Women who take more than a year to conceive are more likely to have a baby boy, according to research published on Friday.
Scientists at Maastricht University in the Netherlands, who studied data on 5,283 women who had babies between July 2001-2003, found that the probability of having a boy was 58 percent if it took longer than 12 months to get pregnant.
The findings did not apply to women having fertility treatment.
"Taking longer to reach lasting pregnancy increases the chances of having male offspring," said Luc Smits, of the department of epidemiology at the university, in a report in The British Medical Journal.
Each additional year it took to conceive was associated with a nearly 4 percent higher chance of having a boy.
Although the proportions of X chromosome, which determines a female, and Y chromosome for male, in sperm are the same, more boys than girls are born.
The scientists suspect that males are more likely after a long pregnancy because sperm carrying the Y chromosome swim better through the viscous liquids in the woman's body to fertilize an egg than sperm with the X chromosome.
Increased viscosity is thought to reduce the chances of conception, so over a longer period the chances are higher of the Y sperm reaching the egg first.
"The time taken to get pregnant is positively related to the chance of having a boy in couples conceiving naturally," Smits added.