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updated 6/7/2011 8:19:00 AM ET 2011-06-07T12:19:00

After exposure to nuclear radiation events, women may be more likely to give birth to boys than to girls, suggests a new study.

The study documented a localized spike in the ratio of boy-to-girl births after the Chernobyl disaster and similar trends from the delayed fallout of atomic bomb tests in the 1960s and 1970s. The researchers also found a smaller but still disproportionate number of boy births close to nuclear facilities in Germany and Switzerland.

It's not yet clear whether radiation interferes with the father's sperm, the mother's body before she gets pregnant, the development of the embryo or fetus, or something else. But because skewed sex ratios can be a sign of other underlying health problems, the study offers a new category of concern about events like the recent nuclear disaster in Japan.

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"The dogma was that this effect was not possible or that there was no effect of this kind," said Hagen Scherb, a biostatistician at the Helmholtz Zentrum München in Munich, Germany. "Now, we can most clearly demonstrate that there is an effect. And this changes our thinking about radiation risks."

Some of the first evidence linking radiation with altered sex ratios among babies emerged after atomic bombs hit Hiroshima and Nagasaki in 1945, though the data was not strong enough to be convincing.

Subsequent bomb tests and animal studies connected radiation exposure and a rise in stillbirths and birth defects. For years, though, detailed records of birth data have been sitting there, just waiting to be analyzed.


Using official and publicly available records, Scherb and colleague Kristina Voigt found that the ratio of male-to-female births in eastern and central Europe had been declining slightly for decades leading up to the Chernobyl disaster in 1986.

But in 1987, they reported in Environmental Science and Pollution Research, there was a sudden spike in male births compared to female births in those countries.

The trend continued upward for years, before leveling off around 2000. In the United States and elsewhere, sex ratios have continued downward for a variety of reasons, with no evidence of a spike in the mid-1980s.

Overall, the shift was tiny -- on the order of less than half a percent -- with somewhere between 1.045 and 1.06 boys born for every girl, depending on when and where you look.

Spread out over the entire population, though, that adds up to an estimated 440,000 missing girls as a result of the Chernobyl explosion, Scherb said. The researchers suspect that the trends points to a disproportionate loss of female pregnancies as a result of radiation exposure.

In both Europe and the United States, the study also found rises and falls in sex ratios that mirrored the frequency of atomic bomb testing. Those echoes showed a several-year lag that, the researchers proposed, corresponded with the delay in radioactive fallout from the atmosphere.

As a final piece of evidence, the researchers documented an uptick in the relative numbers of male births near nuclear facilities in Germany and Switzerland during periods when those facilities were operating.

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The study offers "proof that the low-dose radiation that no one wants to have an effect has an effect," Scherb said. "And this effect is rather large in absolute numbers."

Ionizing radiation -- the kind that comes out of bombs and nuclear power plants -- has been shown to alter sex ratios in fruit flies and mice, said Karl Sperling, of the Institute for Human Genetics at the University of Berlin. He suspects that radiation exposures might affect DNA in the first few cell divisions around the time of conception, which is known to be a highly vulnerable period.

If radiation can influence sex ratios in this way, he added, there might be reason to be concerned about other environmental exposures, too.

"There is an urgent need for analytical epidemiological studies with respect to other environmental hazards," he said. "The international guidelines for radiological safety should be revised accordingly."

© 2012 Discovery Channel

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