Maternal smoking status before and during pregnancy and bronchial asthma at 3 years of age: a prospective cohort study

The association between maternal pre-pregnancy smoking status and asthma risk is unclear. This study aimed to investigate the association between pre- and post-pregnancy maternal smoking status and bronchial asthma at 3 years of age in a large birth cohort. Data of 75,411 mother-child pairs from the Japan Environment and Children's Study (JECS) were analysed using multivariate logistic regression analysis. Overall, 7.2% of the children had bronchial asthma. The maternal smoking status before childbirth was as follows: Never = 60.0%, Quit before recognising current pregnancy = 24.1%, Quit after finding out about current pregnancy = 12.3%, and Still smoking = 3.6%. Children of mothers who sustained smoking during pregnancy had an increased risk of bronchial asthma at 3 years of age even after adjusting for pre- and postnatal covariates (adjusted odds ratio [aOR] 1.34, 95% confidence interval [CI] 1.15-1.56). Children of mothers who quit before (aOR 1.09, 95% CI 1.02-1.18) or after (aOR 1.11, 95% CI 1.01-1.23) recognising the current pregnancy had an increased risk of bronchial asthma at 3 years of age. Maternal smoking throughout pregnancy and smoking exposure pre-pregnancy or in early pregnancy increases the risk of bronchial asthma in children.