Mothers pregnant with boys may be less forgetful than those carrying girls, Canadian researchers said.

The researchers said they found evidence that women who gave birth to boys consistently outperformed moms of girls in tests that specifically taxed memory in areas of listening, computational and visualization skills.

"When we set out to look at the effects of pregnancy on cognition, we weren't thinking of the sex of the fetus, so we were shocked by our results," said study leader Neil Watson, a Simon Fraser University psychology professor.

The 18-month study tracked 39 Vancouver-area women from early pregnancy to several months after birth. The women were given eight tests that were administered repeatedly during pregnancy and after.

In three cognitively-challenging tests, the women pregnant with boys performed significantly better, the researchers said.

Watson said the results suggest that an "unknown fetal-derived factor" that differs between male and female fetuses may have an influence on the mother's cognition.

"The small amount of research that has been done on maternal cognition has generated contradictory results, but our data suggest that some of this discrepancy may be due to the sex of the fetus," said researcher Claire Vanston.

The researchers' findings will be published in the May 12 findings of the journal NeuroReport.