A temporary Th2 skewed immunity is essential for a successful outcome of pregnancy. It is also a hallmark of atopic disease. We recorded the number of siblings to 3667 children in relation to maternal atopy. In all, 65% of the allergic and 56% of the non-allergic mothers had more than one child (p < 0.001). These data support a hypothesis that the atopic genotype may be associated with an increased likelihood for a successful outcome of pregnancy and thus from an evolutionary point of view compensate for the less efficient host defence against microbial infections associated with this type of immunity.