Not 50/50: Science Helps Calculate the Probability of Having a Boy or a Girl

Expecting couples often ask themselves: “Boy or girl?” Well, the probability of having a son or a daughter is not 50/50, scientists say.

From flipping a coin to guessing the answer to a yes or no question, there are plenty of things that have a 50/50 chance of happening, writes the Daily Mail. For years, it was thought that the chance of having a son or daughter also fell into this category. But a shocking new study has found that this is not actually the case.

Scientists from Harvard University have found that some women are much more likely to have children of only one gender. In their study, the team analyzed data from more than 58,000 mothers who had given birth to at least one child. Their results showed that the mother's age plays a key role in determining the sex of the baby.

"Human sex ratios have long been of interest to biologists, statisticians, demographers, sociologists and economists," the researchers explained. "Here we showed that within each family size, sex at birth did not follow a simple binomial distribution and found significant intrauterine correlations in offspring sex."

The sex of the unborn child is determined by the combination of sex chromosomes inherited from the sperm and egg. While the egg always contains an X chromosome, the sperm can contain either an X or a Y chromosome, writes the Daily Mail. If a sperm carrying an X chromosome fertilizes the egg, the embryo will be female, while an embryo with a Y chromosome will be male.

It has long been believed that this ratio is 50/50. However, in their new study, published in the journal Science Advances, scientists have questioned whether this is actually the case.

"Several co-authors have observed cases where friends, colleagues, first-degree relatives, or themselves have produced offspring of only one sex, raising questions about randomness," the team explained.

To figure this out, the researchers analyzed data from 58,007 American women who had at least two children. They looked at the effects of eight maternal traits — height, body mass index, race, hair color, blood type, chronotype (whether you’re a morning person or a night owl), age at first menstruation, and age at first birth. Seven of these traits had no effect on the sex of the child. However, the analysis did find a key link between the age at which a mother had her first child and whether they had boys or girls.

Women who were over 28 when they had their first baby had a 43 percent chance of having babies of only one sex, the Daily Mail reports. But women who were under 23 when they had their first baby had only a 34 percent chance of having babies of only one sex.

"Older maternal age may be associated with higher odds of having same-sex offspring, but other hereditary, demographic and/or reproductive factors are not associated with offspring sex," the researchers explained.

The reason for this link remains unclear, the Daily Mail reports. However, researchers say it may be linked to physiological changes that occur as women age, including a shorter follicular phase and a decrease in vaginal pH.

"A shorter follicular phase tends to favor the survival of the Y chromosome, while a more acidic vaginal environment favors the survival of the X chromosome," the scientists explained. "With age, each woman may have a different predisposition to each of these factors, which may lead to an increased likelihood of having offspring of the same sex. However, these mechanisms remain hypothetical, and more detailed data are needed to confirm these hypotheses."

Overall, the findings suggest that the odds of having a boy or a girl are not 50/50, the Daily Mail concludes.