MedWire News: The conditions experienced by a first-born child in the womb may increase their susceptibility to allergies in later childhood, researchers have found.

"Our findings add to the evidence that allergic reactions are programmed during pregnancy and then affect the disease in later life," said Dr Wilfried Karmaus, from the University of South Carolina in Columbia, USA.

"This finding may partially account for the increasing prevalence of asthma and allergies in children in the last 30 years, primarily seen in the western world, as developed nations' birth rates continue to decline."

Dr Karmaus and team found that, among more than 1200 children born on the Isle of Wight in the UK, those who were first-born were more likely than later-born children to have high levels of immunoglobulin E (a marker of allergy development) in blood taken from their umbilical cords at birth.

First-born children were also more likely than later-born children to have positive skin-prick tests for allergies at ages 4 and 10 years.

The researchers then looked at a particular variant of a gene that encodes interleukin-13, a molecule that contributes to inflammation. This genetic variant is known to increase people's risk of developing an allergy.

In the Isle of Wight children, the genetic variant increased the risk of later allergies only in first-born children.

Dr Karmaus observed that his team's findings may one day have a practical application.

He said: "If we could produce pregnancy conditions for first-born children that are comparable to later-born children, we may be able to prevent 20% to 30% of all cases of asthma and allergy."

The researchers presented their findings at the American Thoracic Society International Conference in Toronto, Ontario, Canada.