If you’re trying to get pregnant, relax and try to keep your stress down.

That sounds like good advice, which your doctor has probably given you, but there has been very little science to back it up – until now.

Researchers, publishing in the journal Human Reproduction, say they have put out the first prospective study showing an association between stress and infertility.

They measured stress using biomarkers in the saliva of women who wanted to conceive, and found a strong correlation with alpha-amylase.

“The women who had the highest levels of this salivary stress biomarker had a 29% decreased probability of pregnancy over time, and that actually translated into a more than two-fold risk of infertility for them by the end of the study,” said lead author Courtney Lynch, director of reproductive epidemiology at The Ohio State University Wexner Medical Center.

The National Institutes of Health already recommends reducing stress while trying to conceive, but this new research actually studied the relationship between stress and fertility, Lynch said.

Four years ago, Lynch and colleagues showed in a previous study that women with higher levels of a stress biomarker in their saliva had a 15% lower chance of getting pregnant in the first cycle.

Keep in mind that these studies show associations, not causes. Researchers did not investigate why stress might lead to infertility, and there could be some other factor that links the two. And there are many other reasons that a couple may be having trouble conceiving.