Objective The Institute of Medicine (IOM) does not provide recommendations for gestational weight gain (GWG) specific to women with pregestational diabetes. We aimed to assess the impact of GWG outside the IOM recommendations on perinatal outcomes. Study Design We performed a retrospective cohort study of all singletons with pregestational diabetes from 2008 through 2013. Women were classified as GWG within, less than, or greater than IOM recommendations for body mass index per week of pregnancy. Maternal outcomes examined were cesarean delivery, preeclampsia, and percentage of visits with glycemic control (>50% blood sugars at goal). Neonatal outcomes were birthweight, small for gestational age (<10th percentile), large for gestational age (LGA) (>90th percentile), macrosomia (>4000 g), preterm delivery (<37 weeks), and birth injury (shoulder dystocia, fracture, brachial plexus injury, cephalohematoma). Groups were compared using analysis of variance and χ2 test, as appropriate. Backwards stepwise logistic regression was used to adjust for confounding factors. Results Of 340 subjects, 37 (10.9%) were within, 64 (18.8%) less than, and 239 (70.3%) greater than IOM recommendations. The incidence of cesarean delivery, preeclampsia, glycemic control, preterm delivery, and birth injury were not significantly different between GWG groups. The incidence of LGA and macrosomia increased as GWG category increased (adjusted odds ratio [AOR], 3.08; 95% confidence interval [CI], 1.13-8.39 and AOR, 4.02; 95% CI, 1.16-13.9, respectively) without decreasing the incidence of small for gestational age (AOR, 0.34; 95% CI, 0.10-1.19). Increases in the risk in LGA and macrosomia were not explained by differences in glycemic control by GWG groups. Conclusion Women with pregestational diabetes mellitus should be counseled to gain within the IOM recommendations to avoid LGA and macrosomic newborns.