The longer it takes to become pregnant, the more likely the baby will be a boy, according to a report in the British Medical Journal. This finding may, in part, explain why male births are slightly more common throughout the world than female ones.

In a study of more than 5000 women who gave birth to a single baby, the proportion of male infants rose from 51 to 58 percent when the time to become pregnant increased from less than 12 months to greater than 12 months. This finding only applied to pregnancies conceived naturally, not to those in which assisted reproductive techniques (ARTs), such as in vitro fertilization, were used.

“Until now, time to pregnancy has not been studied” as a primary determinant of the infant’s gender, said lead author Dr. Luc J. M. Smits, from University Maastricht Hospital in the Netherlands.

One previous study looked at this topic, but the researchers did not differentiate women who became pregnant naturally from those who used ARTs, he added.

The new findings suggest that each additional year a couple attempts to become pregnant, the odds of having a boy increase by nearly 4 percent.

So why might a longer time to conceive raise the chances of having a boy? No one knows for sure, but it may relate to the thickness of the fluids that cover the cervical opening to the womb.

Previous reports have shown that sperm carrying the Y (male) chromosome swim faster through thick cervical fluids than sperm carrying the X (female) chromosome. As it turns out, thick cervical fluids have been tied to difficulty in conceiving. Therefore, a longer time to conceive is associated with thicker fluids that may favor the entry of a Y-carrying sperm over a X-carrying one.

Smits said his group is now planning a study looking at how the time to conceive and infant gender interact to influence various pregnancy outcomes, such as birth weight.

SOURCE: British Medical Journal, December 17, 2005.