Smoking in pregnancy and disruptive behaviour in 3-year-old boys and girls: an analysis of the UK Millennium Cohort Study

Background: Maternal smoking during pregnancy has been consistently associated with disruptive behaviour in male offspring; however, results for girls are inconsistent and little is known about emergent patterns in young children. Additionally, it is unclear whether maternal smoking is independently associated in offspring with hyperactivity-inattention or only when it co-occurs with conduct problems. Further, few studies have controlled for a broad range of maternal psychosocial problems. Methods: Associations between self-reported smoking in pregnancy and maternal reports of externalising behaviour were analysed in more than 13 000 3-year-old boys and girls in the UK Millennium Cohort Study. Conduct and hyperactivity-inattention problems were assessed using the Strength and Difficulties Questionnaire. Results: Boys whose mothers persistently smoked throughout pregnancy were at significant risk of conduct and hyperactivity-inattention problems compared with sons of non-smokers: the effect was stronger for heavy smokers. After excluding children with co-occurring problems, conduct-only problems remained a significant risk for sons of heavy smokers, OR 1.92 (95% CI 1.29 to 2.86); and hyperactivity-inattention only for sons of light or heavy smokers, OR 1.79 (95% CI 1.27 to 2.51) and 1.64 (1.10 to 2.46). Daughters of light or heavy smokers were at significant risk of conduct-only problems, OR 1.73 (95% CI 1.14 to 2.61) and 1.73 (1.06 to 2.83). Relative to non-smokers, daughters of pregnancy quitters had significantly reduced odds of having conduct 0.61(0.39 to 0.97) or co-occurring problems 0.26(0.08 to 0.82), although only 79 and 20 girls met these criteria, respectively. All findings were robust to controlling for key social and psychosocial factors. Conclusions: Associations between maternal smoking during pregnancy and disruptive behaviour in 3-year-old children vary by sex, smoking status and whether or not conduct or hyperactivity problems occur together or separately