We examined the association between sports and other leisure-time physical activities during pregnancy and birth weight of babies born after 37 completed weeks of gestation. All Danish-speaking pregnant women attending routine antenatal care at the Department of Obstetrics, Aarhus University Hospital, Denmark, from August 1989 to September 1991 were invited to participate in the study. A total of 4458 healthy women who delivered after 37 completed gestational weeks participated in this study. The associations between sports (0, 1-2, 3+ h/week) or leisure-time physical activity (sedentary, light, and moderate to heavy) and birth weight were examined by linear and logistic regression and adjusted for potential confounding factors such as smoking, parity, schooling, pre-pregnancy body mass index and gestational age. The results showed that pregnant women who practiced sports or were moderate to heavy leisure-time physical active during the early second or the early third trimester gave birth to infants with a similar birth weight as inactive women. The proportion of newborns with a low (<2500 g) or a high birth weight (>/=4500 g) was also unchanged. In conclusion, in this large population-based study, we found no association between sports and leisure-time physical activity and low-birth weight, high-birth weight, or average-birth weight.