Babies born by Caesarean section are more prone to developing asthma, say Dutch researchers.
In a study of almost 3,000 children, birth by Caesarean was associated with a 80% increased risk of asthma by age eight compared with vaginal birth.
The association was even stronger in children whose parents had allergies, suggesting a genetic predisposition to the disease, it is reported in Thorax.
Previous research has linked Caesareans with the development of allergies.
In total, the team looked at 2,917 children, 247 of whom were born by caesarean.
Around 12% of the children were diagnosed with asthma for which they were treated with inhaled steroids by the time they were eight years old.
The researchers found that the 9% of children who had two allergic parents were almost three times more likely to be asthmatic by the time they were eight compared with children whose parents were not allergic.
Study leader, Dr Caroline Roduit, now based at the Children's Hospital of Zurich, Switzerland, said rates of asthma had soared in industrialised countries in parallel with a rise in Caesarean section births, which have increased from 5% in the 1970s to more than 30% in 2000.
She suggested that one reason for the association between Caesareans and asthma could be the priming of the immune system after birth.
Other research has shown babies born by Caesarean are not exposed to microbes as soon as babies born by vaginal delivery.
Previous studies in this area have produced conflicting results but the authors said the size of the study, the long monitoring period and and the definition of asthma to include inhaled steroids, strengthened the findings.
"The increased rate of Caesarean section is partly due to maternal demand without medical reason," said Dr Roduit.
"In this situation the mother should be informed of the risk of asthma for her child, especially when the parents have a history of allergy or asthma."
Dr Mike Thomas, chief medical adviser to the charity Asthma UK, said previous studies had also suggested that Caesarean section might increase the risk of asthma.
He said: "Sometimes a Caesarean section is needed for medical reasons, but where possible a natural birth is better."