First-born children are more likely to develop asthma and allergies, according to a new study.

Researchers said that it is because they experience different conditions in the womb.

A team from the University of South Carolina looked at more than 1,200 newborns, testing their umbilical cord blood for an indicator of allergic response. They were then tested at age 4 and 10.

They found that first-born children with a gene variation called IL-13 were more likely to show the increased allergic response at all ages than those who had IL-13 but were born later in the order, said Dr. Wilfried Karmaus.

That means that the gene acted differently in first-born children.

"We were not surprised that birth order had an effect on the development of the immune system, but were surprised that this interaction persisted at least through age 10," said Karmaus. "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' birthrates continue to decline."