A team of doctors from several UK universities says fertility clinics are increasingly offering tests to measure natural killer (NK) cells in the blood.

Women are then given powerful drugs which may have serious side effects.

But the British Medical Journal study says there is no scientific basis for offering the tests or treatments.

NK cells are found in the womb, and accumulate in large numbers during early pregnancy, but their function is completely unknown.

They may help to control blood supply to the lining of the womb and to the placenta, but how they do this is not yet clear.

The tests are based on speculation that recurrent miscarriages and infertility problems are in some way linked to a malfunction of NK cells.

'No useful information'

The researchers say that many women who record high levels of NK cells are then being offered powerful treatments, such as steroids or immune suppressant drugs, to try to reduce the cells' numbers and activity.

But the team argues that the tests provide no useful information about what is happening in the uterus.

Not only does nobody fully understand what role the cells play, the tests analyse cells taken from the blood, which are different to those found in the uterus.

In addition, the level of NK cells found in the blood of a healthy person varies greatly, so there is no way of accurately defining what constitutes a high level in women who undertake the test.

At this stage, the researchers say, not only is there no evidence to justify the use of the tests, it is wrong to offer women powerful treatments which are known to carry risk, and which are not licensed for use in reproductive medicine.

Exploitation

Writing in the journal, they say: "This unfortunate group of women are particularly vulnerable to financial exploitation, and of being exposed to powerful treatments that have, as yet, no rational scientific basis."

Lead researcher Dr Ashley Moffett from the University of Cambridge said: "Women in this situation are quite understandably desperate and, because these cells appear to have unique qualities and are found in the uterus during early pregnancy, they have been latched on to as the one thing that something can be done about.

"But as we still don't know what their function is, it is totally premature to be thinking about any sort of intervention."

Dr Mark Hamilton, of the British Fertility Society, agreed with the conclusions of the paper.

He said the validity of the tests was unproven, and that it was wrong to advise women to take on treatment of no proven benefit and potentially hazardous.

"The reproductive medicine community is acutely aware of how desperate couples either with a history of infertility or recurrent miscarriage are to have a baby.

"However as with all medical interventions, it is a fundamental principle of good medical practice that safety concerns are addressed.

"Great care has to be taken not to take advantage of couples merely because they are desperate and offer treatment which is sometimes expensive and often intrusive without adequate justification."