Pregnancy is characterized by insulin resistance and hyperinsulinemia and thus may predispose some individuals to diabetes. These metabolic changes stem from the placental hormones that ensure that the fetus has an abundant and continuous supply of nutrients: growth hormone, cortisol, placental lactogen, prolactin and progesterone. Gestational diabetes mellitus (GDM) is defined as carbohydrate intolerance that begins or is first recognized during pregnancy. The condition is associated with increased maternal, fetal, and neonatal risks. GDM is associated with increased risks for the fetus and the newborn, including macrosomia, shoulder dystocia, birth injuries, hyperbilirubinemia, hypoglycemia, respiratory distress syndrome, hypocalcemia, polycythemia, and childhood obesity. Maternal risks include preeclampsia, polyhydramnios, gestational hypertension, cesarean delivery, and diabetes occurring later in life. This chapter discusses screening, risk factors, differential diagnosis, management, and patient education of gestational diabetes mellitus during pregnancy.