OBJECTIVE: Gestational weight gain consistent with the Institute of Medicine’s recommendations is associated with better maternal and infant outcomes. The objective was to quantify the effect of pre-pregnancy factors, pregnancy-related health conditions, and modifiable pregnancy factors on the risks of inadequate and excessive gestational weight gain. METHOD: A longitudinal cohort of pregnant women (N=1100) who completed questions about diet and weight gain during pregnancy and delivered a singleton, full-term infant. RESULTS: Gestational weight gain was inadequate for 14% and excessive for 53%. Pre-pregnancy factors contributed 74% to excessive gain, substantially more than pregnancy-related health conditions (15%) and modifiable pregnancy factors (11%). Pre-pregnancy factors, pregnancy-related health conditions, and modifiable pregnancy factors contributed fairly equally to the risk of inadequate gain. CONCLUSION: Interventions to prevent excessive gestational gain may need to start before pregnancy. Women at risk for inadequate gain would also benefit from interventions directed toward modifiable factors during pregnancy.