A 28-year-old primigravida at 24 weeks of gestation is diagnosed with gestational diabetes mellitus (GDM) on oral glucose tolerance test. She is counselled on dietary modifications and lifestyle changes, but repeat fasting blood glucose remains 135 mg/dL and postprandial glucose is 185 mg/dL after 2 weeks. What is the drug of choice for pharmacological management of her GDM?

Explanation

## First-Line Pharmacotherapy in GDM **Key Point:** Insulin is the gold standard and first-line pharmacological agent for GDM because it does not cross the placenta and has the longest safety record in pregnancy. ### Why Insulin? 1. **Safety Profile in Pregnancy** - Does not cross the placental barrier (large protein molecule) - No teratogenic effects - No fetal hyperinsulinemia risk - Decades of safe use in obstetrics 2. **Efficacy** - Rapidly achieves glycaemic control - Allows precise titration based on blood glucose monitoring - No risk of hypoglycaemia in the fetus (insulin-mediated) 3. **Types Used in GDM** - **NPH insulin** (intermediate-acting) — most commonly used - **Regular/short-acting insulin** — for postprandial control - **Insulin analogues** (aspart, lispro) — increasingly used; safe in pregnancy - Basal-bolus regimen often required ### Indications for Insulin Initiation | Criterion | Threshold | |-----------|----------| | Fasting blood glucose | ≥ 95 mg/dL (5.3 mmol/L) | | 1-hour postprandial | ≥ 140 mg/dL (7.8 mmol/L) | | 2-hour postprandial | ≥ 120 mg/dL (6.7 mmol/L) | **Clinical Pearl:** The patient in this case has fasting glucose 135 mg/dL and 2-hour postprandial 185 mg/dL — both well above thresholds for insulin initiation. **High-Yield:** IADPSG (International Association of Diabetes and Pregnancy Study Groups) and ACOG guidelines recommend insulin as first-line pharmacotherapy when lifestyle modifications fail in GDM. ## Comparison with Other Agents | Agent | Status in GDM | Reason | |-------|---------------|--------| | **Insulin** | **First-line** | **No placental crossing; gold standard** | | Metformin | Second-line option | Crosses placenta; limited long-term fetal data; emerging evidence of safety | | Glibenclamide | Relative contraindication | Crosses placenta; risk of fetal hyperinsulinemia; associated with higher neonatal hypoglycaemia | | Pioglitazone | Contraindicated | Teratogenic in animal models; inadequate human safety data | **Warning:** While metformin is increasingly used in some centres as a second-line agent (especially in overweight/obese mothers), it is NOT first-line. Glibenclamide, though once used, is now avoided due to placental transfer and fetal effects. [cite:ACOG Practice Bulletin 190; Obstetric Care Consensus 8]