There is limited data on prepregnancy obesity trends specifically among low-income women, a population at high risk for obstetric complications. Using the Pregnancy Nutrition Surveillance System, we assessed prepregnancy obesity [body mass index (BMI) ≥ 30 kg/m(2)] trends among women who participated in the Supplemental Nutrition Program for Women, Infants, and Children in 1999, 2004, and 2008. Prepregnancy BMI was calculated using measured height and self-reported prepregnancy weight. We report unadjusted contributor (state, territory or Indian tribal organization) specific trends, and both unadjusted and adjusted overall trends, to account for changes in maternal age and race-ethnic distributions, using 1999 as the referent. Of the 27 contributors in 1999, 2 had a prepregnancy obesity prevalence <20%, and 1 had a prevalence ≥ 30%. Of the 35 contributors in 2008, none had a prepregnancy obesity prevalence <20%, and 14 had a prevalence ≥ 30%. From 1999 to 2008, the overall prevalence of prepregnancy obesity increased among all racial-ethnic groups, except among American Indian/Alaskan Natives, where it remained high, but stable. Overall prepregnancy obesity increased most rapidly among Hispanics, and remained stable from 2004 to 2008 among non-Hispanic blacks. In 2008, prevalence was highest among American Indian/Alaskan Natives (36.1%) and lowest among Asians/Pacific Islanders (10.8%). The adjusted prepregnancy obesity prevalence increased from 24.8% in 1999 to 28.3% in 2008, a relative increase of 14.1%. Prepregnancy obesity among low-income women increased from 1999 to 2008 and varied by race-ethnicity. These data can be used by obesity prevention programs to better target high-risk women.