THURSDAY, Aug. 12 (HealthDay News) -- Women may have a more difficult time becoming pregnant when they're feeling stressed, according to a new study that found women were less likely to conceive when they showed elevated levels of a stress-related substance called alpha-amylase.

Alpha-amylase is secreted into saliva in order to digest starch. But researchers have begun using the substance as an indicator of the body's response to physical or psychological stress because it's also released when the nervous system produces catecholamines, compounds that initiate a type of stress response.

For this study, researchers from the U.S. National Institutes of Health and the University of Oxford charted the ovulation cycles of 274 English women, ages 18 to 40, who were trying to conceive.

The results showed that women with high alpha-amylase levels were less likely to conceive than those with low levels.

"Overall, the 25 percent of the women in the study who had the highest alpha-amylase levels had roughly an estimated 12 percent reduction in getting pregnant each cycle in comparison to women with the lowest concentrations," study first author Germaine Buck Louis, director of the division of epidemiology, statistics, and prevention research at the U.S. National Institute of Child Health and Human Development, said in a National Institutes of Health news release.

The study, published online in the journal Fertility and Sterility, is the first to show an association between a biomarker of stress and a reduced chance of becoming pregnant.

"It has been suggested that stress may increase with the disappointment of several failed attempts at getting pregnant, setting off a cycle in which pregnancy becomes even more difficult to achieve," Buck Louis said.

The findings suggest that doctors need to identify appropriate ways to help women reduce stress when they are trying to get pregnant.

"The question is, 'What do you do to help women to relax?'" the study author said. "People often turn to alcohol or tobacco to relieve stress, but these substances also reduce the likelihood of pregnancy." Techniques worthy of investigation, she said, include meditation, biofeedback, yoga, and boosting social support.

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