A baby’s sex at birth is generally considered a coin toss, roughly a 50-50 chance of having a boy or a girl- until now. A new study from Harvard T.H. Chan School of Public Health suggests that biology may have more of a say than mere luck.

“If you’ve had two girls or three girls and you’re trying for a boy, you should know your odds are not 50-50,” said Dr. Jorge Chavarro, senior author of the study and professor of nutrition and epidemiology at Harvard Chan School. “You’re more likely than not to have another girl.”

Published on July 18 in Science Advances, the study analyzed more than 146,000 pregnancies reported by 58,000 U.S. nurses over nearly six decades as part of the NIH-funded Nurses’ Health Study. It’s one of the most comprehensive looks to date at what really influences the biological sex of a child at birth.

And what the researchers found turns conventional wisdom on its head.

The study found that some families seem to have a natural tendency toward having children of the same sex. 

So what’s at play here?

The study points to three main factors: maternal age, genetics, and the sex of older siblings.

One striking pattern was linked to the age of the mother. Women who started having children after the age of 28 were slightly more likely to give birth to babies of the same sex.

Perhaps even more intriguing was the discovery of two genes, one found in both men and women, and the other only in women, that were associated with the likelihood of having children of a single sex.

“We don’t know why these genes would be associated with sex at birth,” Chavarro admitted, “But they are, and that opens up new questions.”

The researchers say this study should encourage scientists to dig deeper into other possible influences—lifestyle, nutrition, and environmental exposures, for example.

While the findings won’t help expectant parents “choose” the sex of their next child, they do offer important insights into reproductive biology, and challenge long-held assumptions.

The study was co-authored by Harvard Chan scientists Siwen Wang, Bernard Rosner, Hongyan Huang, Janet Rich-Edwards, Francine Laden, Jaime Hart, and Kathryn Penney.

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