Researchers at Columbia University Vagelos College of Physicians and Surgeons and NewYork-Presbyterian took part in the study.
They tracked 187 pregnant women, ages 18 to 45, to determine how maternal prenatal stress influenced offspring neurodevelopment and birth outcomes.
Researchers say on average, around 105 males are born for every 100 female births. But in this study, the sex ratio in the physically and psychologically stressed groups favored girls.
"The womb is an influential first home, as important as the one a child is raised in, if not more so," study leader Catherine Monk, from the Columbia University Vagelos College of Physicians and Surgeons, said in a statement.
"Other researchers have seen this pattern after social upheavals, such as the 9/11 terrorist attacks in New York City, after which the relative number of male births decreased," says Monk. "This stress in women is likely of long-standing nature; studies have shown that males are more vulnerable to adverse prenatal environments, suggesting that highly stressed women may be less likely to give birth to a male due to the loss of prior male pregnancies, often without even knowing they were pregnant."
The more social support a mother received, the greater the chance she had of having a male baby, researchers said.
The study is published in the journal PNAS, the Proceedings of the National Academy of Sciences.
A new study by @Columbia's @Monk_Lab has identified markers of maternal stress—both physical and psychological—that may influence a baby’s sex and the likelihood of preterm birth. https://t.co/FcfVHh8iTX #WomensHealth pic.twitter.com/jRGruzR4N9— Columbia Med School (@ColumbiaPS) October 15, 2019