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The sex of a child does not depend only on chance: scientists have identified the influence of the mother's age and genetics
Last reviewed: 03.08.2025
Families with multiple children of the same sex are more likely to have a subsequent child of the same sex than of the opposite sex, according to a major study¹ of maternal and genetic factors that influence the sex of offspring.
The results, published in Nature, show that in families with three boys, the chance of having a fourth boy is 61%. In families with three girls, the chance of having a subsequent girl is 58%.
The findings challenge the widely held belief that every pregnancy has a 50-50 chance of having a boy or a girl, says Alex Polyakov, an obstetrician and researcher at the University of Melbourne in Australia. “Based on these findings, couples should be told that their chance of having a baby of a different sex than their previous children is actually lower than 50-50,” he says.
Age affects the sex of the child
Researchers from Harvard University (Boston, Massachusetts) analyzed the sex of children born to 58,007 nurses in the United States from 1956 to 2015, as well as factors that might explain why some families have only boys and others only girls.
They found that families with two children were more likely to have "boy and girl" pairs than "two boys" or "two girls." But families with three or more children were more likely to have children of the same sex than of different sexes.
In the analysis, the scientists excluded data on the last child in the family to minimize the influence of conscious parental choices (for example, some couples stop having children after they have already had both a boy and a girl).
The team also found that women who had their first child at age 29 or older were 13% more likely to have children of only one sex than women who had their first child before age 23.
The authors note that changes in vaginal pH as a woman ages may explain this phenomenon. For example, changes in the environment may influence which sperm (carrying an X or Y chromosome) are more likely to fertilize an egg, Polyakov says.
Genetic influence
The genomic analysis also showed that some of the women had two common genetic variants associated with having children of a certain sex. A change on chromosome 10 in the NSUN6 gene was associated with a higher chance of having only girls, while a single nucleotide change on chromosome 18, near the TSHZ1 gene, was associated with a higher chance of having only boys.
The study did not include male influence, but Polyakov notes that it would be difficult to conduct a similar study involving fathers today, as the number of children in families is declining in most countries. “There simply won’t be enough subjects for this kind of study,” he explains.
Siwen Wang, a Harvard graduate student and co-author of the study, notes that more research is needed to explain how maternal factors, such as age at first pregnancy, influence the sex of the baby. It’s possible that hormonal changes with age play a role, or that maternal age is a proxy for paternal age, which the study didn’t measure, she adds.
Overall, the results are interesting because there was no population-wide bias toward one gender or the other, Polyakov says.
Wang cautions that parents cannot use these results to accurately predict the sex of their unborn child, as they only reflect trends at the level of large groups, but do not explain why a particular woman gave birth to only boys or only girls.