Women's Health May Affect Sex Of Offspring; Other Factors Still At Play

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The sex of a child may be impacted by the health of the mother. Kitt Walker, CC BY-ND 2.0.

When it comes to whether a baby will be a girl or a boy, most believe the determination is left up to chance. For the most part, this is true, but science has also shown the parent whose genes make that decision is the father; having both an X and a Y chromosome, the father can give one of either to the future child, ultimately determining whether it is a boy or a girl. But now, researchers are wondering if the mother’s health may contribute to gender formation. Looking at examples from two species of animals, the researchers from the Swiss National Science Foundation believe this may be so.

For the study, the researchers are looking into the validity of a previous work which found women who have more stressful jobs tend to give birth to more girls than boys. Stressful circumstances possibly affecting the sex of a child is not something new to scientists. Many cases in the animal kingdom also seem to suggest the condition of the mother could contribute to a child’s sex. So in order to find the potential for this in humans, researchers looked to animal examples.

A theory exists, known as the Trivers-Willard hypothesis, which says a mother’s body can adjust the sex of her offspring in concordance with her own health. For example, a mother who is in better health is more likely to give birth to a boy because men have a better chance of being able to produce more offspring in their lives than female children. In doing this, the mother ensures that her own genetic code, which is healthy and strong, gets passed on through more generations. However, when a mother is in poorer condition, she is more likely to give birth to daughters because the mother is not strong enough to produce an alpha male.

But, “it’s not quite as simple as that,” said biologist Peter Neuhaus of the University of Calgary in Canada. To test the theory, Neuhaus and his team of researchers looked at Columbian ground squirrels and Canadian bighorn sheep. They have published their findings in the journal Nature.

Out of both animals, the most telling example, the team found, was with the bighorn sheep. They observed that bighorn ewes only gave birth to one lamb each year, mating specifically with the dominant ram while leaving other, weaker specimen without offspring. Though they found that female ewes in better physical health were more likely to make “supermales,” as Neuhaus points out, healthy females were not found to make any more males than female offspring. The team thus concluded that other things needed to be taken into consideration when evaluating reproductive values. For instance, they found that many males within the herd die before reaching sexual maturity, which ultimately changes reproductive potential.

The researchers are not ruling out that stress in human women may shift a child’s sex, but their main conclusion still finds evolution to be more complex than the Trivers-Willard theory. Considering all factors that contribute to reproductive potential may clue us in more into whether or not a child will be male or female. But other than that, it’s very difficult to tell.

Source: Schindler S, Neuhaus P, Coulson T, et al. Sex‐specific demography and generalization of the Trivers–Willard theory. Nature. 2015. 

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