A study published in this week's online edition of Environmental Health Perspectives reports that during the past thirty years, the number of male births has decreased each year in the Unites States and Japan.
In a review of all births in both countries, the University of Pittsburgh-led study found significantly fewer boys being born relative to girls in the U.S. and Japan, and that an increasing proportion of fetuses that die are male.
In the study, researchers reported an overall decline of 17 males per 10,000 births in the U.S. and a decline of 37 males per 10,000 births in Japan since 1970. They also found that while fetal death rates have generally decreased, the proportion of male fetuses that die has continued to increase. In Japan, among the fetuses that die, two-thirds are male, up from just over half in 1970.
They note that the decline in births is equivalent to 135,000 fewer white males in the U.S. and 127,000 fewer males in Japan over the past three decades and suggest that environmental factors are one explanation for these trends.
"The pattern of decline in the ratio of male to female births remains largely unexplained," said Devra Lee Davis, lead investigator of the study. "We know that men who work with some solvents, metals and pesticides father fewer baby boys. We also know that nutritional factors, physical health and chemical exposures of pregnant women affect their ability to have children and the health of their offspring. We suspect that some combination of these factors, along with older age of parents, may account for decreasing male births."
Dr. Davis explained that environmental factors, such as prenatal exposure to endocrine-disrupting environmental pollutants may impact the SRY gene -- a gene on the Y chromosome that determines the sex of a fertilized egg. Other environmental factors that also may affect the viability of a male fetus include the parents' weight, nutrition and the use of alcohol and drugs.
"Given the importance of reproduction for the health of any species, the trends we observed in the U.S. and Japan merit concern," added Dr. Davis. "In light of our findings, more detailed studies should be carried out that examine sex ratio in smaller groups with defined exposures as a potential indicator of environmental contamination."