Stressed Mothers Have More Girls — a Lot More Girls

New paper supports empiric discovery that after major quakes and terror attacks, more girls are born than boys

The stronger sex.
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Here’s some advice for men who want a son: Be nice to his future mother. Yet more research has now demonstrated that women harrowed by physical and psychological stress during pregnancy are less likely to have a boy.

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How less likely, exactly? In general, human procreation is biased toward males: Globally, 105 boys are born for every 100 girls. This makes evolutionary sense if you figure that young boys are more likely to die than young girls — whether because they’re more daring, more annoying, or more stupid (to sum up: more likely to play chicken). Our long-term evolution did not factor in polygamy.

But physically stressed women produce nine girls to every five boys; that’s almost twice as much. Psychologically stressed women produced three girls for every two boys, report researchers at Columbia University Vagelos College of Physicians and Surgeons and NewYork-Presbyterian.

The statistical study was published this week in the Proceedings of the National Academy of Sciences.

That is quite the difference. It bears adding that the study was a very small one: The team examined 187 healthy women, that’s all. Of these, 32 (17 percent) were found to be psychologically stressed with clinically meaningful levels of depression, anxiety and perceived stress. Thirty (16 percent) were physically stressed, with high blood pressure and overeating; and the majority, 125 (nearly 67 percent), were not stressed.

The stress levels were based on 27 indicators of psychosocial, physical and lifestyle stress based on questionnaires, diaries and daily physical assessments of the 187 women, who were aged 18 to 45.

In other words, this was a really small study — but it adds to other evidence that stressful conditions lead to an increase in girl births.

“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.

There were more than 700,000 births in New York City between January 1996 and June 2002. After the terror attacks on the Twin Towers on September 11, 2001, the birth sex ratio for the city dropped below one.

This latest paper supports the theory touted at the time that the mothers’ stress resulted in a disproportionate loss of male fetuses — especially in the 20th to 24th weeks of pregnancy.

Monk points out that various studies have demonstrated that males are more vulnerable to adverse prenatal environments.

A separate study, published in the Journal of the Turkish-German Gynecological Association, notes the correlation between major quakes not only in Turkey but in Japan, Chile, Greece and Iran, and an increase in the proportion of girl births. “The first sign of deterioration in public health at birth after natural disasters over the years may be a change in sex ratios,” note Emek Doğer and colleagues, which isn’t an insult to girls. Quite the contrary. 

But why might this be? Like so many areas in biology, the truth may be out there (so to speak), but we don’t know what it is. In any case, it seems that if there’s one species that’s a delicate little flower, it’s the male — at least in utero.

In other words, unborn males suffer more. “Converging evidence confirms that infant and early childhood developmental outcomes of male fetuses exposed to prenatal and perinatal adversities are more highly impaired than those of female fetuses,” wrote a separate team in Neuroscience. The bottom line is that when the mother faces adversity, a girl in her womb is more likely to make it than a boy.

Monk’s study also found that the more social support a mother received (which can translate into less stress), the more improved the likelihood of having a boy. “When social support was statistically equalized across the groups, the stress effects on preterm birth disappeared.”

To be clear, it isn’t that the human fetus changes sex in the womb because the mother is stressed. That does not happen in our species. Our gender is determined by our sex chromosomes, xx being female and xy being male, followed by further differentiation in the womb.

Let us not digress on the spectrum of sexuality. There are reptiles and fish where gender at the fetal (egg) stage depends on things like temperature stress, yet again the environmentally stressed offspring tend to be female.

None of this applies to us. Doing the dishes or just not being a dick won’t change a female fetus into a male once pregnancy has ensued. It won’t even assure you of one offspring or the other. All you’ll get is possibly clean tableware and the same kid you were going to get anyway — but it’s more likely to be alive.