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The sex of a baby has traditionally been seen as a matter of pure chance, like flipping a coin. But a new study published in Science Advances challenges that idea. It suggests that some families may have a biological or genetic tendency to have more boys or more girls. Factors like maternal age, certain genes, and even family decisions appear to influence the odds. This could explain why some families seem to have only boys or only girls. The study, which analyzed decades of pregnancy data, points to a more complex story behind baby gender predictions.As evolutionary biologist David Haig puts it, “Different families are flipping different coins with different biases.” The sex of a baby isn’t always a simple 50-50 toss. It may depend on a unique combination of biology, genes, and parental choices—making every family’s story one of science, mystery, and personal meaning.
Harvard researchers finds surprising odds of having all boys or all girls in large families
Researchers from Harvard T.H. Chan School of Public Health studied over 146,000 pregnancies among 58,000 US nurses between 1956 and 2015. They discovered that families with more children had a higher-than-expected chance of having all boys or all girls. For example:
- Families with three girls had a 58% chance of having a fourth girl.
- Families with three boys had a 61% chance of another boy.
According to study author Jorge Chavarro, “If you’ve had two or more girls and want a boy, your chances may not be 50-50. You’re more likely to have another girl.”
Maternal age linked to baby’s gender pattern, says study
The study also found a link between maternal age and birth sex patterns. Women who started having children after age 28 were more likely to have babies of the same sex. One theory is that changes in the female reproductive system, like increased vaginal acidity, might reduce the survival of Y chromosome sperm—which typically results in boys.While paternal factors might also matter, the study didn’t include data on fathers, which researchers admit is a limitation.
Genes may influence baby’s gender, but experts urge caution
Scientists identified two genes that were more common in families with only boys or only girls. Although the exact role of these genes is still unknown, their discovery suggests there could be a hereditary link to birth sex. However, experts like geneticist Iain Mathieson from the University of Pennsylvania urge caution. He noted that the genetic sample size was small, and more research is needed before drawing firm conclusions.Beyond biology, family choices may play a role. The study found that many parents stopped having children after they had one boy and one girl, which could affect broader statistics. To remove this effect, researchers excluded the final child in families and still found that the birth sex pattern remained uneven. This means the bias isn’t only due to family planning—there are likely biological factors involved.
Why some families have all boys or all girls; science says it’s not just luck
These findings may help explain families in fiction and real life—like those in Pride and Prejudice or Malcolm in the Middle—that have children of all one sex. While this may seem rare, the study shows it’s not just random luck. Still, scientists stress that more research is needed to explore other influences like diet, lifestyle, and environmental factors. Variables such as race, hair color, BMI, and blood type showed no connection with birth sex in this study.Because the data was from mostly White, female nurses, the results may not apply to everyone, making further diverse studies essential.Also Read | NASA alert! 95-foot asteroid 2025 ME92 to fly past Earth on July 31 at 11,000 mph; should we be worried
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