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Our objective was to estimate associations between gestational weight gain z scores and preterm birth, neonatal intensive care unit admission, large- and small-for-gestational age birth, and cesarean delivery among grades 1, 2, and 3 obese women.We included singleton infants born in Pennsylvania (2003–2011) to grade 1 (body mass index 30–34.9 kg/m2, n = 148,335), grade 2 (35–39.9 kg/m2, n = 72,032), or grade 3 (≥40 kg/m2, n = 47,494) obese mothers. Total pregnancy weight gain (kg) was converted to gestational age-standardized z scores. Multivariable Poisson regression models stratified by obesity grade were used to estimate associations between z scores and outcomes. A probabilistic bias analysis, informed by an internal validation study, evaluated the impact of body mass index and weight gain misclassification.Risks of adverse outcomes did not substantially vary within the range of z scores equivalent to 40-week weight gains of −4.3 to 9 kg for grade 1 obese, −8.2 to 5.6 kg for grade 2 obese, and −12 to −2.3 kg for grade 3 obese women. As gestational weight gain increased beyond these z score ranges, there were slight declines in risk of small-for-gestational age birth but rapid rises in cesarean delivery and large-for-gestational age birth. Risks of preterm birth and neonatal intensive care unit admission were weakly associated with weight gain. The bias analysis supported the validity of the conventional analysis.Gestational weight gain below national recommendations for obese mothers (5–9 kg) may not be adversely associated with fetal growth, gestational age at delivery, or mode of delivery.