Stressed Pregnant Women Less Likely to Have Boys: 'The Womb Is an Influential First Home'

Stressed pregnant women are less likely to give birth to boys, according to researchers who investigated how a mother's health can affect their child.

The study, published in the journal PNAS, involved 187 pregnant women aged between 18 and 45, who filled out questionnaires and had physical assessments.

By also studying their medical documents, the researchers looked at how 27 indicators of stress affected their labor and offspring. These included potential stressors, like their job, their physical health, and their lifestyle—such as diet, their social support network, and how much their exercised.

The majority of the women—66.8 percent—were deemed healthy when it came to their stress levels. But 17.1 percent were classed as psychologically stressed, reporting they felt under pressure, depressed or anxious. A further 16 percent were physically stressed, showing signs like higher blood pressure or consuming more calories.

Mirroring the findings of previous studies, stress levels appeared to affect the sex of the babies, the reserachers found. The authors explained that in the general population, there are 105 males born for every 100 females. But in the women who were stressed, the ratios shifted to 4:9 for physically stressed mothers, and 2:3 for those under psychological stress.

The more social support the women had the higher their chance of having male baby.

Physically stressed mothers were also 22 percent more likely to give birth 1.5 weeks earlier than expected, compared with 5 percent of those in the healthy group. That is below the U.S. average of 8 to 10 percent. These children were also at a higher risk of having reduced heart rate-movement coupling—a condition that has been linked to slower development of the central nervous system.

Participants who felt under emotional pressure were more likely to experience birth complications, the study showed.

The authors say making sure women have social support could be an approach to improving the health of babies.

"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.

"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."

Speaking to Newsweek, Monk said the findings add to the body of research showing the importance of supporting women and their partners during pregnancy—through public policy and health care systems.

"This is a two-generation and life course perspective on health: How women are doing during pregnancy affects them and the next generation," she said. "Moreover, as we found that both stress groups had higher rates of childhood trauma, we can see that what happens even before pregnancy is carried forward to affect one's future children."

She advised those hoping to become pregnant to "check in" with their physical and emotional health status. Lifestyle habits, such as exercising, as well as seeking mental health care when needed are excellent steps to take for managing stress, she said.

Alison Gemmill, an assistant professor in the Department of Population, Family and Reproductive Health at Johns Hopkins Bloomberg School of Public Health who was not involved in the study, told Newsweek: "Although the findings are not surprising, the novelty of the study appears to be the development of typologies of pregnant women, which is based on 27 variables.

"Whereas previous studies might have focused on only one or two of these variables, this paper considers them all simultaneously. Thus, one takeaway might be that it's perhaps not just one stressor that matters but a combination of stressors.

"This makes intuitive sense—the study is likely identifying the most vulnerable women who are exposed to a host of negative exposures," she said.

Gemmill argued the study was partly limited by a small sample size that is not representative of the general population, as two-thirds of participants identified as Latina, and half were on Medicaid. In addition, those with the highest levels of stress were also those who were the most disadvantaged, she said.

"It appears here that much of the 'stress' the authors capture is likely manifested from broader social determinants of health like economic stability, food security, and quality housing," said Gemmill.

Last month, the authors of a separate study looking at the how the behaviour of parents could affect their children advised men and women to stop drinking alcohol at least six months before they hope to conceive. This could lower their child's risk of being born with heart problems, according to the authors of the paper published published in the European Journal of Preventive Cardiology.

This article has been updated with comment from Alison Gemmill.

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A stock image shows a pregnant woman holding her belly at her desk. Researchers have studied how stress can affect a fetus. Getty