There may be genetic explanations for why some women consistently give birth to boys or girls. This is the reasoning behind it.

According to a new study, women who have multiple children of the same sex are more likely to have another child of the same sex.

Factors such as maternal age and genetics can create bias in the “coin toss” of gender determination, rather than following a pure 50/50 chance for each child.

The study also indicated that older mothers tend to have children of the same sex; we identified two genes that can increase the probability of having all daughters or all sons.

The gender of a child at conception hinges on the sperm’s X or Y chromosomes, suggesting that the metaphorical coin flip for determining a child’s gender isn’t entirely random.

However, doctoral researcher Xiwen Wang from Harvard pointed out that this might not be the complete picture.

“The project started from a casual chat among co-authors and friends about a family with only boys or only girls,” Wang noted in an interview with BBC Science Focus.

“It happened so frequently that we began to wonder: Is that really a possibility?

Wang and her team analyzed data from over 58,007 women who had more than two children.

They found that if a couple has three boys, they have a 61% chance of having another boy. Likewise, after three girls, there’s a 58% chance of having another girl.

This research identified various factors that might tip the scales towards families of exclusively girls or boys.

“Women who have their first child after the age of 28 are about 10% more likely to have only boys or only girls than those who start before 23,” Wang explained. “While this isn’t a huge difference, it is statistically significant.”

Though the study did not delve into the reasons for this association, Wang offered some theories.

“As women age, they go through physiological changes like a shorter follicular phase and lower vaginal pH,” she elaborated.

The follicular phase is the initial stage of the menstrual cycle, which generally favors Y chromosome sperm, while a lower vaginal pH is conducive to X chromosome sperm.

“These effects can vary among individuals; thus, depending on their specific biology, aging might shift the balance toward one gender,” Wang said.

Wang also proposed another potential connection.

“The age of pregnant mothers often correlates with that of older fathers. Unfortunately, we lacked father data for this study,” Wang mentioned.

Researchers also explored genetic markers for the 7,530 women involved in the study, discovering two SNPs: NSUN6 associated with all-female offspring and TSHZ1 correlated with all-male offspring.

The study examined whether behavioral factors, such as couples continuing to have children until a daughter is born following a series of sons, could account for patterns of same-sex offspring.

“We conducted an analysis that excluded the last child for each family, which is likely influenced by parents stopping once they have both genders. Even after this adjustment, we still observed a strong clustering of same-sex siblings,” Wang stated.

About our experts

Siwen Wang is a doctoral student specializing in nutritional epidemiology at Harvard University School of Public Health. Her research investigates how nutrition, lifestyle, and psychosocial elements affect the health of mothers and children.

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