For women hoping to conceive boys, there's an upshot to having a hard time getting pregnant. A new study indicates that the longer it takes a woman to conceive, the higher her chances of having a boy. Researchers say the findings may have something to do with the fact that sperm that make boys swim faster than those that make girls.
If the battle of the sexes started at birth, guys would have a slight advantage: 105 boys are born for every 100 girls. One explanation has to do with the chromosomes in sperm. The Y chromosome, which makes boys, contains less DNA than the X chromosome for girls. That means sperm that bear a Y chromosome swim faster in viscous liquids. Such a liquid exists at the tip of a woman's cervix in the form of a thick mucous.
When cervical mucous is excessively thick, it might be hard for any sperm to reach the uterus. Under such conditions, Y-chromosome sperm might have an advantage. And if that's true, reasoned epidemiologist Luc Smits at Maastricht University in the Netherlands, then longer conceptions should lead to a higher percentage of male babies.
The team looked into the issue by collecting data from 5283 women around the age of 30. Of the 498 women who took longer than 12 months to become pregnant, 56% gave birth to baby boys. For the rest, only 51% produced boys. Using data only from those who conceived naturally, the authors calculated that each additional year of pregnancy attempts raised the chance of having a boy by 4%. The women who used medical help to get pregnant, such as in vitro fertilization or having semen injected directly into the uterus, bypassing the cervix, had an equal chance of having boys or girls, suggesting that the boys' advantage lay in the ability of Y-chromosome sperm to zip faster through the cervical goo.
The data are "really interesting," says anthropologist John Martin of Arizona State University in Tempe, and give some "insight into what determines the sex of the offspring." He wonders whether stress also plays a role in the equation, as it can contribute to thicker mucus around the cervix.
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