LOS ANGELES (Xinhua):The normal growth of a developing foetus may be slowed if pregnant women are exposed to air pollution, a new study has found.

The risk of a small birth-weight baby increased significantly with each increase in particulate matter of 4 micrograms per cubic meter during the first and third trimesters of pregnancy, according to the study.

Each 10 parts per billion increase in nitrogen oxide exposure was also associated with a large increase in the risk of a small birth-weight baby, found the study, conducted by researchers at the School of Public Health, University of Medicine and Dentistry of New Jersey.

The study also found that exposure to particulate matter in later pregnancy was associated with a two-to-five fold increased risk of restricted foetal growth among mothers with separation of the placenta before birth and premature rupture of the membrane, compared with mothers who did not have these complications.

The study was published online in the April issue of the Journal of Epidemiology and Community Health.

In the study, researchers examined data on nearly 336,000 births in New Jersey between 1999 and 2003 and at daily air pollution readings from across the state.

Readings from monitoring sites within six miles (9.6 kilometers) of the mothers' homes were used to calculate average levels of air pollution during their pregnancies.

The findings suggest that traffic pollution or living close to a major road could be linked to restricted fetal growth, said David Rich and colleagues from the department of epidemiology at the school.

It's not clear exactly how air pollution affects fetal growth, the researchers said.

They noted that previous research found that air pollution might alter cell activity or reduce the amount of oxygen and nutrients received by a foetus.