Background:Neonatal body composition likely mediates fetal influences on life long chronic disease risk. A better understanding of how maternal lifestyle is related to newborn body composition could thus inform intervention efforts. Methods:Using Cork participant data (n = 1754) from the Screening for Pregnancy Endpoints (SCOPE) cohort study [ECM5(10)05/02/08], we estimated how pre-pregnancy body size, gestational weight gain, exercise, alcohol, smoking and diet were related to neonatal fat and fat-free mass, as well as length and gestational age at birth, using quantile regression. Maternal factors were measured by a trained research midwife at 15 gestational weeks, in addition to a 3rd trimester weight measurement used to calculate weight gain. Infant body composition was measured using air-displacement plethysmography. Results:Healthy (versus excess) gestational weight gain was associated with lower median fat-free mass [-112 g, 95% confidence interval (CI): -47 to -176) and fat mass (-33 g, 95% CI: -1 to -65) in the offspring; and a 103 g decrease in the 95th centile of fat mass (95% CI: -33 to -174). Maternal normal weight status (versus obesity) was associated with lower median fat mass (-48 g, 95% CI: -12 to -84). At the highest centiles, fat mass was lower among infants of women who engaged in frequent moderate-intensity exercise early in the pregnancy (-92 g at the 95th centile, 95% CI: -168 to -16). Lastly, women who never smoked tended to have longer babies with more fat mass and fat-free mass. No other lifestyle factors were strongly related to infant body composition. Conclusions:These results suggest that supporting healthy maternal lifestyles could reduce the risk of excess fat accumulation in the offspring, without adversely affecting fat-free mass development, length or gestational age.