Tobacco smoke exposure in early childhood and later risk of inflammatory bowel disease: A Scandinavian birth cohort study.

OBJECTIVES: To examine the association between early-life smoking exposure and later risk of inflammatory bowel disease (IBD).

METHODS: We followed 115,663 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 age 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.

RESULTS: During 1,987,430 person-years of follow-up, 444 participants developed IBD (ABIS, n=112; MoBa, n=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 to 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.

CONCLUSIONS: 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.