A new study from the University of Pittsburgh has found that during the past thirty years, the number of male births has steadily decreased in the U.S. and Japan. Perhaps more worryingly, the study, published in Environmental Health Perspectives, also found that an increasing proportion of fetuses that die are male. In Japan, among the fetuses that die, two-thirds are male, up from just over half in 1970.The study 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. Lead investigator Devra Lee Davis said the figures translated to 135,000 fewer males in the U.S. and 127,000 fewer males in Japan.
Davis suggests that environmental factors may be one explanation for the shrinking male birth rate. "The pattern of decline in the ratio of male to female births remains largely unexplained," she explained. "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."
The study notes that 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," said 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."