Pregnant women exposed to high levels of ozone and carbon monoxide are more likely to give birth to children with heart defects, according to a study published yesterday in the American Journal of Epidemiology. The study, which was conducted by the University of California at Los Angeles, looked at birth defect cases in Los Angeles, San Bernardino, Riverside, and Orange counties, and found as much as a threefold incidence in serious infant heart problems where mothers were exposed to air pollution. The researchers hope to continue the study to determine how pregnant women are exposed to smog and how it affects prenatal development. The results could help shape a pending evaluation of California’s carbon monoxide cleanup plan.