Background We previously reported an increased prevalence of any congenital malformation among women experiencing moderate-to-severe asthma exacerbations during the first trimester of pregnancy, based on a study in which 90.1% of the cohort of women were social welfare recipients. This study re-examined the association between asthma exacerbations and congenital malformations in a new large representative cohort of asthmatic pregnant women.

Methods A cohort of 36 587 pregnancies in asthmatic women was reconstructed from Québec Province administrative databases (1998–2009). Occurrences of asthma exacerbations during the first trimester of pregnancy were assessed and categorised into severe, moderate and no such exacerbations. For comparison, we also considered moderate and severe asthma exacerbations combined. Congenital malformations were identified using diagnoses recorded in the hospitalisation database. Generalised estimation equations were used to estimate adjusted ORs of congenital malformations.

Results The prevalence of any congenital malformation was 19.1%, 11.7% and 12.0% among women with severe, moderate and no such exacerbations during the first trimester, respectively. The adjusted OR for all malformations was 1.64 (95% CI 1.02 to 2.64) when women with severe exacerbations were compared with those in the reference group, while no association was seen for moderate exacerbations. Also, no association was observed between cases of moderate and severe asthma exacerbations combined and any congenital malformation.

Conclusions Only severe asthma exacerbations were found to significantly increase the risk of congenital malformations in this representative study. Previous studies possibly overestimated the risk because they were based mainly on women at a lower socioeconomic status.