Background: Inverse associations between allergic disease and sibship have been consistently described and are frequently explained by purported lower rates of early infection among children from small families. Alternative explanations include the possibility that pregnancy itself determines maternal atopic status.
Objective: To test the hypothesis that atopy defined by skin prick test (SPT) declines with increasing numbers of pregnancies.
Methods: At enrollment to a birth cohort, mothers were skin prick tested to three common allergens. Seven years later these women underwent a second SPT and provided information on their reproductive histories. At both visits, information on allergic disease was also sought.
Results: Twenty five (15%) women who were initially atopic were no longer so at the second visit; loss of hayfever symptoms was reported by 33 (29%) women. Women with higher numbers of intervening pregnancies were more likely to 'lose' their atopy (P=0.05) and symptoms of hayfever (P=0.02); this was not true for asthma. The findings could not be accounted for by maternal age.
Conclusion: Successive pregnancies may in part determine a mother's atopic state. Since maternal atopy is a risk factor for childhood atopic disease, this process may affect the atopic state of successive children. These findings suggest an alternative explanation for the sibship effect in allergic disease.