Antibiotic exposure during pregnancy and childhood asthma: a national birth cohort study investigating timing of exposure and mode of delivery

Objective To investigate whether antibiotic exposure during pregnancy was associated with childhood asthma and if this relationship was conditional on timing of exposure and mode of delivery. Design A cohort study using multivariable logistic regression models adjusting for a priori defined confounders. Pregnant women were recruited from 1996 to 2002. Setting The Danish National Birth Cohort. Patients Of the 96 832 children in the cohort, 32 651 children were included in the study population. Main outcome measure Parent-reported childhood asthma at 11 years. Results A total of 5522 (17%) children were born to mothers exposed to antibiotics during pregnancy. In adjusted analyses, children born to exposed mothers had higher odds of asthma (OR 1.14, 95% CI 1.05 to 1.24). There was no association with antibiotic exposure in the first trimester (OR 1.02, 95% CI 0.83 to 1.26), but higher odds were observed for antibiotic exposure in the second to third trimester (OR 1.17, 95% CI 1.06 to 1.28), compared with unexposed children. The overall association between antibiotics during pregnancy and childhood asthma was only observed in vaginally born children (OR 1.17, 95% CI 1.07 to 1.28) but not in caesarean section born children (planned caesarean section: OR 0.95, 95% CI 0.66 to 1.37; caesarean emergency: OR 0.96, 95% CI 0.73 to 1.28). In exposed vaginally born children, the odds for childhood asthma requiring treatment during the preceding year were 34% higher (OR 1.34, 95% CI 1.21 to 1.49), compared with unexposed vaginally born children. Conclusions Antibiotic exposure in mid-to-late pregnancy is associated with higher odds of childhood asthma in vaginally born children. Mode of delivery may modify the association.