First-born children are at a higher risk of developing asthma and allergies than their younger siblings, according to a new study.
Researchers suggest that this happens because of different conditions the kids experience in the womb.
In order to find out whether the higher risk of asthma and allergy in first-born children is the result of a prenatal process of post-natal factors, researchers investigated whether birth order affected several immune markers that are set at birth, including a variation in the DNA of the IL-13 gene, which is linked to allergic development.
They recruited more than 1,200 newborns form the Isle of Wight and ascertained their birth order, then sampled their cord blood IgE, an indicator of allergic development, and conducted skin prick tests at ages four and 10.
"This is the first study to test whether the effect of genes involved in the development of asthma and allergy differs between first- and later-born children," said Wilfried Karmaus, M.D., M.P.H., of the University of South Carolina.
They found that among first-borns, the IL-13 gene variant was associated with a statistically significant higher relative risk of having increased cord-blood IgE, an indication of increased allergic response.
This higher risk seemed to persist in older children; at four and ten the children with increased cord blood IgE were also more likely to have a positive skin prick test. Such associations were not found in children who were not first-born.
"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 ten," said Dr. Karmaus.
This gene-birth order effect is indicative that genes act differently in first-born children.
"Our findings add to the evidence that allergic reactions are programmed during pregnancy and then effect the disease in later life.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," said Dr. Karmaus.
It may also indicate that there are ways to reduce the chances of first-borns developing asthma and allergy by changing the conditions in the uterus.
"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 percent of all cases of asthma and allergy," Dr. Karmaus said.
"Better understanding the mechanism involved in prenatal priming [of immune development] may offer new and exciting avenues of intervention," he added.
The study will be presented at the American Thoracic Society's 2008 International Conference in Toronto.