MedWire News: Birth by Caesarean section increases the risk of allergic sensitisation and hayfever among children with a parental history of allergies, researchers have found.

Some studies among children in the general population have indicated that birth by caesarean section increases the risk of asthma and other allergic diseases, but other studies have found no such association, explain Dr Juan Celedón, from Brigham and Women's Hospital in Boston, Massachusetts, and colleagues.

However, they add that none of these studies specifically assessed the risk of allergic diseases associated with caesarean section among children considered to be at high risk due to having parents with such disorders.

To address this, the team studied the relationship between mode of delivery and the development of allergies in the first 9 years of life among 432 children with a parental history of such disorders.

The children's medical records were examined for diagnoses of asthma and heyfever, and allergic sensitization, called atopy, was defined by at least one positive result on a skin prick test for common allergens.

After accounting for factors such as maternal age at birth, birthweight and exposure to cigarette smoke, the team found that children delivered by Caesarean section were more than twice as likely to become sensitised to common allergens as those born by vaginal delivery.

Children delivered by Caesarean section were also 1.8 times more likely to develop hayfever than those with a vaginal birth.

However, mode of delivery did not appear to affect the children's risk of developing asthma, note the researchers in the Journal of Allergy and Clinical Immunology.

Dr Celedón and team conclude: "Our findings suggest that birth by Caesarean section leads to an increased risk of allergic rhinitis [hayfever] and atopy among children at high risk of atopy in childhood."

They suggest that a lack of contact with microbes present in the mother's vagina among children delivered by Caesarean section could negatively affect their immune systems and increases their risk of allergies. Additionally, reduced/absent labour during Caesarean delivery may also play a part in the association.