Diabetes mellitus is one of the most common medical complications of pregnancy. Two percent to 5% of pregnancies are complicated by diabetes, of which 90% are classified as gestational diabetes mellitus. Prevention of the fetal and maternal complications of diabetes presents unique challenges to all providers of healthcare to reproductive-aged diabetic women. The management of diabetes before, during, and after pregnancy can serve as a model of preventive healthcare. Preconception counseling and metabolic control before pregnancy reduce the rate of congenital malformations and the burden of suffering in offspring.