The objective of the study was to test the association between gestational weight gain, reproductive factors, and postpartum weight retention based on a cohort conducted with 405 women aged 18-45 y with follow-up waves at 0.5, 2, 6, and 9 mo postpartum. The outcome variable, postpartum weight retention, was calculated as the difference between the measured weight at each visit minus the prepregnancy weight. We estimated the statistical associations between the outcome variable and potential explanatory covariates of interest by fitting a longitudinal mixed-effects model. Women with gestational weight gain above the recommendations of the Institute of Medicine (IOM) retained significantly more weight than women with weight gain within or below the recommendations, independently of prepregnancy BMI [weight (kg)/height (m(2))] or body fat at baseline. Women with the highest gestational weight gain and with body fat >/=30 g/100 g at baseline had the highest likelihood of developing maternal obesity. The final longitudinal model showed that 35% of each kilogram of weight gained during pregnancy was retained 9 mo postpartum, even after adjustment for age, prepregnancy BMI, body fat at baseline, and years since first parturition. Each unit of increase in prepregnancy BMI was associated with a decrease of -0.51 kg in postpartum weight retention. In conclusion, gestational weight gain was one of the most important predictors for postpartum weight retention and must be monitored systematically with the aim of preventing postpartum obesity and the diseases that follow.