PITTSBURGH, April 10 (UPI) -- During the past 30 years, the number of male births has decreased each year in the United States and Japan, but researchers are not exactly sure why.
A study in the online edition of Environmental Health Perspectives reports an overall decline of 17 males per 10,000 births in the United States and a decline of 37 males per 10,000 births in Japan since 1970.
University of Pittsburgh researchers said the pattern of decline in the ratio of male to female births is a mystery.
"We know men who work with some solvents, metals and pesticides father fewer baby boys," lead investigator Devra Lee Davis, director of the University of Pittsburgh Cancer Institute's Center for Environmental Oncology, said in a statement.
"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."
