Weight gain during pregnancy may contribute to obesity development. Concerns about possible adverse effects of pregnancy weight gain on later maternal weight and on labor and delivery must be rigorously evaluated in light of possible benefits for fetal growth and development. Birth-weight rises with increased pregnancy weight gain, and perinatal and neonatal mortality fall as birthweight increases in both preterm and term infants. The lowest mortality is observed at 3500 to 4500 g in infants of white women. Although often thought to be at high risk, infants termed "macrosomic" include infants of the lowest mortality rate. Thus, restricting weight gain may be detrimental to the baby. Weight gain that is optimal for the mother and the baby differs according to the mother's prepregnancy weight. Pregnancy weight gain exceeding current recommendations is associated with increases in maternal fat gain, pregnancy complications, and delivery problems and should be discouraged. Postpartum weight loss is essential to prevent permanent weight increase. Smoking cessation during pregnancy, reduced postpartum physical activity, and other lifestyle changes can contribute to increased postpartum weight. Health care providers can help to reduce obesity risk by regularly monitoring women's weight; promoting appropriate prepregnancy weight, pregnancy weight gain, and postpartum weight less; and explicitly encouraging maintenance of an active postpartum lifestyle.