A new study indicates that when a pregnant woman suffers from stress, she's more likely to have a low-birth-weight baby than a non-stressed pregnant woman if both are exposed to the same toxic chemicals.

Published Wednesday in the journal PLOS ONE, the first study examining the combined impact of stress and environmental chemicals on fetal development was based on a systematic review of 17 human studies and 22 animal studies on the links among the three factors.

It also showed that the effects of air pollution on low birth weight were heightened when combined with stress. Exposure to ambient fine particulate matter, a type of air pollution, increased the risk of African American women having a low-birth-weight baby compared to those who were white.

Behind the study was a team of researchers from the University of California, Berkeley, and the University of California, San Francisco.

"It appears that stress may amplify the health effects of toxic chemical exposure, which means that for some people, toxic chemicals become more toxic," said Tracey Woodruff, a professor in the Department of Obstetrics, Gynecology and Reproductive Sciences at UCSF and the senior author of the study.

Taken individually, the adverse effects of stress or environmental chemical exposures on fetal growth are well known, yet their combined effect has not been clear. When combined, researchers found the strongest connection between smoking and low birth weight: highly stressed pregnant women who smoked were about twice as likely to have a low-birth-weight baby as less stressed smokers.