Exposure to air pollution at an early stage of pregnancy can increase the risk of birth defects in newborns, according to a new study from Stanford University School of Medicine.

The study was based on data available from one of the smoggiest places in the U.S. - San Joaquin Valley in California. Researchers found that high levels of air pollution lead to high risk of neural tube defects that affect the brain and spine.

Previous studies have shown that air pollution can increase risk of preeclampsia - a condition where the mother develops high blood pressure - in pregnant women. Air pollution is also associated with increased risk of stillbirth.

The present study included 806 women who had a pregnancy with a birth defect between 1997 and 2006. Researchers examined two types of neural defects, spina bifida, a spinal-column malformation, and anencephaly, an underdeveloped or absent brain, according to a news release from the University. The study also included 849 women whose babies had no such complications.

Researchers then looked at data available for air pollutants around the participants' home during the study period. Specifically, they assessed the levels of pollutants like carbon monoxide, nitrogen oxide, nitrogen dioxide, particulate matter and ozone. They also examined traffic data in the area.

After controlling for other factors that may have resulted in the birth defects, researchers found that exposure to carbon monoxide led to a two times increased chance of the baby being born with spina bifida or anencephaly. An exposure to nitrogen oxide was linked with a three times higher risk of these birth defects.

"Birth defects affect one in every 33 babies, and about two-thirds of these defects are due to unknown causes. When these babies are born, they bring into a family's life an amazing number of questions, many of which we can't answer," said Gary Shaw, PhD, professor of neonatal and developmental medicine and lead author of the study.

The study is published in the American Journal of Epidemiology.