Researchers have claimed that by 2010, a new test would be on the horizon, which could detect the presence of a gene that seems to predict the rate at which a woman's egg supply diminishes.

The test is aimed 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.

With this information, a woman could then decide whether to start a family sooner or later, or freeze some eggs to increase her chances of conceiving later on.

A woman is born with all the eggs she will ever have- 1 to 2 million immature eggs or follicles- out of which 400 will mature and could be fertilised during her reproductive life.

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 thus their fertility, but around 10 per cent of women experience "early ovarian ageing" in their 20s.

Norbert Gleicher of the Center for Human Reproduction in New York has claimed that the new test could predict early ovarian ageing before it happens, 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.

Thus, the researchers thought that 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-Mullerian 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.

The researchers 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.

They concluded 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," New Scientist quoted Gleicher as saying.