'Gender-bending' chemicals could be to blame for a worrying drop in the proportion of boys born in the U.S. over the past 30 years, scientists have claimed.

A massive study found that, the number of boys born for every 100 girls has dropped steadily from 105.5 to 104.5 over the three decades in America.

The decline, while apparently small, shows no sign of levelling out and could have major demographic and sociological implications for the future.

Could chemicals be the reason for the worrying drop in baby boys?

It means that for every 205 births there is now one boy fewer than 30 years ago. So for roughly each million births in the country, 5,000 fewer boys are born.

Figures for Britain show the male-female ratio here has remained fairly constant in the past 20 years, at about 104.9 boys for every 100 girls.

It is considered normal in a large population for the number of baby boys to outnumber girls slightly, by a proportion of about 105 males to 100 females.

This tends to balance out because of the higher mortality rate among baby boys than girls, a phenomenon which has yet to be fully explained.

The study - published in the journal Environmental Health Perspectives - also found the proportion of deaths among male foetuses in the U.S. after the 20th week of pregnancy had steadily risen.

The researchers from the University of Pittsburgh said similar trends existed in other industrialised countries, such as Japan, where chemicals and other pollutants were in widespread use.

Dr Devra Lee Davis, lead scientist on the study, said that while there was no conclusive evidence as yet, it was feasible that the decline was the result of genderbending pollutants in the atmosphere, particularly certain plastics and metals which have been shown to harm male-producing sperm.

The study pointed to the example of a small Indian reservation in Canada surrounded by petrochemical plants, where many of the tribe work. The malefemale births ratio there is falling rapidly, heading to 100 female births for every 50 boys.

"To our knowledge, this is a more significantly reduced sex ratio and greater rate of change than has been reported previously anywhere," said the researchers.