Fertility tests for women are now available via their GPs, thanks to an Oxfordshire biotechnology firm.

Oxford Bio-Innovation, based in Heyford Park, Upper Heyford, is the world's only supplier of the pioneering Inhibin B test.

The test, nicknamed the "egg timer", costs £75 and is not available on the NHS. Results are usually known within five days.

Fertility clinics have already been offering the test to their patients. Now, for the first time, women can find out more about how fertile they are via a simple blood test taken by their doctor.

William Finch, Oxford Bio-Innovation's managing director, said the test was important for women who wanted children because many were under the misguided impression that even when they reached their 40s, it would be easy to conceive.

Mr Finch said: "In truth, only about five per cent of women in their 40s retain natural fertility." Early onset of the menopause can cause lower fertility rates in some women. Experts at Bio-Innovation believe that career-minded women, or women whose mothers entered the menopause early, can gain extra information on planning a baby by taking the test early on.

Mr Finch said: "Early onset of the menopause means that a woman's biological age may not be in line with her actual age. It is very important for a woman to be aware of her particular situation before making life and career decisions."

Inhibin B is the name of a reproductive hormone which is believed to be a reliable indicator of the level of eggs remaining in a woman's body.

Oxford Bio-Innovation has developed a simple blood test to establish this, using technology invented by Prof Nigel Groome at Oxford Brookes University.

Blood samples can now be sent by GPs to OBI's laboratories, where test results are interpreted by specialists.

Mr Finch said: "It will help some women avoid the disappointment of childlessness by encouraging those with a decreased ovarian reserve to start a family earlier than they might otherwise have expected."