More and more women are putting off having children until their 30s and 40s. Yet a growing body of research shows that female fertility begins to decline rapidly in the mid-30s – leaving some women with a difficult choice: bearing children earlier or risking infertility later on.

Lifestyle Choices, a firm based in Sheffield, U.K., has begun selling a consumer test that it claims will help women make this important decision. The test, which went on the market last month, assesses the number of eggs that a woman has left in her ovaries, by measuring three different hormones.

However, some experts say that the results of these kinds of tests can be unreliable – and therefore give women a false sense of security about delaying childbearing.

Women are born with a set number of eggs – a number that declines with age, first slowly, then rapidly after the mid-30s. The rate of decline varies from woman to woman – some can easily get pregnant in their 40s, while for others it’s difficult or even impossible.

“Some women start to go into menopause in their 30s,” says Rogerio A. Lobo, professor of obstetrics and gynecology at Columbia University in New York City. “Without some kind of clinical testing, or a suggestive family history, that [life stage] is impossible to guess.”

Women can go to their doctor for blood or ultrasound tests that will give a sense of their fertility status. But women in the U.K. may have a new option now: a mail-order test. For £179 ($320), they can order the test kit, go to a doctor’s office for a blood test, then send the sample to a lab for analysis. Their blood is analyzed for the levels of three different hormones, which predict the number of eggs a woman has available. To interpret the results further, hormone levels are compared with the average hormone levels for the woman’s age group.

“As tests go, these are as close to the best things we have out there, but there are still a lot of questions and a lot of caveats,” says Lobo, who regularly measures the same three hormones in his clinic. “It can give you some information – but it’s not 100 percent.”

Kim Thornton, director of the division of reproductive endocrinology at Beth Israel Deaconess Medical Center in Boston, thinks “the issue with all ovarian reserve testing is that the test cannot necessarily tell you that it’s okay to delay childbearing. There are a lot of false negatives.” An abnormal result gives a good indication that a woman may have fertility problems; but a normal result does not guarantee that a woman has several years of fertile years ahead, Thornton says.

Indeed, experts say the only way to really gauge the accuracy of the U.K. test is with a long-term clinical trial showing that women who get a “normal” result from the test can get pregnant.

Stuart Gall, commercial director of BioFusion, the parent-company of Lifestyle Choices, says the company has run some clinical trials; however, for proprietary reasons, they are not releasing the results.

The test is not approved by the U.S. Food and Drug Administration. But Gall says they are actively discussing options for getting it approved in the United States.