The number of male babies born in the U.S. is dropping. And environmental factors may have a role to play in the changing demographic pattern, says a new study.
Researchers have found that the male birth is on a steady decline for the last three decades. The decline is equivalent to 135,000 fewer white males in the last 30 years in the U.S. Interestingly the decline is confined to whites only.
In Japan too a similar decline has been reported.
They say the reason for the decrease is unclear, but environmental factors coupled with the rising age of parents giving birth may be playing a role.
"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," says Devra Lee Davis, director of the University of Pittsburgh Cancer Institute's Center for Environmental Oncology, in a news release.
"We suspect that some combination of these factors, along with older age of parents, may account for decreasing male births," the news release states.
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.
In the study, researchers analyzed birth statistics in the U.S. from 1970 to 2002 and in Japan from 1970 to 1999. The results appear in the online edition of Environmental Health Perspectives.
The results showed an overall decline of 17 males per 10,000 births in the U.S. and 37 males per 10,000 births in Japan during the study period.
On the other hand the number of black male births has increased slowly over the last three decades, though the ratio of male-to-female births among blacks remains lower than that of whites.
In addition, blacks have a higher fetal mortality rate, and male black babies are more likely to die than females.
"Given the higher mortality rates for African-American males in the United States, these results re-emphasize the need to determine all factors, including environmental contaminants, which are responsible for this continuing health disparity," notes researcher Lovell A. Jones, director of the Center for Research on Minority Health at the University of Texas M.D. Anderson Cancer Center.
"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."