Gestational weight gain (GWG) is an important predictor of short- and long-term adverse maternal and child outcomes. As interest in long-term outcomes increases, utilization of maternal postpartum report is likely to also increase. There is little data available examining the reliability and identifying predictors of bias in GWG recalled by mothers postpartum. We used data from the Early Childhood Longitudinal Study-Birth Cohort, a national study of U.S. children born in 2001, to compare GWG recalled by mothers approximately 10 months postpartum to GWG recorded on the birth certificate, among 5,650 records. On average, the postpartum estimates were 2.1 lbs higher (standard error, 0.2 lbs.) than the birth certificate report; 54.7% were within 5 lbs, 27.2% were overreported by more than 5 lbs, and 18.2% were underreported by more than 5 lbs. The difference between the two sources increased with GWG reported postpartum and was significantly greater among mothers who were obese prior to pregnancy, had inadequate prenatal care, or were multiparous. Bias also differed by birth outcome, indicating the potential for recall bias. When categorized by adequacy of the 2009 Institute of Medicine GWG recommendations, 70% of women were similarly categorized, and associations between GWG adequacy and small- and large-birthweight-for-gestational-age did not differ meaningfully by source of GWG data. These results suggest that for future studies, mothers' estimates of their GWG, obtained within approximately 1 year postpartum, may be a reliable substitute when birth certificate GWG data are unavailable.