In their detailed analysis of almost 3000 children followed from birth until the age of 8 years, Roduit et al1 showed that children born by caesarean section have a higher risk of asthma than those born by vaginal delivery. Surprisingly, the authors offer only one explanation for this finding—namely, delayed microbial colonisation—whereas we believe other mechanisms cannot be excluded.

As an alternative hypothesis we propose to investigate the possibility of confounding by factors already present at/before birth. This hypothesis is supported by studies showing that immunological parameters in cord blood are different between children born by vaginal delivery and those born by caesarean section.2 One such factor could be head circumference which has been repeatedly found to be related to increased IgE and the development of asthma and related disorders,3 4 5 and babies born by caesarean section probably have relatively high values.