Gestational Diabetes Mellitus

Carbohydrate intolerance first recognized in pregnancy, driven by progressive insulin resistance from placental hormones (human placental lactogen, progesterone, cortisol) unmasking limited maternal pancreatic reserve.

Risk factors include obesity (BMI ≥30), previous GDM, a previous macrosomic baby (over 4.5 kg), family history of diabetes, South Asian, Black, or Middle Eastern ethnicity, PCOS, and previous unexplained stillbirth.

Usually asymptomatic and identified on screening; polyuria and polydipsia occur occasionally. Fetal macrosomia or polyhydramnios on a routine scan can be the first clue.

• 75 g OGTT at 24 to 28 weeks for at-risk women (or universal screening per local protocol), performed as soon as identified if there is a history of previous GDM
• Diagnostic thresholds (WHO/IADPSG): fasting glucose ≥5.1 mmol/L, 1-hour ≥10.0 mmol/L, or 2-hour ≥8.5 mmol/L, any single value meeting or exceeding the threshold confirms diagnosis (NICE uses slightly different cutoffs: fasting ≥5.6 mmol/L, 2-hour ≥7.8 mmol/L)

• First line: dietary modification (carbohydrate counting or low glycemic index approach) and exercise, with self-monitoring of capillary glucose; targets: fasting under 5.3 mmol/L, 1-hour postprandial under 7.8 mmol/L, 2-hour postprandial under 6.4 to 6.7 mmol/L

• If targets are not met within 1 to 2 weeks of lifestyle measures: metformin 500 mg PO od, titrated up to 500 mg to 1 g bd-tds (maximum 2 to 2.5 g/day)

• Insulin added or substituted where metformin is insufficient, not tolerated, or contraindicated, typically basal-bolus or intermediate-acting regimens, with doses individualized and reviewed frequently given rapidly changing insulin requirements through pregnancy

• Glibenclamide (glyburide) is used as an alternative oral agent in some settings, though generally less preferred given evidence of higher rates of neonatal hypoglycemia and macrosomia compared with insulin or metformin

• Fetal surveillance: growth scans every 4 weeks from around 28 to 36 weeks

• Timing of delivery: by 40+6 weeks if diet-controlled and uncomplicated; earlier (37 to 39 weeks) if on pharmacological therapy or complications are present, per local protocol

• Postpartum: stop hypoglycemic agents at delivery; 75 g OGTT at 6 to 13 weeks postpartum to exclude persistent diabetes, and annual HbA1c thereafter given a roughly 50% ten-year risk of progression to type 2 diabetes