Firstborn Children More Likely To Develop Food Allergies, Study Says

VIDEO: A new study shows firstborn kids may have more food allergies.
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From watching "The Brady Bunch," it's clear to see that the eldest girl, Marsha, gets the most attention. But according to a new study, she also might get the most allergies.

Birth order, thought by some to determine everything from romantic compatibility to IQ score, may be the reason your oldest child is allergic to peanuts and your younger children are not.

Watch "World News with Diane Sawyer" for more on this story tonight on ABC.

According to a new study, older children are more likely to suffer from food allergies than their younger siblings. The results indicate the "birth order effect" occurs specifically in food allergies, as compared with other types of allergies.

Among study participants, the prevalence of food allergies was 4 percent in firstborn children, 3.5 percent in second-born children, and 2.6 percent in those born later.

Kara Corridan, the Health Editor at Parents magazine, said the study is too preliminary to get "too excited about this." It may be reassuring to parents whose first child has an allergy.

For parents, raising a child with a food allergy puts them "in a club they really don't want to be a part of," Corridan said. Food allergies can sometimes be life-threatening, and parents may have to monitor everything their child eats and touches in all environments. "Even if there's a small fraction of a chance that younger children don't have [food allergies], that would be great," she said.

Firstborns were also more likely to have allergic rhinitis and conjunctivitis -- conditions that affect the nose and mouth, respectively -- than their younger siblings.

The study's authors say that multiple pregnancies may cause changes in the womb that help build the immune systems of the second and third children. Another contributing factor may be the "hygiene hypothesis:" in preparing for their firstborn to come home, parents may hyper-sterilize their environments. By the time babies 2 and 3 come around, there are more germs, and the younger siblings may develop stronger immune systems.

"The more you are exposed to an allergen, the more likely it is you'll be immune to it," Corridan said. The connection between birth order and allergies had been established by earlier studies, she said, but this study is the first that breaks the connection down by allergic condition.

Birth order does not seem to affect a child's likelihood to develop asthma or atopic dermatitis, a type of eczema.

The study surveyed 13,000 children ages 7 to 15. The researchers compared the incidence of each allergic disease with birth order. The survey also asked parents whether their children experienced wheezing, eczema or food allergies before age 1 -- and researchers saw the same trend: the prevalence of allergies was higher in firstborn children.

Birth Order Can Be An Indicator For Behavioral Characteristics

The findings were presented at the Annual Meeting of the American Academy of Allergy, Asthma and Immunology in San Francisco on March 20.

The birth order effect has been studied in several other areas of children's lives; literature has indicated firstborns excel at school and science, while younger siblings tend to be more exploratory and take more risks.

Dr. Frank Sulloway, a visiting scholar at the University of California, Berkeley, has studied the birth order effect. He said examining children's behavior compared with their birth order opens up the issue of family niches and family dynamics.

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