A Japanese research study has found that firstborn children may be at a greater risk of food allergy and hay fever than subsequent siblings, but birth order does not affect the risk of asthma. Birth order did not appear to have an impact on the risk of eczema in school age children, but it did have a significant impact in infancy. The study was observational so it did not draw causal links, but these findings may help in counseling parents of children with allergies about the risks if they choose to have more children.
One popular explanation for the effect of birth order on allergy is called the hygiene hypothesis, under which the immune system of younger siblings is suspected to benefit from older kids bringing home respiratory infections and endotoxins that expose younger siblings at an early age. However, the fact that in this study the effect was also observed for eczema and food allergy in infancy suggests a prenatal origin.