Stressed out women have more difficulty getting pregnant than women with less stress, according to a new study this week in the journal Human Reproduction.

Although the relationship between stress and trouble getting pregnant has been hinted at before, it had never been scientifically proven before now. This new research marks the first time that scientists have found a direct link between stress and infertility.

"Women should not look at these findings and feel guilty," said Courtney Denning-Johnson Lynch, director of reproductive epidemiology at the Ohio State University Wexner Medical Center, and the lead author of the paper. "They should look at it as an opportunity. Working on stress is something they can try to do on their own to improve their chances of getting pregnant."

She added that stress by no means is the most important factor in whether a woman gets pregnant. "Blocked tubes, or ovulation problems, or the quality of the sperm on the male side has more of an effect than stress," she said.

But the link is there. In a study that followed more than 400 women just as they were starting to try to get pregnant, the researchers found that women with the highest levels of the stress indicator alpha-amylase in their saliva were 29 percent less likely to get pregnant than women with the lowest levels.

They also found that women with the highest levels of alpha-amylase were more than twice as likely to meet the clinical definition of infertility — meaning they did not get pregnant even after a full year of trying.

Alpha-amylase is associated with our fight or flight response, but it can still be chronic, said Lynch.

"When you think ‘fight or flight’ you think acute response, but what most people don’t know is that if you are chronically stressed, your body will learn to keep that system hyperactive," she said. "It has learned you are under attack constantly."

The researchers still do not understand exactly why stress affects a woman’s ability to become pregnant, but this study did rule out some possibilities. For example, they found women with high levels of alpha-amylase had the same amount of sex as some of their less-stressed counterparts. "It’s not that stressed out women have less intercourse," she said.

They also found no correlation between high levels of alpha-amylase and ovulation problems.

One theory the researchers plan to explore in future studies is whether stress changes what Lynch called "the hormonal milieu" of the uterus in such a way that it becomes inhospitable to implantation.

"But that’s still a big question mark," she said.

Another big question mark is exactly what women can do to mitigate their stress while trying to get pregnant. Lynch hopes to study intervention techniques to help women handle stress soon, but in the meantime, she said taking yoga classes, doing mindfulness meditation or just trying to get 20 minutes of exercise a day can help.

"These have all been shown to improve people’s overall health status," she said. "And telling someone to ‘just relax,’ can actually create more stress."