Gestational diabetes

Quick Overview

Gestational diabetes mellitus (GDM) affects 3-5% of pregnancies in the UK and increases risks of macrosomia, shoulder dystocia, neonatal hypoglycaemia, and future type 2 diabetes. NICE NG3 emphasises early identification through risk-based screening, tight glycaemic control, and individualised delivery planning to optimise maternal and fetal outcomes.

Core Facts & Concepts

📊 Diagnostic Criteria (75g OGTT at 24-28 weeks):

• Fasting glucose ≥5.6 mmol/L OR
• 2-hour glucose ≥7.8 mmol/L
• Single abnormal value diagnostic (NICE NG3)

🚩 Risk Factors Requiring OGTT:

• BMI >30 kg/m²
• Previous GDM
• Previous macrosomic baby (≥4.5 kg)
• First-degree relative with diabetes
• Ethnicity: South Asian, Black Caribbean, Middle Eastern
• Previous stillbirth or neonatal death

🎯 Target Blood Glucose Ranges:

• Fasting: <5.3 mmol/L
• 1-hour post-meal: <7.8 mmol/L
• 2-hour post-meal: <6.4 mmol/L

💊 Treatment Escalation:

1. Diet & lifestyle (trial for 1-2 weeks)
2. Metformin (if targets not met)
3. Insulin (if metformin inadequate or fasting ≥7 mmol/L)
4. Glibenclamide (if metformin/insulin declined)

Problem-Solving Approach

Step 1: Risk Assessment

• Identify risk factors at booking appointment
• Offer early OGTT (16-18 weeks) if previous GDM; repeat at 24-28 weeks if normal

Step 2: Monitoring Protocol

1. Self-monitor capillary glucose 4 times daily (fasting + 1-hour post-meals)
2. Weekly review until targets met, then every 1-2 weeks
3. HbA1c NOT recommended for monitoring GDM

Step 3: Treatment Decisions

• Start metformin if targets not met after 1-2 weeks of diet modification
• Add insulin if:
  • Metformin inadequate after dose optimisation
  • Fasting glucose ≥7 mmol/L at diagnosis
• Consider immediate insulin if significant fetal macrosomia or polyhydramnios

🚩 Red Flags:

• Fasting glucose ≥7 mmol/L (immediate pharmacotherapy)
• Estimated fetal weight >75th centile (intensify treatment)
• Pre-eclampsia developing (delivery planning)

⚠️ Warning: Do NOT use HbA1c for diagnosis or monitoring in pregnancy-rapid red cell turnover makes it unreliable

Analysis Framework

Fetal Surveillance Protocol:

• Diet-controlled GDM: Single ultrasound at 36 weeks for growth/liquor
• Medication-treated GDM: Serial scans every 4 weeks from 28 weeks
• Umbilical artery Doppler NOT routinely indicated unless growth concerns

Visual Aid

Delivery Planning Table:

Key Points Summary

✓ Diagnosis: Single abnormal OGTT value (fasting ≥5.6 or 2-hour ≥7.8 mmol/L)

✓ Targets: Fasting <5.3, 1-hour post-meal <7.8, 2-hour <6.4 mmol/L

✓ Treatment ladder: Diet (1-2 weeks) → metformin → insulin (immediate if fasting ≥7)

✓ Fetal surveillance: Diet-controlled = scan at 36 weeks; medication = 4-weekly from 28 weeks

✓ Delivery timing: Diet-controlled ≤40+6; medication-treated 37-38+6 weeks

✓ Postnatal: 6-week 75g OGTT mandatory; annual HbA1c thereafter (50% develop T2DM within 10 years)

✓ Common pitfall: Using HbA1c for GDM diagnosis/monitoring-always use capillary glucose

📌 Remember: 5-7-4 Rule - Fasting <5.3, 1-hour <7.8, 2-hour <6.4 (think descending numbers)