Gestational Diabetes
Glucose intolerance first identified during pregnancy. Affects ~4-5% of pregnancies in the UK. Risk factors include obesity, previous GDM, family history of diabetes, and South Asian/African-Caribbean ethnicity. Diagnosed by OGTT. Management: diet/exercise, then metformin, then insulin if targets not met.
Key Facts
Glucose intolerance first recognised in pregnancy: affects ~4-5% of UK pregnancies; incidence increasing with obesity Risk factors: BMI >30, previous GDM, previous macrosomic baby (>4.5kg), first-degree relative with diabetes, South Asian/African-Caribbean/Middle Eastern ethnicity NICE NG3 screening: 75g OGTT at 24-28 weeks in at-risk women; diagnose if fasting glucose ≥5.6 mmol/L or 2-hour glucose ≥7.8 mmol/L Maternal risks: pre-eclampsia, polyhydramnios, operative delivery, future T2DM (~50% develop T2DM within 10 years) Fetal risks: macrosomia, shoulder dystocia, birth injury, neonatal hypoglycaemia, neonatal jaundice, respiratory distress syndrome Management (NICE NG3): dietary modification + exercise first; add metformin 500mg-2g/day if targets not met; add insulin if metformin insufficient; glibenclamide if both declined Glucose targets: fasting <5.3 mmol/L, 1-hour post-meal <7.8 mmol/L, 2-hour post-meal <6.4 mmol/L
Overview
Key Facts
GDM requires close glycaemic monitoring to minimise maternal and fetal complications. Most resolves postpartum but confers significant long-term risk of T2DM. Postnatal follow-up with OGTT at 6-13 weeks is essential.
Epidemiology
Affects ~4-5% of UK pregnancies (higher in high-risk populations). Incidence increasing with rising obesity rates. Recurrence risk in subsequent pregnancies ~30-50%.
Aetiology
Pregnancy is a state of progressive insulin resistance (human placental lactogen, cortisol, progesterone, growth hormone). GDM develops when beta cell function is insufficient to overcome this resistance. Risk factors: pre-existing insulin resistance (obesity), genetic susceptibility, ethnicity, older maternal age.
Pathophysiology
Placental hormones (human placental lactogen, cortisol, TNF-α, progesterone) create progressive insulin resistance from mid-pregnancy. Beta cells normally compensate with increased insulin secretion. GDM occurs when this compensation is inadequate → maternal hyperglycaemia → glucose crosses placenta → fetal hyperinsulinaemia → macrosomia, organomegaly, neonatal hypoglycaemia.
Clinical Presentation
Often Asymptomatic
- Usually detected by screening (OGTT)
- Rarely symptomatic hyperglycaemia (polyuria, polydipsia, thrush)
Maternal Complications
- Pre-eclampsia (increased risk)
- Polyhydramnios
- Operative delivery (caesarean section, instrumental)
- Perineal trauma (macrosomia)
Fetal/Neonatal Complications
- Macrosomia (birthweight >4kg or >4.5kg)
- Shoulder dystocia and birth injury (Erb palsy, clavicular fracture)
- Neonatal hypoglycaemia (fetal hyperinsulinaemia persists after cord clamping)
- Neonatal jaundice, polycythaemia
- Respiratory distress syndrome
- Stillbirth (if severe/uncontrolled GDM)
Long-Term Risks
- Mother: ~50% risk of T2DM within 10 years; increased cardiovascular risk; recurrence in future pregnancies
- Offspring: increased risk of obesity, T2DM, metabolic syndrome in later life
Red Flags
- Fasting glucose ≥7.0 mmol/L at booking → may be pre-existing T2DM (not GDM)
- Large-for-dates fetus + GDM → close monitoring, consider earlier delivery
- Polyhydramnios → poor glycaemic control
Differential Diagnosis
| Diagnosis | Key Features | Investigation |
|---|---|---|
| Pre-existing T2DM | High HbA1c at booking (≥48), persistent postpartum | HbA1c at booking, postpartum OGTT |
| Pre-existing T1DM | Known diagnosis, autoantibodies, insulin-dependent | History, autoantibodies |
| MODY | Strong family history, young onset, may present in pregnancy | Genetic testing |
| Impaired glucose tolerance (pre-diabetes) | May be pre-existing; identified by OGTT | Postnatal OGTT |
Diagnosis / Investigation
Screening (NICE NG3)
- 75g OGTT at 24-28 weeks: for all women with risk factors (BMI >30, previous GDM, previous macrosomia, first-degree relative with DM, South Asian/African-Caribbean/Middle Eastern)
- Early OGTT (at booking): if previous GDM; repeat at 24-28 weeks if normal
Diagnostic Criteria (NICE)
- Fasting glucose ≥5.6 mmol/L OR
- 2-hour glucose ≥7.8 mmol/L
Monitoring
- Self-monitoring of blood glucose (SMBG): fasting + 1-hour post-meal (NICE NG3)
- Targets: fasting <5.3, 1-hour <7.8, 2-hour <6.4 mmol/L
- Serial USS: growth scans (28, 32, 36 weeks) to monitor for macrosomia, polyhydramnios
Postnatal
- Fasting glucose at 6-13 weeks postpartum (NICE NG3): exclude persistent diabetes
- Annual HbA1c or fasting glucose: lifelong screening for T2DM
Management
Dietary/Lifestyle (First-Line)
- Low glycaemic index diet, regular meals, limit refined carbohydrates
- Exercise: 30 min moderate activity daily
- Trial of diet alone for 1-2 weeks before pharmacotherapy
Pharmacological (NICE NG3)
- Metformin 500mg-2g/day: first-line if diet fails; crosses placenta but no evidence of harm; NICE recommends
- Insulin: if metformin insufficient or fasting glucose ≥7.0 mmol/L at diagnosis (start insulin immediately); basal ± bolus; isophane (NPH), insulin aspart/lispro
- Glibenclamide 2.5-20mg/day: if metformin contraindicated/not tolerated AND insulin refused; less preferred (neonatal hypoglycaemia risk)
Obstetric Management
- Serial growth scans (28, 32, 36 weeks)
- Timing of delivery: NICE recommends delivery by 40+6 (diet-controlled) or 39+0-40+6 (on medication); earlier if complications
- Intrapartum: variable-rate insulin sliding scale if poor glycaemic control; hourly glucose monitoring
Postnatal
- Stop all glucose-lowering medication immediately after delivery (insulin resistance resolves)
- Monitor neonatal blood glucose (risk of neonatal hypoglycaemia)
- Breastfeeding encouraged (reduces future T2DM risk for both mother and child)
- Fasting glucose at 6-13 weeks postpartum: exclude persistent diabetes
- Annual HbA1c screening: lifelong
- Pre-conception counselling: before future pregnancies
Referral Criteria
- Joint obstetric-diabetes clinic: all GDM
- Specialist diabetic midwife/team
- Postnatal: GP for annual T2DM screening
Prognosis
GDM usually resolves after delivery. ~50% of women with GDM develop T2DM within 10 years (hence lifelong screening). Higher risk of T2DM with: obesity, need for insulin in pregnancy, higher glucose levels, earlier gestational diagnosis. Recurrence in subsequent pregnancies ~30-50%. Well-controlled GDM: near-normal pregnancy outcomes. Poorly controlled: increased macrosomia, operative delivery, and neonatal morbidity.
Other Relevant Information
GDM Glucose Targets (NICE NG3)
| Timing | Target |
|---|---|
| Fasting | <5.3 mmol/L |
| 1-hour post-meal | <7.8 mmol/L |
| 2-hour post-meal | <6.4 mmol/L |
OGTT Diagnostic Criteria Comparison
| Organisation | Fasting Threshold | 2-Hour Threshold |
|---|---|---|
| NICE (UK) | ≥5.6 mmol/L | ≥7.8 mmol/L |
| WHO/IADPSG | ≥5.1 mmol/L | ≥8.5 mmol/L |
| ADA | ≥5.1 mmol/L (1-hour ≥10.0) | ≥8.5 mmol/L |