Dutch researchers analysed data for 5,283 women who gave birth to single babies between July 2001 and July 2003.
Among the 498 women who took longer than 12 months to get pregnant, the probability of male offspring was nearly 58%, whereas the proportion of male births among the 4,785 women with shorter times to pregnancy was 51%.
The authors calculate that, for couples conceiving naturally, each additional year of trying to get pregnant is associated with a nearly 4% higher expected probability of delivering a male baby, even after adjusting for factors such as age, smoking status, alcohol use, and variability of the menstrual cycle.
In contrast, sex of the offspring of couples who had received medical help in getting pregnant did not show any relation with time to pregnancy.
These findings support the idea that, in viscous fluids, sperms bearing the Y (male) chromosome swim faster than those bearing the X (female) chromosome, say the authors. 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.
Furthermore, the findings may 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 sperms.