Cesarean section and risk of severe childhood asthma: A population-based cohort study

Objective To explore the possible association between delivery by cesarean section (CS) and later development of asthma. Study design A population-based cohort study of 1,756,700 singletons reported to the Medical Birth Registry of Norway between 1967 and 1998, followed up to age IS years or the year 2002. Exposure was the mode of delivery (spontaneous vaginal, instrumental vaginal, or CS, with planned and emergency CS separately from 1988 onward). Outcome was asthma registered in the National Insurance Scheme, which provides cash benefits to families of children with severe chronic illnesses. We used multivariate Cox proportional hazard models to examine associations between exposure and outcome. Results The cumulative incidence of asthma was 4.0/1000. Children delivered by CS had a 52% increased risk of asthma compared with spontaneously vaginally delivered children (adjusted hazard ratio [HR] = 1.52; 95% confidence interval [CI] = 1.42 to 1.62). Between 1988 and 1998, planned and emergency CS was associated with a 42% (HR = 1.42; 95% CI = 1.25 to 1.61) and 59% (HR = 1.59; 95% CI = 1.44 to 1.75) increased risk of asthma, respectively. Conclusion We found a moderately increased risk of asthma in the children delivered by CS. The possibly stronger association with emergency CS compared with planned CS could be worth pursuing to investigate possible causal mechanisms.