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Big Mamas Have Baby Boys - Fatter mothers more likely to give birth to boys when weight is gained between the first and the second pregnancy
By: Stefan Anitei, Science Editor



Naturally, about 103-106 boys are born, as compared to 100 girls (a birth sex ratio of 1.03-1.06). But as boys are weaker, at the adult age there will be a sex ratio of about 100:100 (or 1).

In the last decades, there has been registered a decrease in the number of baby boys born as compared to girls in industrialized countries, like Canada, Denmark, England, Germany, Japan and the United States, even if there are still more baby boys than girls.

The question is: is there any factor affecting the probability of delivering a baby boy or girl?

A new research made by a team from the Harvard School of Public Health (HSPH) and Karolinska Institutet in Stockholm discovered that mothers who gained weight from the beginning of the first pregnancy to the beginning of the second pregnancy are more prone to have a boy as a second child.

"The results are provocative because few biological factors are known in humans to influence the chances of either conceiving or carrying to term a baby boy or girl. Our study suggests that maternal nutritional factors might play a role," said lead author Eduardo Villamor, assistant professor of international nutrition at HSPH.

The team employed data from the Swedish Birth Registry, recording 220,889 women who had successive pregnancies between 1992 and 2004 (both live births and stillbirths being included). The scientists focused on subjects’ body mass index (BMI) between the first and second pregnancies.

The birth sex ratio of the second pregnancy rose linearly with the gained weight from the first to second pregnancy, from 1.024 in those who lost over 1 unit BMI (this means 102 boys to 100 girls) to 1.080 in subjects who gained at least 3 units (108 boys to 100 girls).

The tendency was not linked to other factors like obstetric issues, mother's smoking, parents' age, length of the interpregnancy period and the sex or survival of the first-born. But as obesity is rampant in many industrialized countries, the lower number baby boys means there are factors still unknown affecting the sex of the newborn.

But the researchers warn that putting on weight in an attempt to influence the sex of the baby is not a wise measure. "Weight gain before pregnancy carries significant risks to the mother and the baby, and should not be practiced to influence the odds of having a boy. Other factors of which weight gain is only an indicator could be at play here." said Villamor. Recent researches showed that higher weight in mothers during pregnancy exposes the child to a higher risk of obesity during the adult life.

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