Erasmus University Rotterdam Dr. Molewaterplein 50 Rotterdam 3000 DR, The Netherlands
In the November 15, 2001, issue of the Journal, Karmaus et al. presented an interesting paper on the relation between immunoglobulin E cord blood and birth order (1). Their findings indicated that an intrauterine mechanism underlies the association between birth order and atopy, as has been hypothesized previously (2, 3). However, the age of the mother could be a confounder in this relation: Of course, birth order and the age of the mother are strongly related, and it is conceivable that the immune system changes with increasing age, thus changing the mother-fetus interaction. We wonder what the results would be if the analyses were adjusted for this characteristic.
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