Some mothers have many sons, while others may continuously have daughters. It has been traditionally understood that the gender of a fetus is determined by a 50-50 chance, similar to flipping a coin, but recent research has contradicted this notion. Some mothers are biologically predisposed to have a higher probability of having children of a specific gender. The age of the first pregnancy and the number of children may also influence this probability.
A research team at Harvard University’s T.H. Chan School of Public Health analyzed the birth records of 58,007 American women who had two or more children from 1956 to 2015 and published the findings in the journal Science Advances on the 18th.
The research showed that in households with two children, it was more common to have one boy and one girl than to have two boys or two girls. Meanwhile, in households with three or more children, it was more typical to have only children of the same gender rather than a mix of genders.
At conception, the gender of a fetus is generally determined by the sex chromosomes (X or Y) carried by the sperm. This is an independent event each time, leading to a probability similar to that of flipping a coin. However, the study results suggest that for certain mothers, the gender of their children may not be determined by a 50-50 chance.
The research team confirmed specific statistical probabilities through their analysis. For instance, a mother with three sons had a 61% probability that the fourth child would also be a son, while a family with three daughters had a 58% probability that the fourth would also be a daughter.
Childbirth is influenced not only by biological factors but also by cultural factors. In some families, they may continue to have children until they have a child of the desired gender, and when they have one son and one daughter, they may stop attempting to have more children. To account for these cultural factors, the research team excluded the 'last child born' from their analysis. Nonetheless, a clear gender imbalance was evident.
Additionally, women who had their first child after age 28 were found to be 13% more likely to have only children of one gender compared to women who gave birth before age 23. The research team analyzed the genomes of over 7,000 of these women and identified gene variations on specific chromosomes.
For example, women who only had daughters often had specific variations in the 'NSUN6' gene located on chromosome 10, while women who only had sons frequently had variations in the 'TSHZ1' gene on chromosome 18. These genes are not known to be directly related to reproduction.
Earlier, in 2024, a study found that specific gene variations on chromosome 10 could influence the fertilization process of eggs, increasing the probability of having daughters by 10%P. Professor Zhangzhan of the University of Michigan, who conducted the research at that time, noted, 'As more biobanks are established, additional gene variations affecting the sex ratio at birth could be discovered.'
References
Science Advances (2025), DOI: https://doi.org/10.1126/science.zw2hhn1
Proceedings of the Royal Society B: Biological Sciences (2024), DOI: https://doi.org/10.1098/rspb.2024.1876