Maternal smoking before and during pregnancy and offspring bone mineral density in early childhood: findings from the Southampton Women's Survey.

Smoking in adulthood increases fracture risk. In a prospective birth cohort study including 1966 children, we found that smoking before and during pregnancy were positively associated with the offspring BMD in childhood, but with attenuation by weight. This highlights the complex relationship between maternal smoking and offspring bone development.

Introduction: Inconsistent relationships between smoking in pregnancy and offspring bone mineral density (BMD) have been reported. We investigated the relationships between maternal smoking before and during pregnancy with offspring bone outcomes measured by dual-energy X-ray absorptiometry (DXA) during early childhood.

Methods: In the Southampton Women's Survey, maternal smoking was determined before pregnancy and at 11 and 34 weeks' gestation. Offspring had DXA at birth (whole body), 4, 6-7 and 8-9 years (whole-body-less-head). Linear regression was used to examine the associations between maternal smoking and standardised bone area (BA), bone mineral content (BMC), BMD and bone mineral apparent density (BMAD). Adjustment for maternal educational qualification, diet, ethnicity, age and parity (all identified by directed acyclic graph) and offspring age and sex was performed. The child's weight was additionally added to assess mediation of the association.

Results: A total of 1966 mother-offspring dyads were included; 25.7%, 15.9% and 15.1% smoked before, in early and late pregnancy, respectively. Pre-pregnancy smoking was positively associated with offspring BMD (4 years: 0.218 SD, 95% CI 0.062, 0.373; 6-7 years: 0.133 SD, 95% CI 0.004, 0.262; 8-9 years: 0.167 SD, 95% CI 0.017, 0.317), except at birth. BMAD had similar associations but not BA or BMC. Smoking in early or late pregnancy were similarly associated. Offspring of mothers who smoked during pregnancy were lighter at birth but heavier at other ages; including weight in the models weakened the associations.

Conclusion: Offspring of mothers who smoked before or during pregnancy had higher BMD, but this relationship may be partly mediated by higher childhood weight.