Environmental Risk and Sex Ratio in Newborns

Abstract

Detecting intrauterine exposure to environmental pollutants, based on an increased number of malformations, has only been successful with a few teratogens. Nor does the number of spontaneous abortions represent a more reliable indicator, since a precise record of early abortions is not available. A greater vulnerability to prenatal damage leading to abortion is evident in male embryos/fetuses than in female. The newborn sex ratio (birth rate of boys/girls) is a very stable parameter in healthy populations. Its decrease has been reported after exposure to some harmful environmental factors. We document a decrease in the male birth fraction in the Czech Republic after the Chernobyl disaster in 1986. The absolute numbers of male and female births were determined in each of 600 consecutive months from 1950 to 1999. There were always more newborn boys than girls, except in November 1986, when the number of male births significantly decreased. This deficit in male births might have resulted from the spontaneous abortion of male embryos/fetuses during weeks 8-12 of pregnancy, as a consequence of their increased exposure to radiation, in particular to the radionuclide iodine131. We propose using the newborn sex ratio as a further tool for the standard evaluation of reproductive quality. Combined analyses of the incidence of newborn malformations, spontaneous abortions and stillbirths, intrauterine growth retardation and the newborn sex ratio will help to compensate for the imperfections associated with each of these parameters individually and will provide a more complete understanding of the extent of prenatal risk induced by environmental factors.