A new study reveals that firstborn children have a higher risk of developing allergies and asthma.

The findings indicate that a firstborn child faces different conditions in the womb than his or her siblings.

The researchers, led by Dr. Wilfried Karmaus of the University of South Carolina, studied more than 1,200 newborns from 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. Furthermore, 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 researchers said that their conclusions prove that genetic development occurs differently in firstborn children, with health effects that last for many years.

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

Karmaus hopes that if scientists can identify the factors that affect immune-system development in the womb, new treatments to alter foetal development could be pursued.

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