Objective To examine whether in utero exposure to local and systemic corticosteroids is associated with asthma development in offspring.

Design Cohort study.

Setting Denmark.

Participants We included all singletons born alive in Denmark between 1996 and 2009. Data on maternal corticosteroid use, asthma in offspring and covariates were obtained from medical registries.

Main outcome measures We compared asthma risks of children prenatally exposed to corticosteroids and of children of former corticosteroid users with that of unexposed children. We computed absolute risks and used proportional-hazards regression to compute adjusted HRs (aHRs). Using logistic regression we compared exposed children with unexposed siblings in a ‘within-mother-between-pregnancy’ analysis. Adjustment addressed varying length of follow-up.

Results We identified 877 778 children, 3.6% of whom were prenatally exposed to systemic (n=5327) or local (n=24 436) corticosteroids. A total of 105 677 children developed asthma during follow-up with a 10-year risk of 18.4% among the exposed and 13.5% among the unexposed. The aHR was 1.54 (95% CI 1.45 to 1.65) for systemic use, 1.45 (95% CI 1.40 to 1.50) for local use and 1.32 (95% CI 1.30 to 1.34) for former use. The adjusted OR of the ‘within-mother-between-pregnancy’ analysis was 1.11 (95% CI 0.98 to 1.25).

Conclusions These population-based data do not support a strong causal association between maternal corticosteroid use during pregnancy and increased asthma risk in offspring.