After the terrorist attacks of Sept. 11, 2001, there was a small but real rise in the number of miscarriages across the country -- especially for women who were pregnant with boys.
The finding adds to evidence that boys are more vulnerable to stress than girls while in the womb. The study also affirms that the tragic events of 9/11 deeply affected people far beyond the limits of New York City.
"The stress of a mother affects the fetus, and it's not just these individual stressors like whether you had a divorce or lost your job, but also these ambient stressors, like the economy and September 11," said Tim Bruckner, a population health researcher at the University of California, Irvine. "The effects resonated across the entire society. We were essentially bereaving what we saw on TV."
In the United States an average of 105 boys are born for every 100 girls. But in times of stress, like after natural disasters or economic collapse, studies have shown that the ratio drops, and fewer boys are born than expected. So far, it hasn't been clear why.
One theory is that stress harms sperm that bear Y-chromosomes. It could also be that people simply have less sex during stressful times, which might favor girls. Or perhaps something happens during pregnancy to affect the birth ratio.
Bruckner and colleagues collected data from around the country about miscarriages that happened after 20 weeks of pregnancy between 1996 and 2002. Most miscarriages happen before 20 weeks, but those aren't usually recorded, and the gender of the baby is rarely known.
The numbers showed a three percent increase in the number of male babies that miscarried after 9/11, the researchers reported in the journal BMC Public Health. Over the next few months, the ratio of boys born compared to girls dropped accordingly. Based on research in guinea pigs and other animals, scientists suspect that exposure to stress hormones kills more male fetuses than females.
"This is an unusually rigorous and careful demonstration of something that I think is intrinsically interesting," said Robert Trivers, an evolutionary biologist at Rutgers University in New Brunswick, New Jersey.
"It is not at all obvious that a traumatic, single-day event to which people are personally unconnected is something that would reverberate all across the United States inside mother's wombs, claiming an extra set of males," he said. "Yet their evidence is very compelling that exactly that happened."
The tendency of an extremely anxious woman to miscarry male babies might have developed in our ancestors as a way to maximize the number of grandchildren she would eventually have, Bruckner said.
When times were flush and food plentiful, the theory goes, women could pour more resources into their developing sons, boosting the chances that they would go on to become alpha males. In many mammals, including red deer, alpha males are more likely to find mates and have babies.
If conditions turned sour during a pregnancy, on the other hand, it might be best for a woman to miscarry a male baby and try again next year when life is less stressful.
The new study doesn't offer any obvious advice to pregnant women about how to avoid miscarriages, Bruckner said. Instead, it highlights one of the ways that collective stress can impact the health of our nation at its core.
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