Objective: To determine the prevalence of obesity and its effect on maternal and fetal outcomes among pregnant women at the Johannesburg Hospital, South Africa. Methods: A retrospective study of 767 pregnant women divided into BMI groups. Complications such as pregnancy-induced hypertension (PIH), gestational diabetes mellitus (GDM), urinary tract infection (UTI), preterm labor, preterm rupture of membranes (PROM), induction of labor (IOL), postdates, fetal weight, and fetal outcome were compared among the BMI groups. Seasonal variation in BMI was measured. Descriptive statistics included mean with 95% confidence interval or median with interquartile range. Inferential statistics included t test, chi(2) test, analysis of variance (ANOVA), and analysis of covariance. Results: Among the study population, 337 (44%) were obese or morbidly obese. The complications of GDM (P<0.001), UTI (P=0.002), and failed IOL (P=0.003) were significantly more common among morbidly obese women. There was no seasonal variation in BMI. Conclusion: The prevalence of obesity in pregnancy was high in South African women and was associated with an increase in complications. (C) 2010 International Federation of Gynecology and Obstetrics. Published by Elsevier Ireland Ltd. All rights reserved.