A new study by researchers at Harvard T.H. Chan School of Public Health suggests that the sex of a baby may not be purely random. The study, published in the journal Science Advances, analyzed data from 58,007 women and their 146,064 children born between 1956 and 2015. The researchers found that women who had their first child at age 29 or older were more likely to have subsequent children of the same sex.
They also discovered that women who had three children of the same sex were more likely to have a fourth child of the same sex, regardless of their age. Siwen Wang, a PhD student in Nutritional Epidemiology and one of the study’s authors, was inspired by the uneven gender distribution in her own family. “I was wondering whether it’s just pure chance or if there was some special biology underlying this phenomenon,” Wang said.
Dr. Bernard Rosner, a co-author of the study, emphasized that the findings do not allow for definitive predictions of a child’s sex.
Maternal age and child sex patterns
“It’s like moving the needle from 50 to 60 percent,” he said. “I don’t think you could use any of this information to definitively predict whether a specific person will have a male or female offspring, but … it’s not necessarily random probability.”
The study focused solely on the age of the mother and did not consider other variables, such as the father’s influence on the child’s sex.
The dataset was also limited in terms of racial diversity, with about 95 percent of the women studied being white. To ensure the robustness of their findings, the researchers conducted several analyses, including excluding women who experienced miscarriages, had a history of divorce, or aimed for gender-balanced families. The finding that mothers older than 28 were more likely to have children of the same sex remained consistent across these analyses.
While the study offers insights into the factors influencing a child’s sex, Wang cautioned against using the findings to predict or manipulate sex at birth. Instead, she believes the study opens the door for further research into maternal influences on offspring sex and the broader study of sex ratios.
