A baby's sex at birth may not be a 50-50 coin toss after all, according to a new study from the Harvard T.H. Chan School of Public Health.
The research, published July 18 in Science Advances, found that birth sex appears to be linked to maternal age, certain genes, and the sexes of older siblings -- challenging the long-held idea that sex determination is completely random.
"If you've had two girls or three girls and you're trying for a boy, you should know your odds are not 50-50," said Jorge Chavarro, professor of nutrition and epidemiology and senior author of the study, in an interview with The Washington Post. "You're more likely than not to have another girl."
Researchers analyzed more than 146,000 pregnancies from 58,000 U.S. nurses who participated in the NIH-funded Nurses' Health Study between 1956 and 2015. The findings showed that some families consistently have children of the same sex -- suggesting that biological and genetic factors may be influencing birth outcomes.
Women who had three or more children were more likely to have all boys or all girls than would be expected by chance, the study found.
Maternal age also appeared to play a role: Women who began having children after age 28 were slightly more likely to have offspring of a single sex.
The research team also identified two genes linked to sex patterns:
* NSUN6, associated with having only daughters
* TSHZ1, associated with having only sons
"We don't know why these genes would be associated with sex at birth, but they are, and that opens up new questions," Chavarro said.
The study concludes that biological factors, including maternal genetics and age, may influence whether a family's next child is a boy or a girl. Still, Chavarro emphasized that further research is needed to explore how lifestyle, nutrition, and environmental exposures may also play a role.
Other Harvard Chan co-authors include Siwen Wang, Bernard Rosner, Hongyan Huang, Janet Rich-Edwards, Francine Laden, Jaime Hart, and Kathryn Penney.
Click here to read the full study.
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