To Örebro University

BACKGROUND: Associations between tobacco smoking during pregnancy and offspring asthma have been observed, but the role of nicotine and familial factors remains unclear.

OBJECTIVE: To estimate the association between tobacco use in pregnancy, both smoking and Swedish oral moist snuff, and asthma/wheeze in the offspring, how it varies by the child's age and explore the influence of measured and unmeasured familial confounding.

METHODS: Register-based cohort study with sibling comparisons. The cohort included 788 508 children, born in Sweden 2005-2012 with information on maternal tobacco use in pregnancy, followed until December 2015. Asthma was based on a validated algorithm using asthma diagnoses from hospital visits and prescribed asthma drugs from nationwide registers, both as incident asthma/wheeze in age 0-8 years and current asthma at ages 2, 3, 4, 5 and 6 years.

RESULTS: For smoking during pregnancy (SDP), we saw a pattern with higher hazard ratios for asthma/wheeze around ages 5 and 18 months. Snuff did not show the same pattern. For current asthma, we saw the strongest association at age 2 years (adjOR=1.22, 95% CI: 1.17-1.28), for snuff it was weaker (adjOR=1.06, 95% CI: 0.96-1.18). When using sibling controls the estimates for SDP were clearly attenuated, albeit with wide confidence intervals.

CONCLUSION AND CLINICAL RELEVANCE: We saw an association between SDP and asthma at early age. The association with snuff was clearly weaker. The associations with SDP were attenuated when adjusting for measured and unmeasured familial factors shared by siblings. Based on those results, nicotine seems to have a limited role in the association between SDP and asthma; rather environmental tobacco smoke and other familial factors seems to explain observed associations.