LATE-BREAKING ABSTRACT: Mode of delivery and asthma at school age in nine European birth cohorts

Background. The rate of cesarean section has continued to increase both in low-, middle-, and high-income countries largely exceeding the recommendations of World Health Organization. At least in high income countries, the increased rate is mainly due to a rise in elective caesarean, often without medical indication. Evidence on the association of modes of delivery with asthma at school age is inconclusive. We assessed the association between specific modes of delivery and asthma in children of nine prospective European birth cohorts. Methods. Cohort-specific crude and adjusted Risk Ratios (aRR) for asthma at 5-9 years were calculated using Poisson regression models and pooled. A sensitivity analysis in children born at term was carried out to reduce confounding due to perinatal factors. Results. The study included 67,613 participants. Cohort-specific rates of cesarean varied from 9.4% to 37.5%. Current asthma varied from 3.3% to 11.3%. Cesarean section, as opposed to vaginal delivery, was associated with an increased risk of asthma (aRR, 1.22; 95% CI: 1.02,1.46). Compared to spontaneous vaginal delivery, the aRR was 1.33 (95% CI: 1.02,1.75) for elective caesarean, 1.07 (95% CI: 0.94,1.22) for emergency cesarean, and 0.97 (95% CI: 0.84,1.12) for operative vaginal delivery. In children born at term, the effect estimates strengthened only for elective cesarean (aRR, 1.49; 95% CI: 1.13,1.97). Conclusions. The large sample size allowed analysis of the effect of specific modes of delivery. The increased risk of asthma associated with elective caesarean, especially in children born at term where confounding is less likely, is relevant to counteract this practice. Causal underlying mechanisms need to be explored.