The aim of this study is to describe the impact of maternal weight gain on infant birth weight among women with Class III obesity. Retrospective cohort of women with body mass index (BMI) ≥40 kg/m at initial prenatal visit, delivered from July 2013 to December 2017. Women presenting 14/0 weeks of gestational age (GA), delivering preterm, or had multiples or major fetal anomalies excluded. Maternal demographics and complications, intrapartum events, and neonatal outcomes abstracted. Primary outcomes were delivery of large for gestational age or small for gestational age (SGA) infant. Bivariate statistics used to compare women gaining less than Institute of Medicine (IOM) recommendations (LTR) and women gaining within recommendations (11-20 pounds/5-9.1 kg) (at recommended [AR]). Regression models used to estimate odds of primary outcomes. Of included women ( = 230), 129 (56%) gained LTR and 101 (44%) gained AR. In sum, 71 (31%) infants were LGA and 2 (0.8%) were SGA. Women gaining LTR had higher median entry BMI (46 vs. 43,  < 0.01); other demographics did not differ. LTR women were equally likely to deliver an LGA infant (29 vs. 34%,  = 0.5) but not more likely to deliver an SGA infant (0.8 vs. 1%,  > 0.99). After controlling for confounders, the AOR of an LGA baby for LTR women was 0.79 (95% CI: 0.4-1.4). In this cohort of morbidly obese women, gaining less than IOM recommendations did not impact risk of having an LGA infant, without increasing risk of an SGA infant.