PURPOSE: Excess weight gain during pregnancy is a serious health concern among young pregnant women in the US. This study aimed to characterize young women at highest risk for gaining over the recommended amount of weight during pregnancy. METHODS: Using a database that is representative of births in large U.S. cities, The Fragile Families and Child Wellbeing Study, we identified mothers of singleton term-infants age 15-24 years at the time of delivery. Institute of Medicine guidelines were used to categorize each mother's weight gain as less than, within, or more than recommended during pregnancy. Multinomial logistic regression models for weight gain category were performed, controlling for age, race/ethnicity, federal poverty level (FPL), health status, and prepregnancy BMI. RESULTS: Among the weighted sample (n = 1,034, N = 181,375), the mean (SD) age was 21 (3) years, 32% were black, 39% were Hispanic, 44% reported income under the Federal Poverty Level, 45% were overweight or obese before pregnancy, and 55% gained more weight than recommended during pregnancy. Women who were overweight or obese before pregnancy were at increased risk for gaining more pregnancy weight than recommended, compared to normal-weight women (adjusted Relative Risk Ratio (RRR) = 3.82, p = 0.01; RRR = 3.27, p = 0.03, respectively). Hispanics were less likely than non-Hispanics to gain more weight than recommended (RRR = 0.39, p = 0.03). CONCLUSIONS: The majority of mothers ages 15-24 gained excess weight during pregnancy, a strong risk factor for later obesity. Prepregnancy overweight or obesity and non-Hispanic ethnicity predicted excess pregnancy weight gain. Interventions and policies should target these high-risk young women to prevent excess weight gain.