Objective - To study the association between gestational weight gain (GWG) and offspring obesity risk at ages chosen to approximate prepuberty (10 years) and postpuberty (16 years). Design - Prospective pregnancy cohort. Setting - Pittsburgh, PA, USA. Sample - Low-income pregnant women (n = 514) receiving prenatal care at an obstetric residency clinic and their singleton offspring. Methods - Gestational weight gain was classified based on maternal GWG-for-gestational-age Z-score charts and was modelled using flexible spline terms in modified multivariable Poisson regression models. Main outcome measures - Obesity at 10 or 16 years, defined as body mass index (BMI) Z-scores ≥95th centile of the 2000 CDC references, based on measured height and weight. Results - The prevalence of offspring obesity was 20% at 10 years and 22% at 16 years. In the overall sample, the risk of offspring obesity at 10 and 16 years increased when GWG exceeded a GWG Z-score of 0 SD (equivalent to 30 kg at 40 weeks); but for gains below a Z-score of 0 SD there was no relationship with child obesity risk. The association between GWG and offspring obesity varied by prepregnancy BMI. Among mothers with a pregravid BMI <25 kg>/m2, the risk of offspring obesity increased when GWG Z-score exceeded 0 SD, yet among overweight women (BMI ≥25 kg/m2), there was no association between GWG Z-scores and offspring obesity risk. Conclusions - Among lean women, higher GWG may have lasting effects on offspring obesity risk.