Environmental factors may be the cause of declining proportion of males born in Japan and the United States, a U.S. statistical study suggests.

The report published Monday in the online edition of Environmental Health Perspectives also says males account for a higher proportion of the fetuses that die.

The researchers from the universities of Pittsburgh and Texas concluded that "sex ratio declines are equivalent to a shift from male to female births of 135,000 white males in the U.S. and 127,000 males in Japan" over 30 years.

The ratio of African-American male babies is increasing, but is still below whites in America, they said.

The trend is consistent with reports from other industrial countries, but remains unexplained.

The report said known and suspected risk factors include avoidable environmental contaminants — such as widespread exposure to materials that can disrupt prenatal development — changes in the parents' ages, obesity, assisted reproduction and nutrition.

However, prenatal exposure to chemicals such as mercury merits further study, the authors said.

They also said that fathers' exposures to some chemicals before conception may be a factor. “We know that men who work with some solvents, metals and pesticides father fewer baby boys," Devra Lee Davis, director of the University of Pittsburgh Cancer Institute’s centre for environmental oncology, said in a release. 

Fetal deaths

Davis and Lovell Jones, director of the centre for research on minority health at the University of Texas M.D. Anderson Cancer Center, reported that worldwide, males account for 51.5 out of every 100 births. 

In the U.S., the proportion fell to 51.17 males in 2002 from 51.34 in 1970. In Japan, the proportion fell to 51.35 males in 1999 from 51.72 in 1970.

Fetal deaths are much higher in Japan than in the U.S., and are much higher for African-Americans than whites. Nevertheless, the numbers of fetal deaths have been falling in both countries since 1970.

But within those changes, male fetal deaths in Japan rose to nearly two-thirds of the total by 1999 from just over half in 1970. In the U.S., African-Americans reported the biggest increase, while the trend for whites was only "slightly upward."

The authors noted that a recent study of the Aamjiwnaang First Nation, a native band in Ontario, reported that sex ratios have dropped from an expected 54 males per 100 births to 45 in the late 1990s and 35 for the 1999-2003 period.

"To our knowledge this is a more significantly reduced sex ratio and greater rate of change than has been reported previously anywhere, and strongly suggests that the sex ratio may prove to be an environmentally sensitive indicator," the authors said.

The Aamjiwnaang residents, who live near a cluster of chemical plants in Sarnia, Ont., are worried that the plants contributed to the drop in the number of male children.