Researchers studying twins have found that other factors may influence previous data that pointed to associations between c-section birth and increased risk of asthma.
In order to investigate [links between c-section and asthma risk], researchers concentrated on pairs of twins, the first of which was born naturally and the second via an emergency C-section. The researchers found 464 of such pairs of twins (out of a total of 42,628 twin births), ie, about 1% of all twin births.
Seventy-two twins suffered from asthma; 30 of them were born the natural way, and 20 via an emergency C-section; the siblings remained healthy in both cases. In 11 sibling pairs, both twins developed asthma. When scientists considered factors that could influence the outcome, such as birth weight, sex, or the need for ventilation, there was no difference in disease incidence between the two groups.
Although this study is not large, it suggests that factors other than Cesarean sections are responsible for the correlations measured in other studies.