In a study published July 18 in the journal Science Advances, Wang and her co-authors came up with a partial answer. They analyzed a dataset of 58,007 women and their children and concluded that, in some cases, a child’s sex correlates with the mother’s age. Women who had their first child at age 29 or older were more likely to have subsequent children of the same sex. The study also found that, irrespective of age, women who had three children of the same sex were more likely to have a fourth child of the same sex. Two-sibling families, they found, were more likely to be gender balanced than larger families, which were more likely to be single-sex.
“It’s like moving the needle from 50 to 60 percent,” said Dr. Bernard Rosner, a co-author of the study. “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.”
People have many theories about what determines a baby’s sex, but most have not been confirmed in large-scale studies. Among the theories Wang and Rosner cite in their paper: good-looking parents are more likely to have daughters; parents who are big and tall are more likely to have sons. Others have suggested that factors related to the mother’s biology, like vaginal pH, body temperature, or the menstrual cycle phase during conception, may affect the viability of sperm with X or Y chromosomes.
For their study, Wang and her colleagues looked at heritable traits like race, natural hair color, blood type, height, and body mass index at age 18, but only the age of the mother was significant when it came to increased probability of having children of the same sex.
They also found that for women of any age who already had three children of the same sex, the probability of having a fourth boy was 61 percent and the probability of having a fourth girl was 58 percent, on average. It’s possible that these women have a trait or lifestyle feature in common, but it was not explored in this study.
The dataset the researchers looked at was drawn from two long-term studies of 58,007 nurses from across the United States and their 146,064 children born between 1956 and 2015. Around 95 percent of the women studied were white. The nurses reported demographic information about themselves, including their blood type, diet, and sleep habits. Rosner said that the nurses studied, while not racially diverse, are representative of the general female population when it comes to prevalence of different conditions and that the study benefited from their conscientiousness in accurately reporting details about their health.
The study only looked at women, so researchers do not have information on how the fathers may have impacted their children’s gender.
The researchers conducted several analyses to make sure the findings were robust. In one of them, they excluded women who had miscarriages so that there were no cases where information about the birth order was incomplete.
They also excluded women with a history of divorce to make it more likely that all the children had the same biological father.
In a separate analysis, researchers also left out obvious “coupon collectors,” women who stopped having children once they had children of each sex, so that the findings would not be confounded by parents trying to have a gender-balanced family.
Wang said the finding that mothers older than 28 were more likely to have children of the same sex remained stable, whether these other variables were included or excluded.
“The findings are very comparable,” Wang said, “That gives us reassurance that [what] we’re seeing is not very sensitive to the interference of these factors.”
Emily Barrett, a professor at the Rutgers School of Public Health who was not involved in the study, said that she was impressed by the analyses that the researchers performed to ensure the findings were rigorous. Barrett, who studies how environmental factors and hormone levels in mothers affect fetuses, was interested in understanding why mothers older than 28 are more likely to have children of the same sex.
Wang said she and her co-authors did not conduct additional studies to confirm why age was significant, but pointed to previous research suggesting that vaginal pH levels and hormone changes could play a role.
Barrett hypothesized that age could be a proxy for hormone levels or older mothers having more economic resources, which could influence the fetus.
“Anecdotally, there are lots of moms who delay reproduction to work on establishing their careers,” Barrett said. “So older age at first birth might be a proxy for moms who are … high achieving in the workforce.”
Andrey Rzhetsky, a professor of medicine and genetics at the University of Chicago, who was not involved in the study, was also interested in understanding the factors underpinning the study’s findings. Rzhetsky co-authored a 2021 study that looked at how sex ratios on a national level are impacted by environmental factors like chemical pollution.
“The real effect is interactions between genetics and environment,” he said. “But I think their results are very useful.”
Wang said that though the study doesn’t include data on fathers, older maternal age is probably highly correlated with older paternal age.
Wang said that she hopes that people do not mistake the study as a tool to predict or manipulate sex at birth, since the observations are group-level trends.
“Instead, we hope that it opens the door for investigation into maternal influences on offspring sex or the study of sex ratio in general,” Wang said.
Angela Mathew can be reached at angela.mathew@globe.com.
