Key Facts

Gestational diabetes mellitus (GDM) is defined as carbohydrate intolerance resulting in hyperglycaemia of variable severity with onset or first recognition during pregnancy. It is one of the most common medical complications of pregnancy and is associated with increased risks for both mother and baby.

Epidemiology

Prevalence is approximately 5-7% of UK pregnancies (higher in some populations — up to 15-20% in South Asian women). Incidence is rising, largely driven by increasing obesity and older maternal age. GDM is the most common metabolic complication of pregnancy.

Aetiology

Pregnancy is a diabetogenic state due to anti-insulin hormones:

• Human placental lactogen (hPL): Main contributor — increases maternal insulin resistance
• Progesterone, cortisol, prolactin, oestrogen: Additional insulin antagonism
• GDM occurs when pancreatic beta cells cannot compensate adequately for pregnancy-induced insulin resistance
• Pre-existing subclinical insulin resistance (obesity, family history, ethnicity) predisposes

Pathophysiology

During normal pregnancy, insulin resistance increases from the second trimester (driven by placental hormones) → beta cells compensate by increasing insulin secretion by 200-250%. In GDM, this compensation is insufficient → hyperglycaemia. Maternal glucose crosses the placenta freely, but insulin does not → fetal hyperglycaemia → fetal hyperinsulinism → macrosomia (insulin is a growth factor), organomegaly, increased fetal oxygen consumption, increased erythropoiesis → neonatal hypoglycaemia after cord clamping (loss of glucose supply but continued high insulin).

Maternal Symptoms

• Often asymptomatic — detected on screening OGTT
• Polyuria, polydipsia (if significant hyperglycaemia)
• Recurrent infections (candidiasis, UTI)
• Increased risk of pre-eclampsia (~12% in GDM vs ~5% in general population)

Fetal/Neonatal Complications

• Macrosomia (birth weight >4kg or >90th centile): ~25% of GDM pregnancies
• Shoulder dystocia: Increased risk with macrosomia
• Neonatal hypoglycaemia: Fetal hyperinsulinism; check blood glucose within 2-4 hours of birth
• Neonatal jaundice: Polycythaemia → increased bilirubin
• Respiratory distress syndrome: Hyperinsulinism inhibits surfactant production
• Polyhydramnios: Fetal polyuria from hyperglycaemia
• Stillbirth: Increased risk with poorly controlled GDM

Red Flags

• Large-for-gestational-age fetus on USS
• Polyhydramnios
• Uncontrolled hyperglycaemia despite maximum oral therapy — start insulin urgently
• Ketonuria — screen for possible T1DM or T2DM presenting in pregnancy
• Reduced fetal movements — urgent assessment

Bedside

• Urine dipstick: Glycosuria (triggers OGTT if persistent) at antenatal visits
• Capillary blood glucose monitoring: Self-monitoring after diagnosis — fasting, 1-hour post-meal
• Blood pressure: Pre-eclampsia screening (increased risk in GDM)

Bloods

• 75g OGTT at 24-28 weeks (NICE NG3 criteria):
  • Fasting glucose ≥5.6 mmol/L = GDM
  • 2-hour glucose ≥7.8 mmol/L = GDM
  • Offer earlier OGTT (16-18 weeks) + repeat at 24-28 weeks if previous GDM
• HbA1c at booking: If ≥48 mmol/mol, suggests pre-existing diabetes
• Renal function, lipids: Baseline assessment
• FBC: Baseline

Imaging

• Growth scans: Serial USS at 28, 32, 36 weeks — estimated fetal weight, abdominal circumference (macrosomia monitoring)
• USS for polyhydramnios: Amniotic fluid index

Special Tests

• Continuous glucose monitoring (CGM): Considered if targets not achieved or hypoglycaemia on insulin
• Fetal surveillance: Increased monitoring — CTG, USS Doppler as clinically indicated

Non-pharmacological

• Dietary modification: First-line — low glycaemic index diet, regular meals, complex carbohydrates, reduce refined sugars
• Exercise: 30 minutes moderate activity daily (e.g., brisk walking)
• Self-monitoring of blood glucose: Fasting + 1-hour post-meal initially; 4-7 times per day
• Targets (NICE NG3): Fasting <5.3 mmol/L, 1h post-meal <7.8 mmol/L, 2h post-meal <6.4 mmol/L
• Trial of diet and exercise for 1-2 weeks before starting medication

Pharmacological

• Metformin (if targets not met with diet): 500mg OD with meals, titrate to max 2g/day in divided doses; reduces insulin resistance; crosses placenta but no evidence of fetal harm (MiG trial 2008)
• Insulin (if metformin insufficient or fasting >7.0 at diagnosis):
  • Rapid-acting with meals (insulin aspart or lispro) if post-prandial hyperglycaemia
  • Intermediate/long-acting (isophane or insulin detemir) if fasting hyperglycaemia
  • Often combined basal-bolus regimen; dose titrated individually
• Glibenclamide: Second-line alternative to insulin if metformin fails (not preferred — increased neonatal hypoglycaemia and macrosomia; limited use in UK practice)

Surgical/Interventional

• Timing of delivery: NICE NG3 recommends advising birth by 40+6 weeks for GDM on diet alone; 37+0 to 38+6 if on medication or with complications
• Induction of labour: Offered if GDM with complications, macrosomia, or at 40+6
• Caesarean section: Offered if estimated fetal weight >4.5kg or other obstetric indications

Referral Criteria

• All women with GDM — joint obstetric/diabetes team management
• Fasting glucose >7.0 at diagnosis — immediate insulin initiation
• Unable to achieve targets on maximum metformin — add insulin
• Evidence of fetal macrosomia or complications — increased obstetric surveillance

• Resolution: GDM resolves after delivery in the vast majority
• Postpartum T2DM: ~50% of women with GDM develop T2DM within 10-20 years; OGTT at 6-13 weeks postpartum then annual HbA1c screening (NICE NG3)
• Recurrence: GDM recurs in ~30-50% of subsequent pregnancies
• Maternal: Increased lifetime cardiovascular risk; increased risk of GDM → T2DM progression if overweight
• Neonatal: Good outcomes with well-managed GDM; macrosomia and hypoglycaemia are main short-term risks
• MiG trial (2008): Metformin non-inferior to insulin for maternal and neonatal outcomes
• Lifestyle modification: Postpartum diet and exercise reduce T2DM progression by ~50% (DPP trial data extrapolated)

NICE NG3 Risk Factors for GDM Screening

Blood Glucose Targets in GDM (NICE NG3)

GDM Management Stepwise Approach