Clinical meaning
Gestational diabetes mellitus (GDM) results from the progressive insulin resistance of normal pregnancy becoming uncompensated in women with insufficient beta-cell reserve. During pregnancy, the placenta produces hormones — human placental lactogen (hPL), progesterone, cortisol, and placental growth hormone — that antagonize insulin action at the post-receptor level by interfering with insulin receptor substrate (IRS) phosphorylation and GLUT4 transporter translocation. This physiological insulin resistance serves an evolutionary purpose: redirecting maternal glucose across the placenta to the fetus via facilitated diffusion through GLUT1 transporters. In normal pregnancy, maternal beta cells undergo compensatory hyperplasia (30-50% increase in mass) driven by prolactin and hPL signaling, increasing insulin secretion 2-3 fold to maintain euglycemia. When beta-cell compensation is inadequate, maternal hyperglycemia develops, typically manifesting in the late second or third trimester when placental hormone production peaks. The oral glucose tolerance test (OGTT) exploits this pathophysiology: a 75-gram glucose load (IADPSG/WHO criteria) or two-step approach (50-gram glucose challenge test followed by 100-gram 3-hour OGTT using Carpenter-Coustan criteria) unmasks the impaired glucose disposal. Fetal consequences of maternal hyperglycemia include fetal hyperinsulinemia (fetal beta cells respond to transplacental glucose), which drives excessive somatic growth (macrosomia), organomegaly, and increased oxygen consumption leading to chronic fetal hypoxia, polycythemia, and the characteristic neonatal complications of hypoglycemia, hyperbilirubinemia, and respiratory distress syndrome.