Dutch researchers writing in the British Medical Journal found that the longer it takes to get pregnant, the more chance there is of having a boy. Their study was based on the medical records of over 5,000 women who gave birth to single babies between July 2001 and July 2003.
In the 500 women who took longer than 12 months to get pregnant, the probability of male offspring was nearly 58 percent, whereas the proportion of male births among the other women with shorter times to pregnancy was 51 percent.
The researchers calculated that, for couples conceiving naturally, each additional year of trying to get pregnant is associated with a nearly 4 percent higher expected probability of delivering a male baby. Interestingly, the gender of the offspring of couples who had received fertility treatment did not show any correlation with time to pregnancy.
The findings support the idea that, in viscous fluids, sperms bearing the Y (male) chromosome swim faster than those bearing the X (female) chromosome, said the researchers. Women whose cervical mucus is relatively viscous would not only have more difficulties conceiving naturally, but also have a higher probability of male offspring if they do get pregnant.
The researchers speculate that the findings may explain why 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 sperms.
Source: British Medical Journal