The blood test measures three key fertility hormones which are markers of how many eggs are left -- an "index of ovarian reserve." It is designed to be a "wakeup call" for women whose fertile years are running out.

'Intrinsically Cruel Process'

The process involves having blood taken by a nurse or doctor which is sent for analysis. The result is then sent back with information on what it means and details of a hotline for advice.

Professor William Ledger, who developed the test, said women "are delaying childbearing for longer and longer," adding, "and when you sit in a fertility clinic and pick up the pieces you wish you could take them back 10 years.

"Men can produce sperm until their seventies but women cannot make eggs then. It is an intrinsically cruel process."

Professor Ledger, a consultant obstetrician and gynaecologist at the Royal Hallam Hospital in Sheffield, said the test is "not definitive" and does not cover other possible obstacles to getting pregnant.

He said: "It does not tell if you can conceive naturally and does not test your partner's sperm quality or how healthy your fallopian tubes are." Nor does it measure egg quality, which deteriorates as a woman ages, making it more difficult to conceive and miscarriage more likely.

Professor Ledger said the test was intended to "help women who are around 30 and want to plan when they want to have a child". He added: "If the result is good, then keep on going with the career and don't worry just now. But for others, it will be a wake up call that they can't really wait much longer."

Couples Are Often Surprised

The test is called the Plan Ahead kit. Visit www.lifestylechoices.com or phone Sheffield (0114) 275 5723 to buy on mail order. It will be in chemists and private clinics later in the year.

The blood test needs to be taken on day two or three of the woman's cycle. She must not be on the Pill and should be having regular periods.

Clare Brown, chief executive of Infertility Network UK, said it would prove useful to women who do not know who long to postpone childbearing. She said couples are often surprised to discover how quickly their natural fertility drops with age.

Ms Brown said: "Plan Ahead is a good method for women to check out whether their ovarian reserve is diminishing or otherwise, allowing them to make a more informed decision."

Healthy Embryos

Meanwhile, it was announced that a new technique has been developed for establishing which embryos fertilized in a test tube are most likely to result in a healthy pregnancy when implanted in the womb.

This could dramatically improve the chance of a successful pregnancy through IVF (in vitro fertilization), reducing the number of costly and stressful rounds of treatment needed to conceive. The test, which should be available in fertility clinics within 18 months, was developed by Professor Henry Leese at York University.