Change in maternal smoking behaviour between the first two singleton live pregnancies and childhood obesity: analysis of a UK population-based cohort

BackgroundMaternal smoking during pregnancy has been linked to offspring adiposity. We examined interpregnancy changes in maternal smoking behaviour and the risk of age-specific and sex-specific obesity (≥95^th centile) in the second child (C2).MethodsWe used a population-based cohort of antenatal health-care records (August, 2004–August, 2014) at University Hospital Southampton, linked to measured child body-mass index (BMI) at 4–5 years obtained from child health records at two community NHS Trusts (Solent and Southern). We analysed the first two singleton live pregnancies of 6515 women using logistic regression to examine interpregnancy changes in self-reported maternal smoking in relation to C2 obesity (adjusting for maternal age, ethnicity, BMI, educational attainment, employment, folate supplementation, previous losses, infertility treatment, pre-existing and gestational diabetes and hypertension, interpregnancy interval, C2's birthweight, caesarean section delivery, and gestation).FindingsUnadjusted C2 obesity prevalence for children whose mothers never smoked, smoked at the start of both pregnancy 1 (P1) and pregnancy 2 (P2), P2 only, P1 only, and stopped smoking when both pregnancies were confirmed was 6·9% (215/3118 children), 12·5% (87/698 children), 12·4% (22/178 children), 10·3% (28/271 children), and 13·1% (29/222 children), respectively. Compared with women who never smoked, smoking at the start of both pregnancies was associated with higher odds of C2 obesity (adjusted odds ratio [aOR] 2·03, 95% CI 1·49–2.78). Women not smoking in P1 who smoked at the start of P2, and those who stopped when each pregnancy was confirmed had increased C2 obesity odds (aOR 1·80 [95% CI 1·09–3·00] and 1·73 [1·11–2·69], respectively). Smokers maintaining cessation having quit by P1 confirmation, P1 smokers who ceased by P2 confirmation, and those who smoked between pregnancies but who quit before C2 conception, did not have higher odds of C2 obesity (aOR 1·21 [95% CI 0·95–1·55], 1·39 [0·89–2·17], and 1·13 [0·78–1·63], respectively).InterpretationA mother smoking at the start of her first two pregnancies has twice the odds of having an obese second child compared with a non-smoker. Smoking in the second pregnancy only and between pregnancies up to the first trimester of P2 is also associated with childhood obesity. The interpregnancy period is an opportunity to intervene on modifiable risk factors such as smoking.FundingNIHR Southampton Biomedical Research Centre and University of Southampton Primary Care and Population Sciences PhD studentship (to EJT) and an Academy of Medical Sciences and Wellcome Trust grant (grant number AMS_HOP0011060; to NAA).