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Exposure in the first two months of pregnancy to air pollution from traffic sharply increases the risk for birth defects, a new study has found.

Researchers used data from two large studies carried out in eight counties in the San Joaquin Valley in California. One has tracked birth defects since 1997, and the other has recorded concentrations of nitrogen dioxide, nitrogen oxide, carbon monoxide and particulate matter at 20 locations in the valley since the 1970s. The results are posted online in The American Journal of Epidemiology.

Setting aside defects attributable to other known causes, there were 849 cases of birth defects. The researchers adjusted for smoking, maternal age and other variables, and compared these cases with 853 healthy control subjects.

They found that a mother living in areas with the highest levels of carbon monoxide or nitrogen oxide concentrations (the top 25 percent) was almost twice as likely to give birth to a child with neural tube defects — severe and often fatal defects of the brain and spinal cord— as one living in areas with the lowest concentrations.

The lead author, Amy M. Padula, a postdoctoral researcher at Stanford, acknowledged that in looking at many variables, some associations were likely to turn up by chance.

“These results need to be taken in context,” she said. “More studies are needed to affirm these associations before we make any clinical recommendations to pregnant women.”