American Journal of Epidemiology Original Contribution Delivery by Cesarean Section and Early Childhood Respiratory Symptoms and Disorders the Norwegian Mother and Child Cohort Study

Studies have indicated that children delivered by cesarean section are at an increased risk of developing wheezing and asthma. This could be the result of an altered immune system development due to delayed gut colonization or of increased neonatal respiratory morbidity. The authors examined the associations between delivery by cesarean section and the development of wheezing, asthma, and recurrent lower respiratory tract infections in children up to 36 months of age among 37,171 children in the Norwegian Mother and Child Cohort Study. Generalized linear models were used in the multivariable analysis. Children delivered by cesarean section had an increased likelihood of current asthma at 36 months of age (relative risk ¼ 1.17, 95% confidence interval: 1.03, 1.32), and the association was stronger among children of nonatopic mothers (relative risk ¼ 1.33, 95% confidence interval: 1.12, 1.58). No increased risk of wheezing or recurrent lower respiratory tract infections was seen among children delivered by cesarean section. Findings were similar among children delivered by acute and elective cesarean section. In conclusion, children delivered by cesarean section may have an increased risk of current asthma at 36 months, but residual confounding cannot be excluded. In future prospective studies, investigators should reexamine this association in different age groups. The prevalences of wheezing and asthma are high, with asthma affecting approximately 300 million people worldwide (1). Prenatal and early life exposures might increase the likelihood of a person's developing respiratory disorders. Two hypotheses exist for why children delivered by cesar-ean section may have an increased susceptibility to wheezing and asthma. One potential explanation is the altered gut colonization seen among these children, which might result in an altered development of their immune system (2–4). Another potential explanation is the increased neonatal respiratory morbidity among children delivered by cesarean section, which is associated with increased risk of developing wheezing and asthma (5, 6). Rising cesarean section rates in Western countries make such a potential relation an important public health concern, and a steady increase in the number of cesarean sections over the last decades has also been seen in Norway (7). Previous studies in which delivery by cesarean section and development of wheezing or asthma were examined have varied in sample size, age group, follow-up time, case definition , and control of confounding factors. A subgroup of previous studies included registry-based or larger cohort studies (8–14). Two meta-analyses of several of these studies found 18% and …