SummaryObjective Rapid weight gain in the first 3 months of life has been associated with an unfavourable cardio-metabolic phenotype in adulthood. However, little is known about the antepartum determinants of this rapid weight gain, which may reflect key developmental exposures that program metabolic pathways. Thus, we sought to characterize the antepartum determinants of rapid weight gain in the first 3 months of life in infants exposed to gestational diabetes mellitus (GDM), a patient population at risk for early cardio-metabolic disease. Design Prospective observational cohort study. Patients Pregnant women with (n = 90) and without GDM (n = 250) underwent detailed antepartum metabolic characterization, followed by assessment of their term offspring at age 3 months. Measurements Rapid infant weight gain in the first 3 months was defined as weight gain ≥ 0Á5 SD. Results No features of maternal metabolic function in pregnancy (including insulin sensitivity, lipid profile, adiponectin, leptin and C-reactive protein) were associated with infant weight gain in either the GDM or non-GDM group. Interestingly, although all infants were born at term (≥37 weeks), length of gestation was inversely associated with weight gain at 3 months in the infants of women with GDM (b = À148Á5, P = 0Á01). In these infants, length of gestation <39 weeks was an independent predictor of rapid weight gain (OR = 7Á9, 95%CI 1Á7-38, P = 0Á009) in the fully adjusted model. These associations were not observed in infants of women without GDM. Conclusions Delivery before 39 weeks is independently associated with rapid weight gain in the first 3 months of life in term infants of women with GDM and hence may be an antepartum marker of future cardio-metabolic risk.