THURSDAY, Dec. 15, 2005 (HealthDay News) -- The longer a woman takes to get pregnant, the more likely she is to have a baby boy.
That's the conclusion of a new Dutch study that appears in the Dec. 17 issue of the British Medical Journal.
Researchers analyzed data on 5,283 women who gave birth to single babies over a two-year period. They found that the 498 women who took longer than 12 months to conceive had a 58 percent chance of having a baby boy, compared with 51 percent for women who took less time to get pregnant.
For couples who conceive naturally, each additional year of trying to get pregnant is associated with a nearly 4 percent greater expected probability of having a baby boy, the study authors calculated. This was true even after the researchers adjusted for factors such as age, smoking status, alcohol use and menstrual cycle variability.
The study found no relation between a baby's gender and length of time getting pregnant among couples who had medical help in conceiving.
The researchers said their findings support the idea that sperm bearing the Y (male) chromosome swim faster in viscous fluids than sperm bearing the X (female) chromosome. They said women who have relatively viscous cervical mucous would have more difficulty getting pregnant and would be more likely to have a male baby if they do get pregnant.
The study may also explain why more boys than girls are born worldwide (105 boys to 100 girls in most countries), even though human semen contains equal amounts of X and Y bearing sperm, the researchers at Maastricht University said.
The American Pregnancy Association has more about pregnancy.