Boys: Worth the wait?

[Japanese]

MAASTRICHT, The Netherlands,
A boy is significantly better than a 50-50 bet when it takes a year or more to achieve natural pregnancy, researchers here reported.


The purported mechanism behind this finding may also explain why slightly more boys than girls are born on average, said Luc J.M. Smits, M.D., and colleagues at Maastricht University in a study published in the Dec. 17 issue of the British Medical Journal.


The Dutch researchers analyzed data from more than 5,000 Dutch women who gave birth to single babies between July 2001 and July 2003. The women's average age was 30.5 years.


Among the 498 (9.4%) of women who took longer than 12 months to conceive, the probability of having a boy was nearly 58%, whereas the proportion of male births to women who conceived more quickly was 51% (P=0.0052).


In fact, with every additional year it took to conceive, the odds of having a boy increased by 4%, the researchers found. So women who took two years to conceive had about a 62% probability of having a boy. For those who took three years, the odds rose to nearly 66%.


These results applied only to natural conceptions. For couples whose conception was medically assisted, there was no relationship between time to conception and sex of the baby.


These findings support the idea that in viscous fluids, sperm bearing the Y chromosome swim faster than those bearing the X chromosome, the study authors said. Women whose cervical mucus is relatively viscous and a more difficult swim for sperm would not only take longer to conceive naturally, but they would also have a higher probability of male offspring once they became pregnant, the researchers explained.


This hypothesis may also explain why throughout the world, more boys than girls are born (105 boys to 100 girls in most countries), despite the fact that human semen holds equal amounts of X bearing and Y bearing sperm, the Dutch team said.