Tobacco Smoke Exposure in Early Childhood and Later Risk of Inflammatory Bowel Disease: A Scandinavian Birth Cohort Study

To examine the association between early-life smoking exposure and later risk of inflammatory bowel disease IBD. We followed 115663 participants from the Norwegian Mother, Father and Child MoBa and All Babies in Southeast Sweden ABIS cohorts from birth 1997-2009 through 2021. IBD was identified through national patient registers. Validated questionnaire data defined maternal smoking during pregnancy, maternal environmental tobacco smoke ETS exposure during pregnancy, and child ETS exposure by ages 12 and 36 months. Cox regression was used to estimate adjusted hazard ratios aHRs for sex, maternal age, education level, parental IBD, and origin. Cohort-specific estimates were pooled using a random-effects model. During 1 987 430 person-years of follow-up, 444 participants developed IBD ABIS, 112; MoBa, 332. Any vs no maternal smoking during pregnancy yielded a pooled aHR of 1.30 95% CI = 0.97-1.74 for offspring IBD. Higher level of maternal smoking during pregnancy (compared with no smoking, average ≥6 cigarettes/day: pooled aHR = 1.60 95% CI = 1.08-2.38) was associated with offspring IBD, whereas a lower smoking level was not (average 1-5 cigarettes/day: pooled aHR = 1.09 95% CI = 0.73-1.64). Child ETS exposure in the first year of life was associated with later IBD (any vs no ETS, pooled aHR = 1.32 95% CI = 1.03-1.69). Estimates observed for child ETS exposure by 36 months were similar but not statistically significant. In this prospective Scandinavian cohort study, children exposed to higher levels of maternal smoking during pregnancy or ETS during the first year of life were at increased risk of later IBD.