Last updated: Sep 15, 2016

Women were 37 percent less likely to get pregnant when they felt stressed during ovulation in a recent study, compared to their chances during less stressful months. That number jumped to 46 percent among women who generally reported the highest stress levels. The findings confirm what scientists (and hopeful parents-to-be) have long assumed: Stress does seem to lower chances of conception.

Fertility specialists routinely advise women to reduce their stress levels while trying to get pregnant. They may point to anecdotes about couples trying for years and finally conceiving naturally after adopting a child, or to an increased probability of pregnancy among IVF patients practicing stress-reduction techniques.

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But to date, the authors of the new study wrote in the Annals of Epidemiology, there has been very little research on how this connection plays out in real life—and none on the effects of stress across different windows of a woman’s menstrual cycle. Without this research, the authors point out, there’s no way to know if stress is more likely to affect ovulation and fertilization (during the ovulatory phase) or implantation (during the luteal phase, a few days later).

To investigate, University of Louisville researchers asked 400 sexually active women to record their daily stress levels on a scale of 1 to 4, and then followed them for about eight months or until they became pregnant. The women, who were all 40 and younger, also recorded information about their lifestyle habits, as well as when they had their periods, had sex, and used contraception.

After they adjusted for factors like age, body mass index, alcohol and cigarette use, and frequency of intercourse, the researchers found that when women reported more stress during their estimated ovulatory period, they were less likely to get pregnant.

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“This effect is large enough to be considered clinically significant, as it translates to more than a 3-month delay in conception,” the authors wrote. “In addition, the effect size is comparable to that of smoking, a widely recognized risk factor for subfertility.” (The American Society of Reproductive Medicine advises women over age 35 years to see a fertility specialist after trying for six months.)

Interestingly, higher stress at the end of a month was associated with increased rates of conception—but the authors hypothesize that this was due to natural hormonal fluctuations in the early stages of pregnancy, or in response to women taking at-home pregnancy tests and learning of their positive results.

The study couldn't prove a cause-and-effect relationship, and more studies are needed to determine exactly how stress hormones may affect ovulation. But until then, says epidemiologist Kira Taylor, PhD, prospective parents can still be informed about this potential consequence of stress—and, hopefully, do something about it.

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“The results imply that women who wish to conceive may increase their chances by taking active steps towards stress reduction such as exercising, enrolling in a stress management program or talking to a health professional,” she said in a press release.

Taylor’s findings also bring legitimacy to a theory that some people have remained skeptical of. “I hope the results of this study serve as a wake-up call for both physicians and the general public that psychological health and well-being is just as important as other more commonly accepted risk factors such as smoking, drinking alcohol, or obesity when trying to conceive,” she says.

This article originally appeared on RealSimple.com.