Sex-specific differences in birth weight due to maternal smoking during pregnancy

We investigated the sex-specific risk of maternal smoking during pregnancy on the birth weight and the proportion of small-for-gestational-age (SGA) newborns in 888,632 (49.9%) of 1,815,318 singleton births (ca. 80% of all singleton births in Germany from 1995 to 1997) in whom data on maternal cigarette consumption were available.

Newborns below the 10th percentile for weight and duration of pregnancy were classified SGA. Maternal smoking during pregnancy lowers the mean birth weight and increases the risk of SGA newborns. The negative effect depends on the daily number of cigarettes consumed, and is greater in girls than in boys. In non-smokers, 9.8% of the newborns were SGA, with a sex-ratio of females:males=1, but this percentage increased with increasing number of cigarettes consumed (p<0.001), as did the sex-ratio, i.e. the negative effect of smoking on growth was greater in girls than in boys. In mild smokers (1–5 cigarettes/day), the risk of giving birth to an SGA girl was 1.7275-fold (95% CI: 1.7266–1.7284) above normal, but was 1.7143-fold (95% CI: 1.7137–1.7150) in boys. More than 21 cigarettes/day increased the risk of SGA 3.15-fold for a boy, but 3.51-fold for a girl (p<0.001).

In conclusion, particularly in heavy smokers, the negative effect of maternal smoking during pregnancy on the mean birth weight and risk of SGA is significantly greater in newborn girls than in newborn boys.