If you haven't been trying to get pregnant for very long, the results could be basically useless.

When a woman is having trouble getting pregnant, a fertility doctor may recommend a test to determine her “ovarian reserve”—a measure of the quality and quantity of eggs remaining in her ovaries. Doctors sometimes offer these tests to women who are wondering about their ability to conceive (whether they’ve been trying or not) because of their age or other factors.

Khloe Kardashian, 33, even had her ovarian reserve tested on a recent episode of Keeping Up With the Kardashians as she considered being a surrogate for her sister Kim.

But a surprising new study suggests that diminished ovarian reserve may actually not be a good indication of infertility. The authors say their research, published this week in JAMA, should discourage some women from relying too heavily on such tests when making decisions about their reproductive health.

An ovarian reserve test generally consists of a blood and/or urine analysis to determines levels of the hormones inhibin B, anti-mullerian hormone (AMH), and follicle-stimulating hormone (FSH). Those first two hormones usually decrease as a woman’s egg count and quality go down, while the third tends to increase.

Lead author Anne Z. Steiner, MD, professor of obstetrics and gynecology at the University of North Carolina, says some evidence shows that women with normal ovarian reserve are more likely to respond to fertility treatments, like in vitro fertilization, compared to those with diminished reserve. But there haven’t been any studies to determine whether the measure plays a role in a woman’s ability to get pregnant naturally.

So Dr. Steiner and her colleagues recruited 750 women, ages 30 to 44, who had been trying to get pregnant for three months or less. The women all provided blood and urine samples, and they were followed for the next six to 12 months.

During that time, 487 of the women conceived naturally. After adjusting for other factors that could have influenced the results—including age, body mass index, and recent contraception use—the researchers found that women’s levels of AMH, FSH, and inhibin B all had no significant effect on who got pregnant and who did not.

The researchers were surprised. “These tests have worked their way into generalist’s offices and more mainstream uses, and we were really hoping to find that they are a good test for women who want more information about their fertility,” says Dr. Steiner. “Ultimately, that’s not what we found.”

Dr. Steiner says the study results do not support the use of ovarian reserve tests for women in their 30s and 40s who have been trying for a baby for three months or less.

An accompanying editorial by Nanette Santoro, MD, professor of obstetrics and gynecology at the University of Colorado, notes that the study’s large size, its community-based sample, and its robust measurement methods “all support the credibility of the findings.”

But Santoro also points out that the study only included couples who have never had fertility problems, and that pregnancy outcomes beyond a positive pregnancy test weren’t available. It’s unknown, then, whether women with low ovarian reserve experienced more miscarriages or other complications.

Dr. Santoro writes that the study’s findings “challenge clinicians to think more carefully” about the true meaning of ovarian reserve. She concludes that women who’ve only been trying for a few months to conceive should not be evaluated the same way as those who’ve been struggling for longer.

“Doing so can not only provide potentially misleading and anxiety-producing results,” she writes, “but may also lead to costly fertility preservation treatments that have no value.”

Dr. Steiner agrees that women with no history of infertility should not put too much stock in these tests. “I would personally discourage it,” she says. Women whose tests reveal a diminished reserve may unnecessarily rush to freeze their eggs, she says.

“At the same time, a woman who’s 35 or 36 who’s thinking about freezing her eggs might be incorrectly reassured if she sees that she has a normal value, and think, ‘Oh I can wait a couple of years,’” she says. “That transition from 35 to 38 makes a big difference in a woman’s fertility, so that’s my biggest concern.”