The prevalence of obesity has been increasing worldwide and has reached epidemic proportions in the United States, where well over 20% of the population have a body mass index (BMI) within the obese range. Obesity is associated with a wide spectrum of obstetric and perinatal complications, including increased risks of fetal mortality and morbidity, congenital malformations, maternal hypertensive disorders, gestational diabetes, excessive fetal growth and cesarean delivery. The odds ratios for these risks increase in direct correlation with the severity of obesity, and are significant even among women who are overweight without meeting criteria for obesity. Although obesity is closely associated with diabetes which, in itself, is associated with similar perinatal complications, diabetes and obesity are independent risk factors for adverse pregnancy outcome. Moreover, improving glycemic control in the pregnant woman with diabetes may mitigate the additive adverse effects of diabetes and obesity on pregnancy outcome.