['To determine whether women with gestational diabetes mellitus (GDM) whose weight gain exceeded the 2009 Institute of Medicine (IOM) recommendations were more likely to have macrosomia.', 'Retrospective cohort study of the association of weight gain in women with Class A1 GDM, with term (≥37 weeks) singleton liveborns and macrosomia (birthweight ≥4000\u2009g). Multivariate logistic regression models were used to adjust for covariates and test for interactions.', 'Of 1502 women studied, pre-pregnancy body mass index (BMI) categories were: normal (39.6%), overweight (28.5%) and obese (31.9%). The mean (±standard deviation ) weight gain (lbs) for these groups was: 27.6±10.9, 24.2±13.0 and 18.8±16.3 (P<0.0001), whereas the occurrence of macrosomia was 7.4, 11.4 and 19.0%, respectively. Women with an obese BMI were twice as likely to have a macrosomic infant compared with women in the normal BMI group (odds ratio, OR 2.0; 95% CI 1.4-3.0; P=0.0005). Independently, women who exceeded the IOM guidelines were three times more likely to have a macrosomic infant (OR 3.0, 95% CI 2.2-4.2, P<0.0001).'] Are pre-pregnancy weight and excess weight gain risk factors for macrosomia in women with gestational diabetes?