Abstract
BACKGROUND: We aimed to investigate whether early-life hygiene-related factors influenced the risk of inflammatory bowel disease (IBD) in a Scandinavian population and test the association's consistency across cohorts. METHODS: This study followed 117 493 participants in the All Babies in Southeast Sweden study and the Norwegian Mother, Father, and Child Cohort Study. IBD diagnoses were defined by national registers. Comprehensive data on hygiene-related exposures, such as having pets, rural living, daycare attendance, and siblings, were retrieved from questionnaires administered from pregnancy until child's age of 36 months. A multivariable Cox regression model yielded adjusted hazard ratios (aHRs) for IBD accounting for socioeconomic status and perinatal factors. Cohort-specific estimates were pooled using a random-effects model. RESULTS: In over 2 024 299 person-years of follow-up 451 participants developed IBD. In pooled estimates children attending daycare up to 36 months of life vs not attending daycare were less likely to develop Crohn's disease (aHR, 0.60; 95% confidence interval [CI], 0.37- 0.98). Children having 1 or more siblings had a modestly increased risk of IBD (aHR, 1.17; 95% CI, 0.96-1.42; aHR for each sibling, 1.12; 95% CI, 1.01-1.24). The other hygiene factors were not significantly linked to later IBD. In the Norwegian Mother, Father, and Child Cohort Study cohort, bed sharing was associated with an increased risk of IBD, most notably for ulcerative colitis (aHR, 1.67; 95% CI, 1.01-2.78). CONCLUSIONS: In this birth cohort study from 2 high-income Scandinavian countries, some early-life hygiene-related exposures were associated with IBD risk. The generalizability of these results to countries of other socioeconomic level is unknown.