Long-term stress may reduce a woman’s ability to conceive by as much as 29 percent, a new study from the Ohio State University Wexner Medical Center finds.

From 2005 to 2009, researchers followed 401 reproductively healthy couples who were trying to get pregnant.  Over a period of 12 months, researchers recorded each couple’s time-to-pregnancy and observed that by five to six months into the study, the couples who had yet to conceive were those with the highest levels of chronic stress.

“When we get to couples starting to have problems [conceiving], stress levels really seem to differ from those who get pregnant [quickly],” lead study author Courtney Lynch, an assistant professor of epidemiology at the Ohio State University Wexner Medical Center told FoxNews.com

To study stress levels, Lynch and her team collected saliva samples from the women at the beginning of the study and after the women’s first menstrual cycles. For each woman, they recorded levels of the biomarker salivary alpha-amylase, which measures the body’s sympathetic adrenomedullary pathway, more commonly known as the “fight or flight” system. Unlike cortisol, a biomarker that measures acute stress, salivary alpha-amylase levels indicate chronic stress levels.

“[People] think of ‘fight or flight’ as an acute response, but as time has gone on, folks have realized if you’re experiencing chronic stress, lots and lots of acute stress, that ‘fight or flight’ [response] never turns off,” Lynch said.

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Researchers found that women with the highest levels of salivary alpha-amylase had a 29 percent reduction in their ability to conceive and more than two-fold greater risk of infertility, compared to women with the lowest levels.

The study, published in the journal Human Reproduction, is the first to observe an association with infertility and salivary stress biomarkers. The U.S. Centers for Disease Control and Prevention (CDC) reports that 6.7 million women have difficulty getting pregnant, representing 10.9 percent of women of reproductive age.

Their findings suggest that stress-reducing methods may be beneficial for some couples trying to start a family. Typically, a couple trying to conceive won’t meet with an infertility specialist until after 12 months of trying, at which point the specialist will start by recommending lifestyle interventions such as quitting smoking, cutting back on caffeine intake and starting stress-reduction techniques. Lynch believes focusing on stress earlier may be beneficial.

“Stress is not an issue until you’ve tried to get pregnant for a while and not succeeded…if you’ve tried for six months the key thing to recommend would be to take a look at your stress level and think about taking up some sort of stress-reduction activity such as yoga, exercising more, mindfulness or meditation,” Lynch said.

While it’s unknown which stress-reducing modality is best for improving fertility, Lynch said they’re all helpful in improving overall health.

As for how exactly stress affects fertility, the study negated two commonly believed mechanisms—that couples who were more stressed had less intercourse and that stress may delay ovulation. Among participants, women with high levels of salivary alpha-amylase were having as much intercourse as those with lower levels. Researchers used fertility monitors to track ovulation and found no delayed ovulation among women with high levels of salivary alpha-amylase.

“We don’t have an answer at this point,” Lynch said.

While stress alone is not the only contributing factor to infertility, Lynch emphasized that it is a factor that couples can control, in order to improve the chances of getting pregnant.

“Stress is not helping you. Whatever you can do to lower that…not only can potentially help you get pregnant faster, but if you start your pregnancy healthier, you’ll be a healthier pregnant woman, a healthier post-partum mother,” Lynch said. “[Lowering stress] ensures people have as healthy and as productive a life as they’re able to have.”