Objective This study aimed to investigate the association between excess and less than recommended gestational weight gain (GWG) and adverse maternal and neonatal outcomes in women with pregestational and gestational diabetes. Study Design We conducted a secondary analysis of the National Institute of Child Health and Human Development (NICHD) Consortium on Safe Labor (CSL) study. We included deliveries >23 weeks of nonanomalous singletons with either pregestational or gestational diabetes. The exposure was GWG greater than or less than compared with the U.S. Institute of Medicine recommendations for total pregnancy weight gain per prepregnancy body mass index. Consistent with the 2020 Delphi outcome for diabetes in pregnancy, maternal outcomes included cesarean delivery and preeclampsia and neonatal outcomes included small for gestational age (SGA), large for gestational age (LGA), macrosomia >4,000 g, preterm birth <37 weeks, stillbirth, and neonatal death. We modeled both absolute GWG and GWG z-scores, standardized for gestational duration. Multivariable logistic regression with generalized estimating equations was used, adjusting for age, race/ethnicity, parity, prior cesarean delivery, chronic hypertension, tobacco use, U.S. region, and delivery year. Results Of 8,322 deliveries (n = 8,087 women) complicated by pregestational or gestational diabetes, 47% were in excess, 27% were within, and 26% were less than GWG recommendations. Deliveries with excess absolute GWG were at higher adjusted odds of cesarean delivery, preeclampsia, LGA, and macrosomia, compared with those within recommendations. Similar results were observed when using standardized GWG z-scores, in addition to higher likelihood of preterm birth and neonatal death. Less than recommended GWG was associated with a lower likelihood of these adverse outcomes but higher SGA. Additionally, less GWG by z-score was associated with a lower likelihood of stillbirth. Conclusion Excess GWG increases the risk of adverse maternal and neonatal outcomes for women with pregestational and gestational diabetes. Less GWG than recommended may decrease this risk. Key Points