Objective To provide a current estimate of the prevalence of prepregnancy obesity in the United States. Methods We analyzed 2004–2005 data from 26 states and New York City (n = 75,403 women) participating in the Pregnancy Risk Assessment Monitoring System, an ongoing, population-based surveillance system that collects information on maternal behaviors associated with pregnancy. Information was obtained from questionnaires self-administered after delivery or from linked birth certificates; prepregnancy body mass index was based on self-reported weight and height. Data were weighted to provide representative estimates of all women delivering a live birth in each particular state. Results In this study, about one in five women who delivered were obese; in some state, race/ethnicity, and Medicaid status subgroups, the prevalence was as high as one-third. State-specific prevalence varied widely and ranged from 13.9 to 25.1%. Black women had an obesity prevalence about 70% higher than white and Hispanic women (black: 29.1%; white: 17.4%; Hispanic: 17.4%); however, these race-specific rates varied notably by location. Obesity prevalence was 50% higher among women whose delivery was paid for by Medicaid than by other means (e.g., private insurance, cash, HMO). Conclusion This prevalence makes maternal obesity and its resulting maternal morbidities (e.g., gestational diabetes mellitus) a common risk factor for a complicated pregnancy.