Introduction
The incidence of allergic diseases has recently increased in different regions around the world.1 In Japan, allergic diseases are a serious health concern, with high rates of wheezing (16.4%), asthma (10.5%), and atopic dermatitis (21.5%) observed in children at 5 years of age.2 Concurrently, cesarean delivery rates have been increasing worldwide,3–5 and have approximately doubled in Japan since 1990, reaching 18.6% in 2014.6 The increased rate is mainly due to a rise in elective cesarean delivery. Some recent reports describe associations between cesarean delivery and allergic diseases.7-9Given that the increases in allergic disease rates have occurred over a short period of time, they may be associated with environmental factors rather than genetic factors. Compared with children born by vaginal delivery, those born by cesarean delivery have different gut flora and cytokine profiles,10–12 which are thought to be related to allergic diseases. In contrast, some reports showed that delivery by cesarean section was not associated with the development of allergic diseases.13-15 Therefore, the association between cesarean delivery and allergic diseases is controversial. In this study, we investigated the association between infants born by cesarean delivery and the development of asthma, atopic dermatitis, wheezing, and eczema using data from a large sample size cohort study of the Japan Environment and Children’s Study (JECS).