Gestational Diabetes

A pregnant patient's fasting glucose of 97 mg/dL might look nearly normal — but in pregnancy, that number already meets one of the diagnostic thresholds for gestational diabetes.

Gestational diabetes mellitus (GDM) develops when pregnancy hormones — primarily human placental lactogen — create progressive insulin resistance that the pancreas cannot overcome. Screening occurs at 24–28 weeks via a two-step process: a 1-hour 50-g glucose challenge test (GCT) with a threshold of 130–140 mg/dL, followed by a confirmatory 3-hour 100-g oral glucose tolerance test (OGTT). Diagnostic thresholds for the 3-hour OGTT (Carpenter-Coustan): fasting ≥95, 1-hr ≥180, 2-hr ≥155, 3-hr ≥140 mg/dL — two or more abnormal values confirm the diagnosis. Management starts with medical nutrition therapy: three meals plus two to three snacks daily, emphasizing complex carbohydrates and limiting simple sugars, with carbohydrate distribution across meals. Blood glucose self-monitoring targets are fasting <95 mg/dL and 2-hour postprandial <120 mg/dL. If diet fails to control glucose within 1–2 weeks, insulin is the pharmacologic standard; oral agents like metformin or glyburide may be used per provider preference. Fetal risks include macrosomia, shoulder dystocia, neonatal hypoglycemia, hyperbilirubinemia, and polyhydramnios. Maternal risks include preeclampsia and future type 2 diabetes — the patient needs a 75-g OGTT at 4–12 weeks postpartum to rule out persistent disease.

Key Distinctions

Don't confuse GDM screening thresholds with preexisting diabetes criteria — a fasting glucose ≥126 or A1C ≥6.5% at the first prenatal visit suggests pregestational diabetes, not GDM. Students mix up the 1-hour screen (50 g, non-fasting OK) with the diagnostic 3-hour OGTT (100 g, fasting required). Neonatal hypoglycemia after delivery results from the newborn's continued high insulin production once the maternal glucose supply stops — it's rebound, not deficiency.

Clinical Pearl

Think of the baby as 'overfed and over-insulinized' in utero — the moment the cord is cut, that extra insulin has no glucose to work on, and the newborn crashes.