A meta-analysis of the association between Caesarean section and childhood asthma

Background Children born by Caesarean section have modified intestinal bacterial colonization and consequently may have an increased risk of developing asthma under the hygiene hypothesis. The results of previous studies that have investigated the association between Caesarean section and asthma have been conflicting. Objective To review published literature and perform a meta-analysis summarizing the evidence in support of an association between children born by Caesarean section and asthma. Methods MEDLINE, Web Science, Google Scholar and PubMed were searched to identify relevant studies. Odds ratio (OR) and 95% confidence interval (CI) were calculated for each study from the reported prevalence of asthma in children born by Caesarean section and in control children. Meta-analysis was then used to derive a combined OR and test for heterogeneity in the findings between studies. Results Twenty-three studies were identified. The overall meta-analysis revealed an increase in the risk of asthma in children delivered by Caesarean section (OR=1.22, 95% CI 1.14, 1.29). However, in this analysis, there was evidence of heterogeneity (I-2=46%) that was statistically significant (P < 0.001). Restricting the analysis to childhood studies, this heterogeneity was markedly decreased (I-2=32%) and no longer attained statistical significance (P=0.08). In these studies, there was also evidence of an increase (P < 0.001) in the risk of asthma after Caesarean section (OR=1.20, 95% CI 1.14, 12.6). Conclusions In this meta-analysis, we found a 20% increase in the subsequent risk of asthma in children who had been delivered by Caesarean section.