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Pregnancy Timing May Affect Baby's Sex

The longer it takes for a woman to get pregnant, the more likely she’ll give birth to a boy, according to a new study.

Researchers say the findings may help explain why more boys than girls are born worldwide, despite the fact that human semen contains equal amounts of X (female) and Y (male) chromosome-bearing sperms.

In the study, published in the BMJ, researchers compared information on more than 5,000 Dutch women who gave birth between July 2001 and July 2003.

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The results showed that among the 498 women who took longer than one year to get pregnant, the percentage of male babies was nearly 58 percent, compared with 51 percent among the women who took less time to get pregnant.

Researchers estimate that for couples conceiving naturally, each additional year of trying to get pregnant is associated with a 4 percent increase in the likelihood of having a boy, even after adjusting for other factors, such as age, smoking status, alcohol use, and menstrual cycle variations.

But for couples using fertility treatments to help become pregnant, the study showed there was no link between time to pregnancy and the baby’s sex.

Researchers say the results support the idea that sperms bearing the Y (male) chromosome swim faster than those bearing the X (female) chromosome in thick and sticky fluids and are more likely to fertilize an egg when the women’s cervical mucus is difficult to penetrate.

Previous studies have shown that women with thick and sticky cervical mucus take longer to become pregnant and are more likely to give birth to boys.

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By Jennifer Warner, reviewed by Brunilda Nazario, MD

SOURCES: Smits, L. BMJ, Dec. 16, 2005; vol 331: pp 1437-1438. News release, BMJ.