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A few years ago, while examining the effects of pregnancy hormones on the "baby brain," Neil Watson, PhD, of Simon Fraser University in British Columbia and his student Claire Vanston discovered something that made their jaws drop: Women who had carried male fetuses performed significantly better on the most difficult tests of working memory and spatial ability than those carrying female fetuses.

The study, published in the May 12, 2005, NeuroReport (Vol. 16, No. 7), tested 39 pregnant women at five points, starting just after they got pregnant and ending when their periods returned a year or more after delivery, and compared them with 13 nonpregnant controls. The cognitive effects were still present at the end of the study.

It's unclear why fetal sex should have such a big effect, says Watson. Although a logical candidate would be that male fetuses generate testosterone that enters the woman's bloodstream and brain and influences cognitive ability, that was not the case here, he says.

He is now working on a study to replicate and extend these findings. For one thing, he is looking into the possible influence of other factors related to the fetus or placenta that may differ between male and female fetuses and has some promising leads, he says.