Objective Studies in neonates have demonstrated an adverse relationship between maternal smoking during pregnancy and fetal bone mineral accrual. We aimed to investigate an intrauterine influence of maternal smoking in pregnancy on offspring bone mass during childhood.

Design Prospective birth cohort study: the Avon Longitudinal Study of Parents and Children (ALSPAC).

Setting Mothers were resident in Avon, UK during pregnancy and had expected delivery dates between April 1991 and December 1992.

Participants and Methods Our analysis included 7121 children attending a research clinic at mean age 9.9 years. We used multiple regression models to compare associations of maternal and paternal smoking at the time of pregnancy with measures of bone size and density in the children, since a greater maternal association would provide evidence of a direct intrauterine effect on the fetus. Multivariate multiple imputations were used to account for missing data.

Main Outcome Measures DXA measured total body less head (TBLH) and spine bone mineral content (BMC), bone area (BA) and bone mineral density (BMD).

Results Maternal smoking in any trimester of pregnancy was associated with increased TBLH BMC, BA and BMD in girls (mean difference (95% CI) (using sex-specific SD scores): 0.13 (0.05 to 0.22), 0.13 (0.04 to 0.21) and 0.13 (0.04 to 0.22), respectively) but not in boys (0.01 (−0.07 to 0.09), 0.00 (−0.08 to 0.08), 0.04 (−0.05 to 0.12)) in confounder-adjusted models. It was also associated with spine BMC, BA and BMD in girls (0.13 (0.03 to 0.23), 0.12 (0.03 to 0.22), 0.10 (0.00 to 0.21)) but not in boys (0.03 (−0.06 to 0.12), 0.00 (−0.09 to 0.09), 0.05 (−0.04 to 0.14)). Paternal smoking associations were similar, with no statistical evidence for a difference between maternal and paternal associations (all p values >0.15). Maternal associations increased on adjustment for offspring birth weight and gestational age but attenuated to the null on adjustment for the child's height and weight at age 9 years. Further investigation indicated that these relationships were driven mainly by offspring weight rather than height.

Conclusions There was little evidence of a relationship between maternal smoking during pregnancy and bone mass in boys. Our parental comparisons and multivariable analyses suggested that the positive associations between maternal smoking and bone mass in girls were likely to be attributable to shared familial characteristics related to offspring adiposity rather than an intrauterine mechanism.