Birth Order May Affect Risk of Allergies

March 21, 2011 -- First-born children may be more likely to develop certain types of allergies than their younger brothers or sisters, a study suggests.

Researchers found the prevalence of several types of allergies, such as allergic rhinitis (hay fever or seasonal allergies), allergic conjunctivitis (eye inflammation due to allergies), and food allergy decreased as birth order increased in a large group of Japanese schoolchildren.

For example, 4% of first-borns had some type of food allergy compared with 3.5% of second-borns and 2.6% of third-borns.

Researchers say it's the first study to show an effect of birth order on food allergy.

Benefit of Being the Youngest

Researchers surveyed the parents of more than 13,000 schoolchildren in Japan aged 7 to 15 about the prevalence of specific allergic diseases.

Based on the results, researchers calculated the prevalence of each allergic disease based on birth order.

The results showed there was no significant difference in the prevalence of asthma or atopic dermatitis (a type of eczema) according to birth order.

But the prevalence of allergic rhinitis, allergic conjunctivitis, and food allergy decreased based on birth order -- from oldest to youngest.

“Individuals with increased birth order have a smaller risk of allergy," explained Takashi Kusunoki, MD, PhD, of the Shiga Medical Center for Children in Moriyama, Japan, in a news release. "However, the significance of the effect may differ by allergic diseases.”

Prenatal Origins of Allergies?

The study also showed that in infancy symptoms such as wheezing increased and food allergies decreased as birth order increased.

Researchers say these findings suggest that food allergies may have their origins in the prenatal period in the womb.

The results were presented at the Annual Meeting of the American Academy of Asthma, Allergy and Immunology in San Francisco.

This study was presented at a medical conference. The findings should be considered preliminary as they have not yet undergone the "peer review" process, in which outside experts scrutinize the data prior to publication in a medical journal.