Twenty three studies (n=1,206,679) were included. Sample sizes ranged from 131 to 863,233 participants; 112,081 caesarean sections were performed in the whole sample. Eighteen studies (17 publications) were cohort studies, four were case control studies and one was a cross-sectional study. Studies were undertaken in Europe or the United States (except one from Iraq). The assessment of publication bias revealed no marked departure from symmetry in the funnel plot and the Trim and Fill method for calculating pooled estimates reported similar summary estimates, which suggested that any publication bias had little effect on results.
Overall analysis: There was a significant increase in the risk of asthma after Caesarean section (odds ratio 1.22, 95% CI 1.14 to 1.29, p<0.001). There was significant heterogeneity (I2=46%, p<0.001).
Subgroup analysis: If asthma was ascertained before age 18 years there was a significant increase in the risk of asthma after Caesarean section (odds ratio 1.20, 95% CI 1.14 to 1.26, p<0.001) with a reduction in heterogeneity (I2=32%, p=0.08). The authors reported that other subgroup analyses (by study design, asthma ascertainment and geographic region) reported results similar to the overall analysis.