Gestational Diabetes Mellitus (GDM)

• Timing: Typically identified via screening at 24-28 weeks of gestation, though it can be diagnosed earlier if risk factors are present.
• Pathophysiology: Usually mirrors type 2 diabetes - pregnancy causes progressive insulin resistance (via placental hormones), and GDM develops when beta-cell insulin secretion cannot compensate. A small subset of "GDM" cases are actually early type 1 diabetes unmasked by pregnancy.
• Prevalence: Affects roughly 5-9% of pregnancies (up to ~8% cited in Harrison's).
• Diagnosis (two common approaches):
  • Two-step (US standard): 50-g oral glucose challenge test (non-fasting); if 1-hour glucose >130-140 mg/dL, follow with a 100-g, 3-hour OGTT. Two or more abnormal values (fasting <95, 1h <180, 2h <155, 3h <140 mg/dL) confirm GDM.
  • One-step (IADPSG criteria): 75-g, 2-hour OGTT with a single abnormal value diagnostic - fasting ≥92 mg/dL, 1-hour ≥180 mg/dL, or 2-hour ≥153 mg/dL.
• Clinical importance: Untreated GDM raises risks of preeclampsia, macrosomia (birth weight >4000 g), shoulder dystocia, and neonatal hypoglycemia. Treatment reduces these risks. Women with GDM also have substantially increased future risk of developing type 2 diabetes (and a smaller subset develop type 1 diabetes).