This is especially true for women who breathe polluted air during the first 3 months of pregnancy or during the last months and weeks before delivery.

Importantly, researchers report in the American Journal of Epidemiology, the association between air pollution and increased risk of preterm birth persists after accounting for other factors that might influence preterm birth risk such as smoking, exposure to second-hand smoke and alcohol use.

"Air pollution in Los Angeles County remains a major public health problem affecting everybody, particularly pregnant women," Dr. Beate Ritz from the School of Public Health at the University of California, Los Angeles noted in comments to Reuters Health.

Ritz and colleagues collected detailed information for more than 2,500 women who gave birth in Los Angeles County in 2003. By conducting one-on-one interviews with the women, the researchers were able to separate the air pollution risk from other preterm birth risk factors.

"Our research group had previously reported on the effect of carbon monoxide and fine particles, but because we relied on birth certificates, we did not have detailed information about other risk factors that some people suspected might bias our research findings," Beate explained.

This new study, she said, "helps confirm the results we reported previously - that air pollution mainly caused by vehicle traffic increases the risk of preterm birth even when we take other risk factors into account."

Research that identifies the harmful effects of pollution, Beate added, can help policymakers "in weighing the costs and benefits of reducing air pollution, both in terms of dollars and human health."