The purpose of this study was to determine the effect of maternal superobesity (body mass index [BMI], ≥ 50 kg/m(2)) compared with morbid obesity (BMI, 40-49.9 kg/m(2)) or obesity (BMI, 30-39.9 kg/m(2)) on perinatal outcomes.We conducted a retrospective cohort study of birth records that were linked to hospital discharge data for all liveborn singleton term infants who were born to obese Missouri residents from 2000-2006. We excluded major congenital anomalies and women with diabetes mellitus or chronic hypertension.There were 64,272 births that met the study criteria, which included 1185 superobese mothers (1.8%). Superobese women were significantly more likely than obese women to have preeclampsia (adjusted relative risk [aRR], 1.7; 95% confidence interval [CI], 1.4-2.1), macrosomia (aRR, 1.8; 95% CI, 1.3-2.5), and cesarean delivery (aRR, 1.8; 95% CI, 1.5-2.1). Almost one-half of all superobese women (49.1%) delivered by cesarean section, and 33.8% of superobese nulliparous women underwent scheduled primary cesarean delivery.Women with a BMI of ≥ 50 kg/m(2) are at significantly increased risk for perinatal complications compared with obese women with a lower BMI.