Background: Male sex has been reported as a risk factor for perinatal mortality. It can be hypothesized that smoking during pregnancy and low maternal pre-pregnancy body mass index (BMI) would lead to a higher proportion of growth restricted male compared to female newborns. The objective of the study was to analyze sex-specific differences in birth weight related to smoking and maternal BMI. Method: Data on maternal and newborn characteristics were obtained from the German Perinatal Survey. We analyzed data on 508,926 singleton pregnancies that had been collected in eight German federal states between 1998 and 2000. Women were classified according to the number of cigarettes smoked per day (non-smokers, 1-7, 8-14, ≥ 15) and by maternal BMI (< 18.5, 18.5-24.99, 25-29.99, ≥ 30 kg/m2) at the first obstetric consultation. Newborns were classified as small, appropriate, or large for gestational age based on birth weight below the 10th, between 10th and 90th, or above the 90th percentile, respectively. The proportional sexual dimorphism (PSD) was calculated as the female mean divided by the male mean ×100. Results: Smoking is related to a decrease in mean birth weight and an increase of in the SGA rate. Maternal BMI is positively related to the birth weight. However, the proportional sexual dimorphism for birth weight was not different with smoking or BMI. Conclusion: Smoking during pregnancy is related to lower birth weight, and high BMI is related to higher birth weight with no change in PSD.