Gestational diabetes mellitus (GDM) is glucose intolerance first recognised during pregnancy, typically appearing in the second or third trimester. Globally, GDM affects 7–14% of pregnancies — with rates as high as 20% in some Asian and Middle Eastern populations. Untreated GDM carries significant risks for both mother and baby: pre-eclampsia, macrosomia (birth weight >4 kg), birth trauma, neonatal hypoglycaemia, stillbirth, and a sharply elevated long-term risk of type 2 diabetes in the mother (50–70% within 10–20 years). The good news: with proper screening, diet, monitoring, and medication when needed, the vast majority of GDM pregnancies have excellent outcomes.

From about week 20, the placenta secretes hormones — human placental lactogen, progesterone, oestrogen, cortisol, and growth hormone — that antagonise maternal insulin action. Most women compensate by producing more insulin. In GDM, pancreatic beta-cells cannot keep up, and blood glucose rises. GDM is essentially type 2 diabetes unmasked by the stress of pregnancy — which is why women with GDM are at such high risk of developing T2D later.

All pregnant women are screened at 24–28 weeks, but women with risk factors should be screened at the first antenatal visit:

- BMI ≥30 (or ≥25 for Asian women)

- Previous GDM or macrosomic baby (>4 kg)

- Family history of type 2 diabetes (first-degree relative)

- Polycystic ovary syndrome (PCOS)

- Ethnic background: South Asian, Middle Eastern, Black Caribbean, Hispanic

- Glycosuria, age >35, twin pregnancy, hypertension

- Fasting ≥5.1 mmol/L (92 mg/dL) → diagnosed

- 1-hour ≥10.0 mmol/L (180 mg/dL) → diagnosed

- 2-hour ≥8.5 mmol/L (153 mg/dL) → diagnosed

- Any one value meets criterion.

HbA1c is NOT a reliable screening test in pregnancy (physiological red cell turnover). It's used only to rule out pre-existing diabetes at first visit (HbA1c ≥6.5% = pre-existing DM, not GDM).

| When | Target |

|---|---|

| Fasting | <5.3 mmol/L (95 mg/dL) |

| 1-hour post-meal | <7.8 mmol/L (140 mg/dL) |

| 2-hour post-meal | <6.7 mmol/L (120 mg/dL) |

| HbA1c (2nd/3rd trimester) | <6.0% (42 mmol/mol) |

Diet and exercise control 70–85% of GDM cases. Core principles:

- Carbohydrate distribution: 175–200 g/day (40–45% cal), split 3 meals + 2–3 snacks. Breakfast most problematic — start 15–30 g carb

- Low-GI carbs: oats, legumes, whole grains; limit refined sugar, juice, white bread

- Protein with each meal: 15–20 g slows gastric emptying

- Fibre 25–30 g/day

Exercise: 30 min moderate activity most days. A 10-minute walk after each meal reduces post-prandial glucose by 25–30%.

Women with GDM check capillary glucose 4 times daily: fasting + 1–2h after each main meal. Continuous glucose monitors (CGMs) increasingly used — reveal hidden patterns (nocturnal highs, snack spikes) and reduce macrosomia in trials.

When to escalate: >30% of readings above target after 1–2 weeks of strict diet → add pharmacotherapy.

Insulin does not cross the placenta and has the strongest safety record. First-choice in ACOG, NICE, IADPSG.

Basal-bolus regimen:

- Basal: NPH at bedtime OR detemir 1–2×/day OR glargine 100 U/mL

- Bolus: aspart or lispro before meals

Insulin requirements rise ~5–10% per week from weeks 18–36, then plateau or slightly decrease in the last 2–4 weeks.

Starting dose (weeks 28+): 0.7 units/kg/day. Split ~50% basal, ~50% boluses across 3 meals.

Metformin crosses the placenta but decades of use show no increase in congenital malformations. Favoured in: refusal of insulin, severe insulin resistance (obesity, PCOS), resource-limited settings.

Dosing: 500 mg once daily with dinner; titrate to 2000–2500 mg/day over 1–2 weeks.

MiG TOFU 9-year follow-up: slight increase in offspring adiposity vs insulin, no neurodevelopmental difference. ~46% of women on metformin still need supplemental insulin. NICE 2015/2020 and ADA 2024 accept as first-line.

Glibenclamide / glyburide — once popular but higher rates of macrosomia and neonatal hypoglycaemia vs insulin. ACOG 2018 downgraded — no longer first-line.

- Ultrasound every 4 weeks from week 28 — watch for macrosomia (EFW ≥90th centile) or polyhydramnios

- Low-dose aspirin 75–150 mg/day from 12 weeks — reduces pre-eclampsia risk (USPSTF/ACOG)

- Delivery timing: well-controlled GDM: 39–40 weeks; EFW ≥4500 g: discuss elective CS at 39 weeks

- Labour: maintain maternal glucose 4–7 mmol/L. Neonatal glucose checked 2h after birth, then 2–6h for 24 h

Insulin requirements fall dramatically within hours of placental delivery. STOP insulin immediately after delivery (or restart pre-pregnancy T2D regimen). Metformin is resumed only if persistent T2D confirmed.

75 g OGTT at 6–12 weeks postpartum is essential:

- Normal: repeat every 1–3 years, lifestyle counselling

- Impaired fasting glucose or IGT: annual OGTT; metformin can be considered

- Overt diabetes (~10%): transition to T2D care

Breastfeeding reduces T2D risk by 30–50% in women with GDM and is strongly encouraged.

- 50–70% lifetime risk of T2D — highest in first 5 years

- 40–60% recurrence in subsequent pregnancies

- Increased cardiovascular disease risk

- Children: 2× risk of childhood obesity, metabolic syndrome, T2D

Lifestyle intervention (Mediterranean diet, 150 min/week exercise, 5–7% weight loss) can reduce progression to T2D by up to 50% (Finnish DPS, DPP).

- Blood glucose >13.9 mmol/L (250 mg/dL) with symptoms (thirst, frequent urination, fatigue)

- Ketones on dipstick with normal/low glucose — possible diabetic ketoacidosis, rare but life-threatening

- Reduced fetal movements

- Severe headache, visual disturbance, epigastric pain — pre-eclampsia

- Fever, dysuria, wound discharge — infections worsen glucose control

Gestational diabetes is common, manageable, and — critically — a window into your future metabolic health. With modern screening, diet, glucose monitoring, and (when needed) insulin or metformin, over 95% of GDM pregnancies have healthy outcomes. The work doesn't end at delivery: the 6-week OGTT and lifelong lifestyle changes are the most important things you can do for yourself and your future pregnancies.

More information: insulin, metformin, glibenclamide. Always coordinate care with your obstetrician, endocrinologist, diabetes educator, and pharmacist.