PURPOSE OF THE STUDY.
To determine the relationship between birth order and the prevalence of allergic diseases in childhood.
The study population included 11 454 children 7 to 15 years of age in Kyoto, Japan.
A validated survey was administered to parents of 14 669 children to examine prevalence of allergic rhinitis (AR), atopic dermatitis in childhood and infancy, allergic conjunctivitis (AC), bronchial asthma, food allergy (FA) in childhood and infancy, and wheezing in infancy. The impact of birth order was examined for 11 454 children with known birth order. FA was categorized as current (at the time of survey) or past; FA during infancy was defined according to symptomatic FA with avoidance of major food allergens at <1 year of age. FA was further subdivided into the following: late FA (onset after age 1 year), early tolerant (gained oral tolerance before age 3 years), and prolonged FA (persistence of disease beyond age 3 years). Logistic regression models were used to compare disease prevalence between different birth order groups. Adjustments were made for confounding variables, including age, gender, gestational age, birth weight, nutrition during infancy, day care attendance before age 1 year, and family history of allergic disease.
First-born children were less likely to be completely breastfed during infancy and less likely to attend day care before age 1 year compared with later-born children. Birth order did not affect the incidence of atopic dermatitis or bronchial asthma. Wheezing in infancy was significantly higher among second-, third-, and later-born children compared with first-born children. Increasing birth order was associated with a statistically significant decrease in prevalence of AR, AC, and FA in childhood and infancy. The relation between birth order and FA showed a decreasing trend in prevalence between first-, second-, and third-born children (4%, 3.4%, and 2.6%, respectively). Prevalence of late FA, early tolerant FA, and prolonged FA was significantly lower in second-, third-, and later-born children compared with first-borns. Higher prevalence remained consistent for first-born children when specific foods were examined.
The prevalence of atopic dermatitis and bronchial asthma was not affected by birth order whereas the prevalence of AR, AC, and FA decreased with increasing birth order. A reverse trend was demonstrated for wheezing in infancy.
This cross-sectional examination found a significant protective effect of increasing birth order on some allergic diseases (AR, AC, and FA); however, this effect was not observed for all allergic disorders. Limitations of the study included parental recall bias, lack of confirmation of diagnoses, and limited knowledge of pregnancy and birth history of subjects involved. The trend of decreasing prevalence of FA among later-born children remained consistent regardless of past versus current FA, early- versus late-onset FA, and early tolerant versus persistent FA, and was independent of the food allergen. These findings suggest that future studies should prospectively examine maternal/fetal immunologic changes during first and subsequent pregnancies, the impact of maternal exposures during first and subsequent pregnancies, and the immunologic effect of early childhood exposures to allergens and/or pathogens on first-born versus later-born siblings.
- Copyright © 2012 by the American Academy of Pediatrics