Background: To test the hygiene hypothesis, previous studies have assessed the relationship between mode of delivery at birth and asthma incidence, but the results have been inconsistent because of potential selection and ascertainment biases.
Objective: To assess the relationship between mode of delivery at birth and asthma by following all children born in Rochester, Minn, between 1976 and 1982.
Methods: From the birth certificate, we determined mode of delivery (cesarean section vs vaginal delivery). Asthma status during the first 7 years of life was ascertained from comprehensive medical record reviews. The association between mode of delivery and asthma status was evaluated in a proportional hazards model adjusted for sex, birth weight, maternal education, and maternal age.
Results: The cumulative incidence rates of asthma among children who were born by cesarean section and vaginal delivery were 3.2% versus 2.6%, 4.6% versus 4.6%, 4.6% versus 5.8%, and 5.7% versus 6.7% at the 1st, 3rd, 5th, and 7th years of life, respectively. The adjusted hazard ratios for cesarean section in predicting asthma and wheezing episode were 0.93 (95% CI, 0.6-1.4; P=.71) and 0.93 (95% CI, 0.7-1.3; P=.67), respectively.
Conclusion: Mode of delivery is not associated with subsequent risk of developing childhood asthma or wheezing episodes. Because the effect of mode of delivery on a risk of developing asthma or wheezing episodes varies over time (ie, age), selection of the study subjects according to their ages may have influenced the findings of previous studies with a shorter follow-up period.