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A preliminary study from New Jersey has found that pregnant women exposed to certain air pollutants may be at higher risk for stillbirths.

When fetal death occurs after 20 weeks of pregnancy, it is called stillbirth. These tragic deaths occur in about 1 in 160 pregnancies (1). Most stillbirths occur before labor begins. The pregnant woman may suspect that something is wrong if the fetus suddenly stops moving around and kicking. A small number of stillbirths occur during labor and delivery.

The study was published online July 18 in the American Journal of Epidemiology.

Researchers used statewide birth data from 1998 through 2004. They compared live births with stillbirths, looking at mothers who lived within 10 kilometers (a little over 6 miles) of New Jersey's 25 pollutant-monitoring stations. The stations monitored nitrogen dioxide, sulfur dioxide carbon monoxide. The researchers then compared the pollutant concentrations during a woman's three trimesters of pregnancy, and took into account factors known to affect stillbirth risk, including the mother's age, smoking, ethnicity and prenatal care.

Based on the results, researchers say they found that different pollutants are harmful in different trimesters of pregnancy and exposure to high levels of nitrogen dioxide was harmful during the first trimester.

Researchers found that for every 10 parts per billion increase in nitrogen dioxide levels, risk of stillbirth increased 27 percent. For every increase of 10-ppb in the pollutant during the first trimester, the stillbirth risk increased by 16 percent. For sulfur dioxide, the stillbirth risk increased 13 percent with every 3-ppb increase in the first trimester, and 26 percent for every 3-ppb increase in the third trimester. Carbon monoxide was associated with a 14 percent increase in stillbirth risk in the second and third trimesters for every 400-ppb increase in concentration.

However, researchers said they have not figured out whether reducing pollution alone could prevent stillbirths or how many could be avoided.

The study was not as strong as it would have been had researchers been able to measure each woman's level of pollution exposure. But in following up on the few studies that have looked at air pollution and maternal health, it does contribute to a trend, experts said.