Objective: To discriminate the effect of maternal obesity and gestational diabetes on birth weight and adipose tissue of the newborn. Methods: Normal BMI women (group N, n = 243; 18.5≤ BMI<25 kg/m2) and obese women (group Ob, n = 253; BMI≥30 kg/m2) were recruited in a prospective study between 15 and 18 weeks of gestation. All women were submitted to a 75g oral glucose tolerance test in the second and third trimester. First trimester fasting blood glucose was also obtained from Ob women. All women with one measurement above normal values were considered positive for gestational diabetes and first treated by dietary intervention. When dietary measures were not efficient, they were treated by insulin. Neonatal anthropometrics, sum of skinfolds and cord serum hormones were measured. Results: 222 N and 226 Ob mothers and their newborns were included in the analysis. Diabetes was diagnosed in 20% and 45.2% of N and Ob women, respectively. Birth weight was not statistically different between groups (boys: 3456g±433 and 3392g±463; girls: 3316g±402 and 3391g±408 for N and Ob, respectively). Multivariate analysis demonstrated that skinfold thickness and serum leptin concentrations were significantly increased in girls born to women with obesity (18.0mm±0.6 versus 19.7mm±0.5, p = 0.004 and 11.3ng/mL±1.0 versus 15.3ng/mL±1.0, p = 0.02), but not in boys (18.4mm±0.6 versus 18.5mm±0.5, p = 0.9 and 9.3ng/mL±1.0 versus 9.0ng/mL±1.0, p = 0.9). Based on data from 136 N and 124 Ob women, maternal insulin resistance at 37 weeks was also positively related to skinfold in girls, only, with a 1-point increase in HOMA-IR corresponding to a 0.33mm±0.08 increase in skinfold (p<0.0001). Conclusions: Regardless of gestational diabetes, maternal obesity and insulin resistance were associated with increased adiposity in girls only. Persistence of this sexual dimorphism remains to be explored during infancy.