs between weeks 3–8, and GDM typically develops after 20 weeks — by the time GDM manifests, organ formation is complete. However, if a woman has undiagnosed pre-existing DM that is first detected during pregnancy (and misclassified as GDM), congenital malformation risk IS increased because she was hyperglycaemic during the critical first trimester. Also, if GDM develops very early (i.e., really overt DM first detected in early pregnancy), the risk exists.
Pathophysiology	Hyperglycaemia during organogenesis → ↑ reactive oxygen species (ROS) production → oxidative stress → activation of apoptotic pathways in embryonic cells → disruption of normal organogenesis. Also, hyperglycaemia → depletion of myo-inositol (a precursor for phosphatidylinositol signalling) → impaired neural tube closure. Additionally, hyperglycaemia → arachidonic acid pathway dysregulation → abnormal prostaglandin synthesis → vascular disruption in developing organs.
Types of malformations	Risk is 3–5× higher in pre-existing DM: (1) Cardiac (most common): VSD, ASD, TGA, coarctation of aorta; (2) Neural tube defects: anencephaly, spina bifida (why high-dose folic acid 5 mg is given); (3) Caudal regression syndrome (sacral agenesis) — pathognomonic of diabetic embryopathy though very rare (~1% of diabetic pregnancies); (4) Renal: renal agenesis, duplex collecting system; (5) GI: duodenal/anorectal atresia; (6) Skeletal: limb reduction defects
Risk correlation	Directly proportional to peri-conceptional HbA1c: HbA1c < 6.5% → near-normal risk; HbA1c > 10% → risk approaches 20–25%. Good antenatal care including pre-pregnancy diabetic control ^[5] is the primary prevention strategy.

High sugar level associated with higher risk of miscarriage. ^[5]

Aspect	Detail
Relevance	Primarily pre-existing DM, not typical GDM (same reasoning as congenital malformations — miscarriage occurs in the 1st trimester before GDM typically manifests)
Pathophysiology	Hyperglycaemia → oxidative stress → impaired trophoblast invasion and placentation → defective implantation → early pregnancy loss. Also, hyperglycaemia → direct embryotoxicity. The risk correlates with peri-conceptional HbA1c — women with HbA1c > 8% have significantly higher miscarriage rates.
Prevention	Pre-pregnancy diabetic control ^[5] — optimise HbA1c to < 6.5% before conception

Stillbirth. ^[9]

Aspect	Detail
Incidence	2–5× higher in diabetic pregnancies. Mostly occurs in the third trimester (particularly > 36 weeks), which is why GDM-specific surveillance and timely delivery are critical.
Pathophysiology	Multiple contributing mechanisms: (1) Fetal hyperinsulinaemia → ↑ fetal metabolic rate → ↑ oxygen consumption → exceeds the placenta's ability to deliver oxygen → chronic fetal hypoxia → eventual fetal death. Think of it as the fetus "burning fuel too fast" — insulin drives cellular metabolism, consuming oxygen faster than it can be supplied. (2) Placental vasculopathy — hyperglycaemia damages placental blood vessels (same mechanisms as diabetic microangiopathy: non-enzymatic glycation, thickened basement membrane) → impaired oxygen and nutrient transfer. (3) Acute hyperglycaemic crises — maternal DKA carries ~10–20% fetal mortality rate. (4) Pre-eclampsia/abruption — associated complications.
Prevention	Tight glycaemic control, serial growth USS, fetal movement monitoring from 28 weeks, CTG from 32–36 weeks, timely delivery (38–39 weeks for insulin-treated, up to 40+6 for diet-controlled).

Aspect	Detail
Incidence	~15–25% of neonates born to mothers with GDM; higher in poorly controlled or insulin-treated cases
Pathophysiology	This is the most direct consequence of the Pedersen hypothesis: throughout pregnancy, maternal hyperglycaemia → fetal hyperglycaemia → fetal pancreatic β-cell hyperplasia and hypertrophy. At birth, the umbilical cord is clamped → maternal glucose supply ceases abruptly. But the neonate's hyperplastic β-cells continue secreting insulin (they don't know the glucose supply has stopped) → hyperinsulinaemia without glucose supply → rapid ↓ in neonatal blood glucose → hypoglycaemia. This typically occurs within the first 1–2 hours of life and may persist for 24–72 hours.
Clinical features	Jitteriness, tremors, poor feeding, lethargy, hypotonia, seizures, apnoea, cyanosis. Can be asymptomatic (which is why routine screening is mandatory).
Management	Early feeding (within 30–60 minutes of birth), neonatal blood glucose monitoring at 1h, 2h, then pre-feeds for 24–48h. If glucose < 2.0–2.6 mmol/L despite feeds → IV dextrose 10% infusion. If persistent → exclude hyperinsulinism (extremely rare in this context).

Respiratory distress syndrome. ^[9]

Aspect	Detail
Pathophysiology	Fetal hyperinsulinaemia directly inhibits surfactant production by type II pneumocytes. The mechanism: cortisol normally stimulates surfactant synthesis (via phosphatidylcholine production). Insulin antagonises the action of cortisol on the type II pneumocyte → ↓ surfactant protein (SP-A, SP-B) and ↓ phospholipid production → ↓ surfactant → alveolar surface tension remains high → alveolar collapse at end-expiration → atelectasis → RDS. This means that diabetic neonates can develop RDS at later gestational ages (35–37 weeks) where it would not normally occur, because their surfactant maturation is delayed.
Clinical features	Tachypnoea, grunting (trying to maintain PEEP), nasal flaring, intercostal/subcostal recessions, cyanosis — presenting within the first few hours of life
Why this is especially important	If preterm delivery is needed (e.g., for pre-eclampsia or fetal compromise), these neonates are at even higher RDS risk. Antenatal corticosteroids for fetal lung maturation are given (betamethasone 12 mg × 2), but remember: (1) steroids worsen maternal glycaemic control; (2) the very mechanism of RDS in diabetes (insulin opposing cortisol) may partially blunt the benefit of exogenous steroids.

Complication	Pathophysiology	Clinical Features
Neonatal hypocalcaemia	Likely multifactorial: (1) Fetal hyperinsulinaemia may suppress PTH secretion (functional hypoparathyroidism); (2) Maternal hypomagnesaemia (common in DM) → fetal hypomagnesaemia → impaired PTH release (Mg²⁺ is required for PTH secretion). Typically occurs at 24–72 hours of life.	Jitteriness, irritability, seizures, QT prolongation on ECG, Chvostek/Trousseau signs
Neonatal polycythaemia	Chronic fetal hypoxia (from ↑ O₂ consumption due to hyperinsulinaemia) → ↑ fetal erythropoietin → ↑ RBC production → polycythaemia (venous Hct > 65%).	Plethoric (ruddy) appearance, ↑ blood viscosity → sluggish blood flow → organ ischaemia (renal, cerebral, mesenteric). Risk of neonatal stroke, renal vein thrombosis, NEC.
Neonatal hyperbilirubinaemia / jaundice	Consequence of polycythaemia: excess RBCs are broken down → ↑ unconjugated bilirubin → exceeds immature neonatal liver's conjugation capacity → jaundice. Also, macrosomic babies may have bruising from difficult delivery → additional haem breakdown.	Yellow skin/sclerae, poor feeding, lethargy. Risk of kernicterus if severe and untreated.
Hypertrophic cardiomyopathy	Fetal hyperinsulinaemia → ↑ glycogen deposition in the interventricular septum and ventricular walls → asymmetric septal hypertrophy → potential outflow tract obstruction (similar to HOCM). Usually transient — resolves within weeks to months as insulin levels normalise.	Usually asymptomatic; may cause transient heart failure. Detected on fetal/neonatal echocardiography.

C. LONG-TERM COMPLICATIONS

These are often overlooked but are critically important — not just short-term complications; the final point is crucial. ^[5]

Aspect	Detail
Risk	Up to 50% of women with GDM will develop overt T2DM within 5–10 years. GDM is one of the strongest predictors of future T2DM. Previous conditions: pre-diabetes, gestational DM are listed as T2DM risk factors ^[3].
Pathophysiology	GDM revealed a pre-existing subclinical β-cell deficiency. After delivery, insulin resistance resolves → glucose normalises. But the underlying β-cell dysfunction remains. Over time, with age-related β-cell decline, weight gain, and ongoing insulin resistance (from obesity/metabolic syndrome), the β-cells eventually fail again → overt T2DM. GDM is essentially a "failed stress test" — the pregnancy exposed the pancreas's inadequacy, and the disease process continues after pregnancy ends.
Prevention	Lifelong screening every 1–3 years (OGTT, FPG, or HbA1c). Intensive lifestyle intervention (diet + exercise) reduces T2DM risk by ~50% (Diabetes Prevention Program study). Metformin can also reduce progression (by ~30%) in high-risk women. Breastfeeding is associated with ↓ future T2DM risk (improves glucose tolerance + promotes weight loss).

Aspect	Detail
Risk	Women with a history of GDM have a ~2× increased risk of cardiovascular events (MI, stroke) over 10–20 years, even if they don't develop T2DM.
Pathophysiology	GDM is a marker of underlying metabolic syndrome: insulin resistance, endothelial dysfunction, dyslipidaemia, chronic inflammation. These persist after pregnancy and contribute to atherosclerosis. Metabolic syndrome manifestations: hypertension, dyslipidaemia, NAFLD. ^[3]
Prevention	CV risk factor modification: weight management, exercise, smoking cessation, BP and lipid monitoring.

Aspect	Detail
Risk	30–84% recurrence in subsequent pregnancies (higher in those with greater BMI gain, shorter inter-pregnancy interval, or insulin-requiring GDM)
Prevention	Pre-pregnancy diabetic control ^[5] — weight loss between pregnancies, optimise metabolic parameters, early screening in subsequent pregnancies

Evidence suggests the concept of fetal programming — the baby may be more predisposed to other metabolic and cardiovascular diseases. ^[5]

Aspect	Detail
Concept	Intrauterine exposure to hyperglycaemia causes epigenetic modifications in the fetus — changes in gene expression (DNA methylation, histone modification) that persist into adulthood without altering the DNA sequence itself. These changes "programme" the offspring's metabolic set points, predisposing them to metabolic disease later in life.
Evidence	Children born to mothers with GDM have: (1) ↑ risk of childhood obesity (2–3×); (2) ↑ risk of T2DM in adolescence/early adulthood; (3) ↑ risk of metabolic syndrome; (4) ↑ risk of cardiovascular disease; (5) ↑ risk of GDM themselves when they become pregnant (perpetuating an intergenerational cycle). The Pima Indian studies provided landmark evidence: children born to mothers who were diabetic during pregnancy had significantly higher rates of obesity and T2DM than siblings born before the mother became diabetic (same genetics, different intrauterine environment → proving it's the environment, not just genetics).
Mechanism	Fetal hyperinsulinaemia → (1) alters hypothalamic appetite-regulation circuits → programming toward obesity; (2) alters adipocyte differentiation → predisposition to central adiposity; (3) alters pancreatic β-cell development → altered insulin secretion capacity; (4) epigenetic changes in genes regulating glucose and lipid metabolism.
Prevention	Tight maternal glycaemic control during pregnancy → ↓ fetal hyperglycaemia → ↓ hyperinsulinaemia → ↓ epigenetic programming. Breastfeeding (promotes appropriate growth, associated with ↓ childhood obesity). Healthy childhood diet and exercise.

The Intergenerational Cycle of Diabetes

This is a crucial concept: a mother with GDM → her child is exposed to intrauterine hyperglycaemia → the child is epigenetically programmed toward obesity and insulin resistance → as an adult, this child (especially if female) has a higher risk of GDM when she herself becomes pregnant → her children are again exposed → and so on. This creates a vicious intergenerational cycle that amplifies diabetes prevalence across generations, independent of genetics. Breaking this cycle through tight GDM control is a public health imperative.

Due to worsened control of DM during pregnancy → increased risk of complications involving cardiovascular, renal and optic systems. ^[5]

Complication	Effect of Pregnancy
Diabetic retinopathy	Worsens transiently during pregnancy. ^[13] Mechanisms: (1) Haemodynamic changes (↑ cardiac output, ↑ retinal blood flow → ↑ shear stress on already-damaged retinal vessels); (2) Rapid improvement in glycaemic control paradoxically worsens retinopathy transiently (thought to be due to relative retinal ischaemia when glucose levels drop quickly → ↑ VEGF release → neovascularisation); (3) Growth factors (IGF-1, VEGF) are ↑ in pregnancy. Screening: dilated fundoscopy at booking, each trimester, and 1 year postpartum. Dilated fundus examination annually. ^[3]
Diabetic nephropathy	Pregnancy ↑ GFR by ~50% → ↑ protein filtration → worsening albuminuria (though this may partially reverse postpartum). Pre-existing nephropathy (microalbuminuria or macroalbuminuria) carries ↑ risk of pre-eclampsia, preterm delivery, and accelerated decline in renal function. Women with creatinine > 180 μmol/L should be counselled about the high risk of irreversible renal deterioration during pregnancy.
Diabetic neuropathy	Autonomic neuropathy can worsen gastroparesis → erratic gastric emptying → unpredictable glucose absorption → very difficult glycaemic control. Cardiovascular autonomic neuropathy → blunted heart rate response → may mask symptoms of intrapartum compromise.
Macrovascular disease ^[5]	Increased risk of complications involving cardiovascular systems. ^[5] Pre-existing coronary artery disease (rare in reproductive age but possible in T1DM with long duration) → pregnancy ↑ cardiac workload (↑ cardiac output 30–50%) → risk of MI, heart failure.

Category	Complication	Timing	Key Mechanism
Maternal	Pre-eclampsia	Antepartum	Shared insulin resistance → endothelial dysfunction
	UTI / Candidiasis	Antepartum	Glycosuria + urinary stasis
	Preterm labour	Antepartum	Polyhydramnios, infection, iatrogenic
	Polyhydramnios	Antepartum	Fetal osmotic diuresis
	Operative delivery	Intrapartum	Macrosomia → CPD
	PPH	Postpartum	Uterine atony (overdistension) + trauma
	Hypoglycaemia	Antepartum/Intrapartum	Insulin treatment + erratic glucose
	DKA	Any time	Insulin deficiency + ketogenesis
	Future T2DM	Long-term	Pre-existing β-cell dysfunction unmasked
	Future CVD	Long-term	Metabolic syndrome persistence
Fetal/Neonatal	Macrosomia	Antepartum/delivery	Pedersen: hyperglycaemia → hyperinsulinaemia
	Shoulder dystocia	Intrapartum	Asymmetric macrosomia
	Congenital malformations	1st trimester (pre-existing DM)	Hyperglycaemia-induced teratogenesis
	Miscarriage	1st trimester (pre-existing DM)	Embryotoxicity
	Stillbirth	3rd trimester	↑ O₂ consumption + placental vasculopathy
	Neonatal hypoglycaemia	Immediate postpartum	Persistent hyperinsulinaemia after cord clamp
	RDS	Immediate postpartum	Insulin inhibits surfactant synthesis
	Hypocalcaemia, polycythaemia, jaundice	Neonatal period	Metabolic consequences of hyperinsulinaemia
	Hypertrophic cardiomyopathy	Neonatal (transient)	Glycogen deposition in septum
Long-term (offspring)	Fetal programming ^[5]	Lifelong	Epigenetic modifications → metabolic disease

High Yield Complications Points:

Good glycaemic control prevents complications — every complication correlates with degree of hyperglycaemia.

Pedersen hypothesis explains ALL fetal complications: maternal hyperglycaemia → fetal hyperglycaemia → fetal hyperinsulinaemia.

Congenital malformations and miscarriage are primarily risks of pre-existing DM, not true GDM (organogenesis precedes GDM onset).

Neonatal hypoglycaemia = most common immediate neonatal complication (hyperplastic β-cells keep secreting insulin after cord clamp).

RDS in diabetic neonates occurs at LATER gestational ages (insulin inhibits surfactant synthesis).

Macrosomia in DM is ASYMMETRIC (trunk > head) → higher shoulder dystocia risk than non-diabetic macrosomia.

Fetal programming: baby may be predisposed to metabolic and cardiovascular diseases lifelong ^[5].

Up to 50% of GDM women develop T2DM within 5–10 years — lifelong screening is mandatory.

Diabetic retinopathy worsens during pregnancy ^[13] — needs surveillance each trimester.

High Yield Summary

Maternal Complications: Pre-eclampsia (2–4× risk, shared insulin resistance mechanism), UTI/candidiasis (glycosuria), preterm labour (polyhydramnios, infection, iatrogenic), polyhydramnios (fetal osmotic diuresis), operative delivery (macrosomia → CPD), PPH (uterine atony from overdistension), maternal hypoglycaemia (treatment complication), DKA (mainly pre-existing DM — ~10–20% fetal mortality).

Fetal/Neonatal Complications (Pedersen Hypothesis): Macrosomia (asymmetric — truncal overgrowth from insulin as growth factor), shoulder dystocia (disproportionate shoulders), congenital malformations (sugar is teratogenic — mainly pre-existing DM; cardiac, NTDs, caudal regression), miscarriage (1st trimester hyperglycaemia), stillbirth (fetal hypoxia from ↑ O₂ consumption + placental vasculopathy), neonatal hypoglycaemia (persistent hyperinsulinaemia after cord clamp — most common immediate complication), RDS (insulin inhibits surfactant — occurs at later GA), hypocalcaemia, polycythaemia/jaundice, hypertrophic cardiomyopathy (transient septal hypertrophy).

Long-term Complications: Mother — T2DM (up to 50% within 5–10 years), CVD (2× risk), recurrent GDM (30–84%). Offspring — fetal programming / epigenetic modifications → childhood obesity, T2DM, metabolic syndrome, CVD in adulthood → intergenerational cycle of diabetes.

Pre-existing DM in pregnancy: Worsening of retinopathy, nephropathy, neuropathy, and macrovascular disease due to haemodynamic and metabolic changes of pregnancy.

Active Recall - GDM Complications

References

^[3] Senior notes: Ryan Ho Endocrine.pdf (p77, 81, 94, T2DM risk factors including previous GDM, hypoglycaemia clinical features and management, follow-up evaluation, complication screening) ^[4] Senior notes: Maksim Medicine Notes.pdf (p88, hypoglycaemia management, DM complications pathophysiology) ^[5] Lecture slides: Block C - I am pregnant_ medical problems complicating pregnancy.pdf (p10–13, effect of pregnancy on DM, fasting hypoglycaemia, sugar is teratogenic, fetal programming, pre-pregnancy control, worsening of diabetic complications) ^[9] Lecture slides: Block C - O&G Theme Case 1.docx.pdf (p8–9, complications list: miscarriage, congenital malformation, stillbirth, RDS, macrosomia, pre-eclampsia, polyhydramnios) ^[13] Senior notes: Ryan Ho Opthalmology.pdf (p70, diabetic retinopathy worsens during pregnancy)

Gestational Diabetes Mellitus

Definition

Epidemiology

Global

Hong Kong Context

Key Epidemiological Trends

Risk Factors

Anatomy and Physiology: The Placenta as an Endocrine Organ

Normal Carbohydrate Metabolism in Pregnancy

First Trimester (Anabolic Phase)

Second and Third Trimesters (Catabolic / Diabetogenic Phase)

Why GDM Occurs

Glucose Transport Across the Placenta

Aetiology and Pathophysiology

Aetiology (Focus on Hong Kong)

Pathophysiology — A Step-by-Step Walkthrough

Classification

By Relationship to Pregnancy

By Severity / Treatment Required (White Classification — Historical but still referenced)

By Screening Approach

Clinical Features

Important Context

Symptoms (with pathophysiological basis)

A. Maternal Symptoms

B. Symptoms Suggesting Complications (usually more relevant to pre-existing DM, but can occur in severe/uncontrolled GDM)

Signs (with pathophysiological basis)

A. General Examination

B. Obstetric Examination

C. Signs of Complications

Fetal/Neonatal Clinical Features (Why These Occur)

Maternal Complications (Clinical Features That May Be Present)

Distinguishing GDM from Pre-Existing DM in Pregnancy

Impact Concept Summary

Active Recall - Gestational Diabetes Mellitus (Definition, Epidemiology, Pathophysiology, Clinical Features)

References

Why Differential Diagnosis Matters in GDM

Conceptual Framework for Differential Diagnosis

Detailed Differential Diagnosis

1. Pre-Existing Type 2 Diabetes Mellitus (Undiagnosed)

2. Type 1 Diabetes Mellitus / Latent Autoimmune Diabetes of Adults (LADA)

3. Maturity-Onset Diabetes of the Young (MODY)

4. Secondary Causes of Diabetes in Pregnancy

a. Drug-Induced Hyperglycaemia

b. Endocrinopathies

c. Pancreatic Diseases

d. Genetic Syndromes

5. Physiological (Non-Pathological) Considerations

6. Other Forms of Diabetes Presenting in Pregnancy

Systematic Approach to Differentiating GDM

Summary Table: Key Differentials of Hyperglycaemia in Pregnancy

Active Recall - Differential Diagnosis of GDM

Why GDM Uses Different Diagnostic Criteria

Diagnostic Criteria

A. IADPSG / WHO 2013 Criteria (Currently Adopted in HK / HA)

B. ADA 2024–2025 Criteria

C. Distinguishing GDM from Overt DM First Detected in Pregnancy

D. OGTT Procedure — How It Is Done

Screening Strategy and Diagnostic Algorithm

Who to Screen and When

Comprehensive Diagnostic Algorithm

Investigation Modalities

1. Diagnostic Investigations

a. 75g Oral Glucose Tolerance Test (OGTT)

b. Fasting Plasma Glucose (FPG)

c. HbA1c

2. Monitoring Investigations (After GDM Diagnosis)

a. Home Blood Sugar Monitoring (HBSM) / Self-Monitoring of Blood Glucose (SMBG)

b. Continuous Glucose Monitoring (CGM)

3. Investigations for Complications and Comorbidities

a. Maternal Assessment

b. Fetal Assessment

4. Postpartum Investigations — Confirming the Diagnosis

Summary Table: All Investigations at a Glance

Active Recall - GDM Diagnostic Criteria, Algorithm, and Investigations

Overarching Principles

Management Algorithm

Step 1: Multidisciplinary Team Approach

Step 2: Glycaemic Targets in Pregnancy

Step 3: Medical Nutrition Therapy (MNT) — First-Line Treatment

Dietary Principles