NEW YORK (Reuters Health) - High exposure to urban smog early in pregnancy may raise a woman's odds of having a baby with certain heart defects, California researchers report.

Their review of birth defects and air pollution data in four California counties revealed that the risk of these cardiac conditions climbed in tandem with maternal exposure to ozone and carbon monoxide pollution during the second month of pregnancy.

These heart problems--which include defects of the aortic artery, certain heart valves and the wall separating the heart's upper chambers--occur in fewer than 2 births per 1,000, with around 900 in California each year.

In this study, the infants of women living in areas with the highest carbon monoxide levels had a three times higher risk of ventricular septal defects than those of women from areas with the lowest levels. Women with moderately high exposure had lesser increases in this risk.

A similar pattern emerged when the researchers looked at maternal ozone exposure and birth defects of the aortic artery and valve, and anomalies called conotruncal defects. There was also a less-clear association between ozone levels and the risk of pulmonary artery and valve defects, the report indicates.

Beate Ritz and colleagues at the University of California Los Angeles (UCLA) report their findings in the January issue of the American Journal of Epidemiology.

Exactly why these components of air pollution were linked to the cardiac defects is unclear, but Ritz's team notes that the fetal heart is still developing in the second month of pregnancy--when pollution exposure was key in this study. Other birth defects such as cleft palate were not related to a mother's exposure to air pollution.

``More research needs to be done, but these results present the first compelling evidence that air pollution may play a role in causing some birth defects,'' Ritz said in a statement released by UCLA.

The researchers were unable to factor in other environmental players in birth defect risk, including whether mothers smoked, took vitamin supplements or had occupational exposures that could be damaging to the fetus.

The authors also speculate that carbon monoxide and ozone are not alone in this possible connection between air pollution and birth defects.

``There has been a big reduction in the levels of...air pollutants like ozone and carbon monoxide over the years,'' Ritz noted. ``There may be some other chemical culprit in tailpipe emissions, which we can't identify at this time, that is causing the problem.''

In cities, most carbon monoxide emissions come from motor vehicle exhaust. Ground-level ozone occurs when certain other pollutants react in the presence of sun and heat; this type of ozone is a key component of smog.

In the current study, Ritz's team looked at birth defects occurring in four southern California counties, including Los Angeles, between 1987 and 1993. They analyzed 11 types of birth defects, with around 100 to 600 children or stillborn fetuses in each group. These cases were compared with more than 9,000 infants and fetuses with no birth defects.

SOURCE: American Journal of Epidemiology 2002;155.

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