A revolutionary technique will reveal reproductive life-span. Laura Tennant reports

Every woman who wants children must face the same dilemma: just how long has she got? Should she spend her 20s and early 30s carving out a career, finding Mr Right and enjoying some hard-won adult pleasures and freedoms, only to risk discovering infertility when it's too late to do anything about it?

Every woman who wants children must face the same dilemma: just how long has she got? Should she spend her 20s and early 30s carving out a career, finding Mr Right and enjoying some hard-won adult pleasures and freedoms, only to risk discovering infertility when it's too late to do anything about it?

Make procreation the priority of your 20s and the results are likely to be financial hardship, dismal job prospects, and no more nights out with the girls - and that's assuming you have found a father for your babies.

But the gamble that many women have had to take on their future fertility is about to become rather less hair-raising. A new procedure developed by Dr Hamish Wallace, of the Royal Hospital for Sick Children in Edinburgh, and Dr Tom Kelsey, of the University of St Andrews, can now predict the onset of the menopause, which varies widely. (The average age is 51.)

The technique uses the ultrasound technology developed for scanning the womb in pregnancy to measure the size of ovaries. It is then possible to calculate how many eggs a woman has left, and hence her "reproductive life span".

The team claims the method will "revolutionise the treatment of women seeking assisted conception, those who have had treatment for childhood cancer and those who wish to delay a family for personal or professional reasons". Dr Kelsey said: "Supposing a 25-year-old were to have the procedure to establish the condition of her ovaries. The test would not be able to tell her if she was going to encounter one of the many problems that interfere with conception - low sperm count, the viability of individual eggs, or the genetic make-up of the father - but it would be able to give her a window for assisted conception. We know that, whatever the problems causing infertility, it can't be treated unless the woman is producing eggs."

For children and young women undergoing treatment for cancer who may have suffered a drop in fertility, the need to establish a time frame for pregnancy is especially pressing. "We are cautiously optimistic," said Dr Kelsey, "that we can provide an answer to that question." If the test becomes widely available more women will be having ultrasounds more often and Dr Kelsey hopes more cancers will be detected early on.

Knowing the date of the menopause might also help those who are unsuccessful to step off the heartbreaking treadmill of IVF treatment, in which there always seems to be one more medical advance around the corner. The television presenter Anthea Turner recently made her decision on this. "I didn't want to destroy what was important in my life," she said. "I have seen couples destroy their relationships because of their obsession with having a baby."

Experts aim to help couples to achieve "resolution", either by giving birth or facing the fact that they never will and beginning a grieving process. The deadline provided by the new test could help.

Just as reliable contraception gave women control over their own fertility, so "the Wallace-Kelsey method" should provide them with forward-planning powers our mothers could only dream of. At least that's the theory. One friend who is 40 this year told me that knowing she was due to become menopausal at, say, 46, would make her want to jump off a bridge. And what if prospective partners demanded to see "time remaining" on a woman's biological clock along with a certificate of her sexual health? Yet among the women I talked to the consensus seemed to be - knowledge equals power.

Christine Campbell, 44, London

For women suffering early menopause, the information provided by the test is invaluable. Christine, a former film editor, was treated for thyroid cancer at 32 and began to suspect she was undergoing the menopause two years ago. Doctors failed to warn her that she was at risk and she was "angry and upset" that lack of information had closed down options for her. Her daughter Rachel is five, but had she known time was limited she would have tried for a second child sooner. "I could also have frozen my eggs," she says. "Now the only way forward for us is egg donation."

Anne Seymour, 42, Pontypridd, South Wales

The lecturer in mental health nursing at the University of Glamorgan gave birth to Carys four months ago, after six cycles of IVF. "They discovered my eggs were not of very good quality," she explains, "and there was also a problem with my husband's sperm. I was very career-orientated in my 20s and I went back into full-time education at 28, so I didn't even get a mortgage until I was 34. I wasn't sure I even wanted to have children or get married until I met my husband. Even if I'd known in my 20s that I might have problems, I didn't have the right relationship and I was living in rented accommodation without a salary."

Eleanor Wyld, 15, London

At present the new technology is able to test the fertility of women between 25 and 50, yet it is only a matter of time before girls as young as Eleanor are able to establish exactly how many years of fertility they have. For Eleanor, who would like to be an actress and have "three or four kids" after establishing herself in her 20s, that could make all the difference. In a profession in which youth is at a premium, many actors face agonising decisions about when to interrupt their careers. "If I was told I wasn't very fertile at 30, I would put that dream job on hold and have a family."

Kate Irvine, 32, Aberdeen

"My husband was recently diagnosed with diabetes, and I suffer from polycystic ovarian syndrome which can increase your risk of type two diabetes," says my GP, "so we've had to think very hard about the health of any children we might have. Knowing how much time we had to make up our minds would be very useful. On top of that, because I'm adopted, I don't know my own genetic history, so I have no idea when women in my family hit the menopause." Knowing she could put off a family until her late 30s would allow Kate to enjoy the fulfilment offered by her career.

Chloe Fox, 28, London

A journalist on Vogue, Chloe is married and hopes to get pregnant in the next two years. "I know it's easy to say when you have time on your side," she says, "but I can't help feeling that there's something cold about planning your life with a test like that. Also I think my generation isn't interested in being 'career women' in the same way as our mothers were." She foresees one benefit of the test. "Perhaps it will take the heat off the rampant man-hunt my contem-poraries have been up to. I can think of several people marrying people they really shouldn't."

Allison Bucknell, 45, Wiltshire

"I reacted quite badly to news of the test," says Allison. "It feels so unnatural." After three IVF cycles Allison realised she had left it too late to have children. She admits that she probably would have acted on information on her biological clock and had a baby in her 20s. "But many other avenues would have been cut off for me and I would have ended up a more selfish person." Now Allison and her husband foster a 12-year-old boy.