The long-standing higher male birth fraction is considered an indicator of reproduction stability and health. In contrast, a decrease in the male birth fraction has been reported after prenatal exposure to environmental chemical factors. There is generally higher vulnerability of boys to prenatal damage by environmental stress. We formulated a hypothesis that the Chernobyl disaster might also have had a greater negative impact on male than on female fetuses, leading to their selective loss and to a decrease in the male birth fraction. To test this hypothesis, we examined demographic data on monthly natality in the territory of the Czech Republic from 1950 to 1999. The male birth fraction was higher in the Czech Republic each month between 1950 and 1999 except November 1986, when it was significantly (P< 0.05) reduced. This finding suggests a selective negative effect of the Chernobyl accident on male fetuses during the 3rd month of prenatal development.

Department of Teratology Institute of Experimental Medicine Academy of Sciences CR Videnska 1083 142 20 Prague 4 Czech Republic

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