About This Page
This is a clinician-written, evidence-based summary aligned to the 2026 MLA Content Map. It is intended for medical students and junior doctors preparing for the UKMLA. Always cross-reference with NICE guidance, local protocols, and clinical judgement.
The Bottom Line
- Diagnosed by 75 g OGTT at 24–28 weeks: fasting ≥5.6 mmol/L or 2-hour ≥7.8 mmol/L (NICE NG3 criteria)
- Risk factors for screening: BMI >30, previous GDM, previous macrosomic baby (≥4.5 kg), FH diabetes, South Asian/Black/Middle Eastern ethnicity
- Management: diet and exercise first. Metformin if targets not met in 1–2 weeks. Insulin if metformin insufficient
- BG targets: fasting <5.3 mmol/L, 1-hour postprandial <7.8 mmol/L
- Deliver by 40+6. Consider earlier delivery if complications. Discontinue glucose-lowering treatment after delivery
- Postnatal: fasting glucose at 6–13 weeks postpartum to exclude ongoing T2DM. Annual HbA1c thereafter
Overview
Gestational diabetes mellitus (GDM) is defined as glucose intolerance first recognised during pregnancy. It results from insulin resistance driven by placental hormones (human placental lactogen, cortisol, progesterone), combined with inadequate compensatory insulin secretion. GDM is associated with adverse maternal outcomes (pre-eclampsia, caesarean delivery, future T2DM) and fetal outcomes (macrosomia, birth injuries, neonatal hypoglycaemia, and polycythaemia). It typically resolves after delivery but confers a 50% lifetime risk of developing type 2 diabetes.
Epidemiology
GDM affects approximately 4–5% of pregnancies in the UK, though prevalence varies with ethnicity and diagnostic criteria. It is more common in women of South Asian, Black Caribbean, and Middle Eastern origin. Risk factors include BMI >30, age >25, previous GDM, previous macrosomic baby, first-degree relative with diabetes, and PCOS. GDM incidence is rising in line with increasing obesity rates.
Clinical Features
Symptoms
Often asymptomatic — detected on screening OGTT
Polyuria, polydipsia (less common than in T2DM)
Recurrent infections (urinary, vaginal candidiasis)
Large-for-gestational-age fetus on scan
Polyhydramnios (excess amniotic fluid)
Signs
Symphysis-fundal height greater than expected for dates (macrosomia)
Glycosuria on routine urine dipstick (non-specific — renal threshold drops in pregnancy)
Obesity (BMI >30)
Acanthosis nigricans (insulin resistance marker)
Investigations
First-line
75 g OGTT at 24–28 weeksOffer to women with risk factors. Diagnostic thresholds (NICE NG3): fasting ≥5.6 mmol/L OR 2-hour ≥7.8 mmol/L
Blood glucose monitoringSelf-monitoring: fasting, and 1-hour post-meal. Targets: fasting <5.3, 1-h post-meal <7.8
Second-line
HbA1c at bookingIf previous GDM — identifies undiagnosed pre-existing T2DM (HbA1c ≥48 mmol/mol = T2DM)
Early OGTT at 16–18 weeksIf previous GDM. If normal, repeat at 24–28 weeks
UltrasoundSerial growth scans from 28–36 weeks — assess fetal size, liquor volume, macrosomia
Specialist
Retinal screeningNot routinely needed for GDM (unlike pre-existing diabetes). Offer if not previously screened
Management
NICE NG3 (Diabetes in pregnancy), 20151
Lifestyle modification (first-line)
- Dietary advice: low glycaemic index diet, regular meals, reduce refined carbohydrates
- Moderate exercise: 30 minutes/day walking or swimming
- Trial for 1–2 weeks with diet and exercise alone
2
Pharmacotherapy
- If fasting glucose 5.6–6.9 and diet fails: metformin first-line (500 mg OD, titrate to 2.5 g/day)
- If fasting glucose ≥7.0 at diagnosis: offer immediate insulin ± metformin
- If metformin contraindicated or not tolerated, or targets not met: add insulin
- Insulin regimen: typically intermediate-acting at bedtime ± rapid-acting with meals
- Glibenclamide only if metformin not tolerated and insulin declined
3
Monitoring and delivery
- Self-monitoring BG: fasting and 1-hour post-meal
- Serial growth scans at 28, 32, and 36 weeks
- Deliver by 40+6 at the latest. Offer induction of labour or CS if not delivered by then
- Consider earlier delivery if complications (macrosomia, pre-eclampsia, poor glycaemic control)
- BG monitoring during labour (target 4–7 mmol/L). Sliding-scale insulin if needed
4
Postnatal
- STOP all glucose-lowering medication immediately after delivery
- Monitor neonatal blood glucose (risk of neonatal hypoglycaemia — feed within 30 min of birth)
- Fasting plasma glucose at 6–13 weeks postpartum to exclude T2DM
- Annual HbA1c screening thereafter — 50% lifetime risk of T2DM
- Offer early OGTT in future pregnancies (16–18 weeks)
Complications
- Fetal macrosomia: Increased risk of shoulder dystocia, birth trauma, instrumental/CS delivery
- Neonatal hypoglycaemia: Fetal hyperinsulinaemia persists after cord clamped — feed early
- Polyhydramnios: Osmotic diuresis from fetal hyperglycaemia
- Pre-eclampsia: Increased risk with GDM
- Neonatal jaundice and polycythaemia: From chronic fetal hypoxia and hyperinsulinaemia
- Future T2DM: 50% lifetime risk for the mother. Offspring also at increased metabolic risk
UKMLA Exam Tips
- 1OGTT diagnostic thresholds: fasting ≥5.6 OR 2-h ≥7.8. Lower than WHO criteria — NICE-specific
- 2Diet first, then metformin, then insulin. This stepwise approach is very commonly tested
- 3If fasting glucose ≥7.0 at diagnosis → offer insulin immediately
- 4STOP all GDM medications after delivery — glucose normalises when placenta is removed
- 5Neonatal hypoglycaemia: feed baby within 30 minutes of birth. Check neonatal BG
- 6GDM confers 50% lifetime risk of T2DM — annual HbA1c screening recommended
- 7Previous GDM: early OGTT at 16–18 weeks in next pregnancy, repeat at 24–28 if normal
practicetest your knowledge on gestational diabetesApply what you've learnt with UKMLA-style questions from the iatroX Q-Bank — obstetrics and beyond.
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