A pregnant woman's exposure to ozone and carbon monoxide, air pollutants commonly found in urban areas, may increase the risk that her baby will develop certain cardiac birth defects, according to a study released Saturday.

In the study, due to be published in the January 1 issue of the American Journal of Epidemiology, researchers found that pregnant women living in areas of Los Angeles with elevated levels of ozone and carbon monoxide were up to three times as likely to bear children with pulmonary or aortic artery/valve defects or other heart defects.

That risk increased in women exposed to high levels of the pollutants in their second month of pregnancy, when the heart and other organs start to develop.

"The greater a woman's exposure to one of these two pollutants in the critical second month of pregnancy, the greater the chance that her child would have one of these serious cardiac birth defects," head researcher Beate Ritz, an epidemiologist at the University of California -- Los Angeles, said in a statement.

She said the results "present the first compelling evidence that air pollution may play a role in causing some birth defects."

After analyzing information gathered by the California Birth Defects Monitoring Program on more than 9,000 babies born in the Los Angeles area between 1987 and 1993, researchers compared the air quality around the homes of infants born with birth defects with the air quality near healthy children's homes.

For women in areas with the highest amounts of ozone and carbon monoxide, the risk of birth defects tripled, while in areas with "moderately higher" levels of the pollutants, it doubled.

Although the study examined Los Angeles and the surrounding area, Ritz said the findings have implications for most urban areas in the United States -- especially in high-traffic areas where vehicles create much of the air pollution.

But she stressed that more research is needed, as researchers were unable to evaluate other potential risks such as maternal smoking, diet and obesity, or determine whether other chemicals in the air were to blame.

"There may be some other chemical culprit in tailpipe emissions, which we can't identify at this time, that is causing the problem," Ritz noted. "Since carbon monoxide is released in motor vehicle exhaust along with these other pollutants that we don't measure, these other pollutants also may be important."

Gary Shaw of the California Birth Defects Monitoring Program, one of the co-authors of the study, agreed: "The dose-response aspect of this study certainly strengthens the findings and underscores the need for additional research," he said.

"Unlike other health factors like diet or lifestyle, a pregnant woman has almost no control over the quality of air she breathes. We need answers."

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