NEW YORK, July 22: For generations, it has been widely assumed that the chance of having a baby boy or girl is a simple 50/50 proposition — a biological coin flip. However, a groundbreaking new study by researchers at Harvard University has revealed that this belief may not hold true.

Published in the journal Science Advances, the study challenges conventional wisdom by demonstrating that some women are significantly more likely to give birth to children of a single sex. The researchers found a notable link between maternal age and the probability of producing offspring of only one gender.

The study analyzed data from 58,007 American women who had given birth to at least two children. The researchers explored whether certain maternal traits — including height, BMI, race, hair color, blood type, sleep patterns, and reproductive age—could influence the sex of their children.

Interestingly, only one factor stood out: the age at which a woman gave birth to her first child. Women who were over the age of 28 at the time of their first childbirth had a 43% likelihood of having children of only one sex. In contrast, women who became mothers before age 23 had a significantly lower probability—just 34%.

Traditionally, the sex of a baby is believed to be determined at conception. While all eggs carry an X chromosome, sperm may carry either an X or a Y. An X-bearing sperm results in a girl; a Y-bearing sperm leads to a boy.

Although this process appears to be a 50/50 genetic split, the researchers note that anecdotal evidence—families having multiple children of the same sex—prompted a closer look.

“Several coauthors observed friends, colleagues, or even themselves producing only sons or only daughters, which raised questions about whether chance alone was responsible,” the team explained.

While the exact reason for the age-related pattern remains unknown, the researchers offered several possible biological explanations. As women age, they undergo hormonal and physiological changes, such as a shorter follicular phase and lower vaginal pH—both of which could subtly favor the survival of either X or Y chromosome-bearing sperm.

“A shorter follicular phase may favor Y chromosome survival, while a more acidic vaginal environment could benefit X chromosomes,” the study noted. These changes could influence whether a woman tends to conceive boys or girls consistently.

However, the researchers cautioned that these mechanisms remain theoretical and further studies are required to confirm any causal relationships.

The findings offer fresh insight into family planning, particularly for parents hoping for a child of a different sex after having two or more children of the same gender.

“Families desiring offspring of more than one sex who have already had two or three children of the same sex should be aware that when trying for their next one, they are probably doing a coin toss with a two-headed coin,” the researchers concluded.

The study has drawn interest across multiple disciplines, including biology, sociology, and public health, offering a new perspective on one of the oldest mysteries in human reproduction.