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Firstborn children have a higher risk of developing allergies and even asthma, new research says.

The findings, to be presented Wednesday at the American Thoracic Society's 2008 International Conference in Toronto, indicate that a firstborn child faces different conditions in the womb than his or her subsequent siblings.

The researchers, led by Dr. Wilfried Karmaus of the University of South Carolina, studied more than 1,200 newborns from the Isle of Wight in the United Kingdom. The team analyzed whether the immune system's genetic markers, which are determined at birth, would develop differently in firstborn children.

They were specifically interested if being firstborn influenced the development of a DNA variation of the IL-13 gene, which is associated with the development of allergies.

Researchers first measured the babies' cord blood for IgE, a marker for allergy development, and then gave the children the common allergy skin-prick test at ages four and 10.

They found that in firstborns, the IL-13 gene variant was linked to a higher risk of having more IgE in umbilical cord blood. As well, the children with increased IgE in the cord blood had a greater likelihood of having a positive skin prick test for allergies both at ages four and 10.

The findings were not duplicated in children who were not firstborn.

The researchers said that their conclusions are proof that genetic development occurs differently in firstborn kids, with health effects that last for many years.

Karmaus told CTV.ca that during a mother's first pregnancy, she is fighting a foreign body, the baby, in her womb. Her body will produce antigens, or an immune response to this foreign body. That immune response could lead to the IL-13 gene being expressed in her and her baby.

The researchers point out that birth rates are in decline worldwide while allergy and asthma rates are on the rise. This means that there are more firstborns and fewer subsequent babies.

But Karmaus hopes that if scientists can identify the factors that affect immune-system development in utero, then new treatments to alter fetal development could be developed.

"The idea is how can we simulate the first pregnancy to be as if it were the second or other pregnancies," study author Karmaus told CTV.ca.

"That's the idea in the long run."