IT IS a dilemma facing a growing number of young women: can I delay having a baby until my career is more established? A genetic test that could make this decision less of a gamble might be on offer by next year, thanks to the discovery of a gene that seems to predict the rate at which a woman's egg supply diminishes.

No one is yet sure how useful the test will be. But the aim is to tell a woman in her early 20s whether she is at high risk of early menopause. If she is, monitoring her egg supply will confirm whether her fertility is in early decline. Armed with this information she could then decide whether to start a family sooner or later, or freeze some eggs to increase her chances of conceiving later on.

Meanwhile, other hormonal and genetic markers that could help to predict egg supply more reliably are also in the works.

A woman is born with all the eggs she will ever have - 1 to 2 million immature eggs or follicles. Over her reproductive life, approximately 400 of these will mature and could be fertilised. By puberty, a girl has around 400,000 follicles left, and this continues to decline until menopause, when only a few hundred remain. The number of follicles a woman has left at any time in her life - her "ovarian reserve" - approximately reflects how many eggs she will release.

After 35, most women experience a sharp drop in their ovarian reserve, and, as result their fertility, but around 10 per cent of women experience "early ovarian ageing" in their 20s . Hormonal tests can provide a rough snapshot of a woman's ovarian reserve - and flag up declining fertility. The challenge is to predict early ovarian ageing before it happens.

Norbert Gleicher of the Center for Human Reproduction in New York reckons he can do just that, using a gene already implicated in reproductive ageing.

Women with a version of the Fragile X or FMR1 gene that contains more than 200 repeats of the DNA sequence CGG are likely to have Fragile X syndrome, which causes mental impairment. Some women have 55 to 200 CGG repeats: they don't have mental impairment, but are at increased risk of early menopause.

Intrigued by this relationship, Gleicher wondered whether the number of repeats in FMR1 might also contain clues about early ovarian ageing, which is more common and less dramatic than early menopause.

He analysed the FMR1 genes in 316 women attending his fertility clinic, and took a snapshot of their ovarian reserve by measuring levels of a hormone called anti-Müllerian hormone (AMH), an indicator of how many eggs are currently maturing in the ovaries.

In women with between 28 and 33 repeats, he found normal levels of AMH. But in women with repeats both above and below this range, AMH levels indicated early ovarian ageing. His team calculated that for every decrease of five CGG repeats below this range, the risk of a diminishing ovarian reserve increased by 40 per cent, while each increase of five CGG repeats above the range upped this risk by 50 per cent (Reproductive Biomedicine Online , vol 19, p 385).

Gleicher concludes that the number of CGG repeats predicts whether a woman is likely to experience premature ovarian ageing. "We can take an 18 or 20-year-old girl and check her Fragile X and make a pretty good prediction of whether she's at risk," he says. As FMR1 is known to help regulate the transition of immature eggs to the mature state, this makes sense.

Gleicher hopes to start offering such a test next year as a tool to identify women at high risk of early ovarian ageing. Not all women flagged up by the test will struggle to conceive in their 30s - only hormone tests will confirm whether their egg reserve really is depleting faster than normal. "Then you can sit down and have a discussion about her reproductive life plan," Gleicher says. "In other words, 'do you want to have your kids before you get your PhD, or afterwards?' If the answer is 'afterwards', OK, but maybe you want to freeze some eggs."

Gleicher says he still needs to establish baseline levels for the hormones that are currently used to measure ovarian reserve, in women of different ages who aren't having fertility problems.

But many think he is jumping the gun. One criticism is that although AMH is one of the best markers of ovarian reserve going, it doesn't provide a direct measurement of the number of immature eggs or follicles that remain. "If you really want a long-term predictor for when you will enter menopause, a marker for the primary follicle pool will be more important," says Sharon Lie Fong of Erasmus Medical Center in Rotterdam, the Netherlands. "He also has to prove whether these women have lower fertility."

Although early ovarian ageing encompasses both a decline in quality as well as quantity of follicles, women with a lower reserve may not necessarily struggle to conceive, says Frank Broekmans of the University Medical Center in Utrecht, the Netherlands. He says Gleicher's results need to be confirmed using a larger group of women who are monitored for some years, to determine whether CGG repeats affect the likelihood of a pregnancy.

"You need some kind of follow-up studies on whether this test really predicts long-term events." Stephanie Sherman of Emory University in Atlanta, Georgia, who also studies FMR1 says Gleicher's results are exciting, but she would also like to see them replicated before his test is used in the clinic.

What could be done right now, she says, is to screen women with a family history of fragile X to see if they have the telltale 55 to 200 CGG repeats, which puts them at high risk of premature menopause. "It's clear that this is an indicator of ovarian reserve."

An FMR1 test might also help women who don't become pregnant naturally, says Bill Ledger of the University of Sheffield, UK. "Many women with early ovarian failure are curious as to why it has happened and this may help explain the phenomenon in some cases.

FMR1 isn't the only factor that predicts premature ovarian ageing. Broekmans recently discovered that healthy women with low levels of AMH for their age group seemed to reach menopause earlier than the median age of 51 ( Journal of Clinical Endocrinology and Metabolism , DOI: 10.1210/jc.2007-2093). However, these women ranged from 25 to 45 years of age when they had the AMH test, and such predictions remain to be proven for younger women, who are likely to show more variability in their AMH levels.

Another option is to look at genes other than FMR1 . Last month, Kutluk Oktay of New York Medical College told the American Society for Reproductive Medicine that women with the BRCA1 mutation, which diminishes the ability of DNA to repair itself and raises the risk of breast and ovarian cancer, may also be at higher risk of premature ovarian ageing. Oktay is now planning larger clinical studies to confirm the finding as well as molecular studies to explore the underlying mechanisms. "DNA repair may be an important component of ovarian ageing," he says.

Meanwhile, other genetic studies have recently found a handful of common genetic mutations that influence the age of menopause (Nature Genetics , DOI: 10.1038/ng.387).

Most researchers agree that assessing a woman's reproductive lifespan in her early 20s could be beneficial and that Gleicher's test is a first step. But a test reliable enough to transform the lives of a large number of women will likely involve a series of genetic and hormonal markers. It will also need rigorous testing to ensure woman aren't burdened with anxiety - or given false hope.