Health Science Reviews

By Sasekani Ntshanga

The sex of a baby has long been thought to be determined by chance, like tossing a coin. However, you might have observed that some families or couples tend to have children of only one sex.

Researchers at Harvard University in Boston, Massachusetts, challenged the idea that for each pregnancy there is a 50:50 chance of having a boy or a girl. Findings from this study showed that the families with multiple children of the same sex have higher chances of having another child of the same sex than having a child of opposite sex.

The researchers analysed birth records of 58 007 United States female nurses (95% were white) with at least two single babies born in 1956 – 2015 to determine the factors that might help understand why some had only boys and some had only girls. The results showed that baby sex does not follow a perfectly random pattern. This pattern was even clearer when researchers excluded each woman’s lastborn child, to avoid the effect of parents who stop having kids once they get one of each sex.

They found that the odds of having only female or male children were higher in women who had their first child when they were 29 years of age or older. As women age during their reproductive years, they undergo physiological changes such as a shorter follicular phase and lower vaginal pH which determine whether the X- versus Y-chromosome sperm succeeds in fertilising the egg.

The study showed that maternal genetics may also affect the sex of the baby. The GWAS identified the SNPs NSUN6 and TSHZI to be associated with a higher probability of having only female children or only male children respectively.
In summary, this research study showed that while the chance of having either a boy or girl child is close to 50:50, some families are more likely to have children of the same sex, and this is partly due to maternal age and genetics. Although it is a strong study, more research in different groups and including fathers would help fill in the rest of the picture.

REFERENCE:
Wang, F.et al.2025. Is sex at birth biological coin toss? Insights from a longitudinal and GWAS analysis. ScienceAdvances.10(10):1-10