More women are waiting longer to have children as they pursue college degrees and careers or simply enjoy young adulthood without kids in tow.

That trend could have sharp repercussions for women and couples eventually hoping to start a family. Those who wait too long could have trouble getting pregnant. Wouldn't it be nice for a woman to know in advance how many fertile years she has left?

Researchers in the United Kingdom say they've found a revolutionary way to read the tea leaves. By measuring a woman's ovarian volume using ultrasound and plugging that information into a computer model, it should be possible to predict when a woman will enter menopause and how much time is left on her biological clock, they said.

At least that's the theory.

"We are now looking for funding for a clinical study to prove our hypothesis that there is a very strong relationship between ovarian volume and ovarian reserve/age at menopause," said Dr. W. Hamish Wallace, a consultant pediatric oncologist at the Royal Hospital for Sick Children in Edinburgh, Scotland.

He and colleague Thomas W. Kelsey, a senior research fellow at the University of St. Andrews in Scotland, first unveiled their methodology last June in the journal Human Reproduction.

The occasional headline-grabbing tale of a 50-something woman becoming a new mother does not erase the fact that age remains a critical factor in determining female fertility. As a woman grows older, her ovaries produce fewer eggs. Fertility problems affect about a third of couples in which the woman is over 35, according to the National Women's Health Information Center.

A woman who delays pregnancy into her late 30s or 40s could miss the boat entirely if menopause precedes motherhood. When her monthly periods stop for good, she can no longer produce eggs or become pregnant.

Women who've had their ovaries surgically removed may experience menopause at any age. For most women, menopause occurs naturally around age 51, when the ovaries stop producing estrogen, according to the American Menopause Foundation.

But determining the precise onset of menopause is tricky. "Some women enter menopause prematurely at very young ages -- even less than 30," said Dr. Robert Schenken, president of the American Society for Reproductive Medicine and chairman of obstetrics and gynecology at the University of Texas Health Science Center at San Antonio.

Having an accurate model for predicting when a 39-year-old will become menopausal could influence her decision about when to become pregnant, Schenken observed. "I think it's certainly worthy of further study," he said.

Physicians already use a variety of tests to evaluate female infertility, including a blood test that detects levels of follicle-stimulating hormone (FSH).

"That can tell us how hard the brain has to work to stimulate the ovaries," explained Dr. Alan Copperman, director of reproductive medicine at Mount Sinai Medical Center in New York City.

FSH and other hormone tests are generally administered as part of the workup of candidates for assisted reproductive techniques, including in vitro fertilization. But no current test is sufficiently accurate in predicting the precise age of menopause, fertility experts said; existing tests provide only pieces of the puzzle.

And there is a larger question: How many women really want to know when their baby-making days are over? "I think a lot of people don't want to know or, certainly, don't want to know bad news," Copperman said.

On the other hand, now that modern medicine has made it possible to freeze eggs for future use, a predictive test could be quite useful to a young woman who learns that she's less likely to be fertile in her mid- to late-30s, he conceded. She might consider putting away some of her eggs "almost as an insurance policy," he suggested.

Even if further studies validate Wallace's method for predicting menopause, it could be quite some time before it is recognized as a standard diagnostic tool. As far as he knows, no physicians or clinics in the United States or the United Kingdom are currently using it.

If you're trying to conceive, don't despair.

"I do not believe this paper gives any more information to women presently planning therapy than already exists," said Dr. William D. Schlaff, immediate past president of the Society for Reproductive Endocrinology and Infertility and chief of endocrinology and infertility at the University of Colorado Health Sciences Center in Denver.