A test that predicts how many eggs women have in their ovaries, helping them determine when they should try for a baby, could be on drugstore shelves later this year, British scientists said Thursday.

The test, called Plan Ahead and available now only by mail-order, will sell for $320.

It measures three hormone levels in the blood, and assesses the number of eggs in a test-taker's ovaries compared to levels expected for other women the same age, researchers at the University of Sheffield in northern England said.

Because the test forecasts how many eggs will be in the woman's ovaries for the next two years, women can "make an informed decision as to whether, or how long, they can potentially delay before trying to conceive," a news release from the researchers said.

They warn, however, that Plan Ahead is not a fertility test; rather, it's a tool women can use to help determine if they may need professional help to conceive.

"When you get your report back after you do the test, it's been written very clearly that women shouldn't forget about the dignity of the fallopian tubes, the health of the womb and the partner," said Bill Ledger, a professor of obstetrics and gynecology at the university. "It's not a fertility test in that sense."

Dr. Stuart Lavery, a consultant gynecologist at London's Hammersmith Hospital who was not associated with the product's development, said he thought the test could help fertility experts as they try to help women have babies.

"The biggest challenge we face is that people come to us too late," Lavery said. "I think anything that gets people thinking about babies and trying to conceive early is good. If it alerts people to things that might not be straightforward and gets them to seek medical intervention, that's good."

But Lavery said he was concerned that the test might also give women a false sense of security, when there are a number of other factors that could prevent them from conceiving.

The test requires a small amount of blood, which must be drawn by a doctor or nurse practitioner on the second or third day of the woman's period, the researchers said. The blood is then mailed to a lab, which returns results in two weeks along with a guide to fertility booklet and a telephone number for questions.