Many people might think that the likelihood of women having sons versus daughters is split evenly, akin to a coin toss. However, it appears that some women tend to have children of only one gender.

“Based on my personal observations, I have friends who only have girls or boys,” states Xiwen Wang from Harvard University. “This prompts the question of whether it’s all mere chance or if there are underlying biological factors at play.”

At a broader population level, the ratio of boys to girls at birth is close to 50:50. This is largely due to the fact that male sperm have an approximately equal probability of carrying either X or Y chromosomes that ultimately determine the child’s sex.

Nonetheless, Wang and her colleagues were curious about whether mothers might also have an influence. To investigate, they reviewed data from over 58,000 mothers in the U.S. who were part of two distinct studies focusing on contraception and maternal health. Among these mothers, 61% had two children, 30% had three, 8% had four, and the rest had more than five children.

The researchers examined the gender of female offspring in relation to eight maternal traits: height, weight, race, hair color, blood type, chronotype (time of day they feel most alert), age of first menstruation, and the age at which they had their first child between the ages of 13 and 48.

The findings revealed that women who were over 28 at the time of their first birth had a 43% greater likelihood of having only children of the same sex compared to a 34% chance observed in mothers under 23.

“Our research indicates that the age at which a woman has her first child is correlated with [an increased chance of] her having only sons or daughters,” Wang explains. No other characteristics were found to be associated with the sex of the children.

The reason behind this age-related effect remains uncertain; however, it may point to different biological changes that occur in women’s bodies over time, Wang suggests. Prior research indicates that the initial phase of the menstrual cycle tends to diminish with age. This could correlate with a higher likelihood of birthing boys, while vaginal pH decreases with age, which might benefit the survival of X chromosome-carrying sperm, according to Wang.

However, these findings might also stem from behavioral elements. Joshua Wild from Oxford University suggests that older women may be more inclined to plan their family size. As a result, even if they desire children of both sexes, they may choose to stop after having two boys or two girls, whereas younger women might be less concerned with family size and therefore more likely to have children of different sexes.

Further investigations are necessary to explore how behavioral, environmental, and biological factors influence the sex of newborns, Wild notes.