Background: Studies have shown an inverse association between birth order and allergic disease risk; some but not all have shown an inverse association between cord blood immunoglobulin E (IgE) and birth order. We further examined the relationship between birth order and cord blood IgE in a racially diverse birth cohort.
Methods: Women were interviewed about their pregnancy history, and their babies' cord blood was collected to measure total IgE using a high-sensitivity protocol (lower detection limit 0.01 IU/ml). We analyzed cord IgE as both a continuous and categorical measure.
Results: Of the 733 children, 171 (23%) were first born, 92 (13%) were first born with the mother having prior pregnancies but no live births, and 470 (64%) were born second or later. By birth order, the geometric means +/- standard deviations were: first born 0.26 +/- 4.2 IU/ml, first born after prior pregnancies 0.35 +/- 3.9 IU/ml, second born 0.30 +/- 4.8 IU/ml, third born 0.28 +/- 5.1 IU/ml, and fourth born or greater 0.28 +/- 4.5 IU/ml (trend p = 0.51). Other factors considered (maternal allergic disease history, age, race, exposure to smoking and cats/dogs during pregnancy, fetal gender, season of delivery) neither modified nor confounded these relationships.
Conclusions: Unlike some previous reports, there was no association between total cord IgE level and birth order. Mechanisms other than cord IgE should be studied in the quest to understand the role of birth order in allergic disease risk. Categorization of a continuous measure of IgE may incorrectly create statistically significant results.
2007 S. Karger AG, Basel