Obesity has been associated in the literature with other pregnancy risks such as hypertension and diabetes mellitus, but disagreement persists about the expected course and complications of labor. Also, the effects of obesity on intrauterine growth and gestational duration have not been well defined. This study of 2746 consecutive deliveries used a computer-based uniform perinatal record to compare 300 pregnancy risk and outcome factors for obese and nonobese patients. The 279 obese women (more than 90 kg at some time during the pregnancy) were found to be older and of higher parity than the 2467 who were not obese. Those in the obese group were at increased antepartum risk and had increased frequencies of chronic hypertension, inadequate pregnancy weight gain, twin gestation, and diabetes mellitus. Oxytocin induction and repeat cesarean sections were performed more frequently for the obese patients, with no increase in complications during the current labor. The frequency of labor abnormalities, oxytocin augmentation, and primary cesarean section was similar to that of the comparison group. Examination of infant outcome revealed similar Apgar scores and perinatal mortality in the 2 groups, but fewer low-birth-weight infants (under 2500 g) and more macrosomic babies (over 4000 g) occurred in the obese population. This increase in birth weight was accounted for not only by an increase in the birth weight percentile, but also by a significant lengthening of the period of gestation.