For decades, it has been widely believed that the sex of a baby is determined purely by chance, with every pregnancy carrying an equal 50% chance of resulting in a boy or a girl. However, a new study published in Science Advances suggests the reality may be more complex.
Researchers have found that some families appear to have a naturally higher likelihood of having children of the same sex, with factors such as maternal age and genetics potentially influencing the outcome. The findings provide fresh insight into why some parents have all sons or all daughters, rather than an equal mix.
Study Suggests Baby’s Sex May Not Always Be a 50-50 Chance
Scientists from the Harvard T.H. Chan School of Public Health analyzed data from more than 146,000 pregnancies involving over 58,000 women in the United States between 1956 and 2015.
The researchers discovered that families with several children were more likely than expected to have children of the same sex, indicating that birth outcomes may not always follow the traditional 50-50 probability.
For example:
- Families with three daughters had a 58% chance of having another girl.
- Families with three sons had a 61% chance of welcoming another boy.
These findings suggest that the probability of having a child of a particular sex may vary slightly from one family to another.
Previous Births May Influence Future Odds
According to Dr. Jorge Chavarro, one of the study’s authors, parents who already have multiple children of the same sex should not automatically assume their next pregnancy has equal odds of producing a child of the opposite sex.
For instance, couples with two or three daughters may actually be slightly more likely to have another daughter than a son. Likewise, families with several boys may have a higher chance of welcoming another boy.
Although the difference isn’t dramatic, the research indicates that biological factors may subtly influence the likelihood of having a boy or a girl.
Maternal Age Could Be an Important Factor
The study also found a possible connection between a mother’s age and the sex of her children.
Researchers observed that women who had their first child after the age of 28 were more likely to have children of only one sex.
Dr. Chavarro suggested that age-related biological changes could contribute to this pattern. One possible explanation is that changes in the reproductive environment, such as increased vaginal acidity with age, may affect how well Y chromosome-carrying sperm survive.
However, researchers stress that this remains a scientific hypothesis and requires further investigation before any firm conclusions can be made.
Researchers Identified Possible Genetic Links
Another key finding was the identification of two genes that appeared to be associated with families having only boys or only girls.
Scientists do not yet understand how these genes might influence a baby’s sex, but their discovery raises the possibility that genetics could play a role in determining birth patterns within families.
Despite these findings, experts urge caution.
Geneticist Dr. Iain Mathieson from the University of Pennsylvania noted that the genetic portion of the study involved a relatively small sample size, meaning additional research is needed before confirming any hereditary connection.
Family Planning Choices May Also Affect the Pattern
The researchers found that biology isn’t the only reason some families appear to have both boys and girls.
Many parents choose to stop having children once they have one son and one daughter, which naturally influences overall family patterns.
To account for this, researchers excluded the last child from family records and repeated their analysis. Even after making this adjustment, they found the same trend—families still showed a higher-than-expected likelihood of having children of the same sex.
This suggests the observed pattern cannot be explained solely by family planning decisions.
Factors That Showed No Link to Baby’s Sex
The study also examined several other characteristics but found no meaningful association between the sex of children and factors such as:
- Race
- Hair colour
- Body Mass Index (BMI)
- Blood group
However, the researchers acknowledged an important limitation. Since approximately 95% of the participants were White women and all were nurses, the findings may not fully apply to more diverse populations around the world.
What Does This Study Mean for Parents?
The research does not mean parents can predict or influence whether they will have a boy or a girl.
Instead, it suggests that while chance remains the biggest factor, genetics, maternal age, and possibly other biological influences may slightly shift the odds in certain families.
Researchers also believe future studies should examine additional factors such as nutrition, lifestyle habits, environmental exposures, and paternal influences to better understand how a baby’s sex is determined.
As evolutionary biologist David Haig described it, every family may be “flipping a different coin,” with each coin carrying its own subtle bias.
Final Thoughts
The new study challenges the long-standing belief that every pregnancy has identical odds of resulting in a boy or a girl. While the differences are relatively small, the findings suggest that family genetics and maternal characteristics may influence birth patterns more than previously thought.
Although more research is needed before these findings can be applied broadly, the study offers a fascinating scientific explanation for why some families seem to have all sons or all daughters over multiple generations.
Disclaimer: This article is based on findings published in Science Advances and is intended for informational purposes only. The research identifies associations rather than proving cause and effect. Individual pregnancy outcomes remain unpredictable, and additional studies are needed to confirm these findings across different populations.