"A body of evidence is emerging from several countries on the adverse consequences of ambient air pollution on fetal/birth outcomes, including pre-term birth and fetal growth restriction," said Prof David Rich, of the University of Medicine and Dentistry of New Jersey in the United States.

The findings, published in the Journal of Epidemiology and Community Health, is based on almost 336,000 births in New Jersey between 1999 and 2003 and regular monitoring of air pollution around the state from the US Environmental Protection Agency. It suggested that restricted fetal growth may be linked to traffic pollution.

"These findings suggest ambient air pollution, perhaps specifically traffic emissions during early and late pregnancy and/or factors associated with residence near a roadway during pregnancy, may affect fetal growth," said Prof Rich.

According to the study, reported in the Daily Telegraph, there was significant increase in the likelyhood of delivering a baby of small weight with each rise in pollution levels during the first three months and final three months of pregnancy.

Earlier research indicates that air pollution may alter cell activity - or cut the amount of oxygen and nutrients a baby receives while in the womb, the report said.