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 fetus.

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Editor:Yang Jie