By Michelle Roberts
BBC News health reporter in Copenhagen
Taking steroid tablets while trying to conceive could reduce the risk of miscarriage for some women, scientists at Liverpool University suggest.
The steroid is thought to quash an abnormal immune reaction
Doctors there have already helped some women with histories of recurrent miscarriages to have healthy babies.
The steroid is thought to work by blocking immune cells that, in some women, appear to go into overdrive in the womb and stop embryos implanting.
A European fertility conference in Denmark heard more research is planned.
Previous research has shown that there are increased numbers of immune cells called natural killer (NK) cells in the wombs of women who suffer from recurrent miscarriages - when three or more pregnancies fail to go to term.
The condition affects two in every 100 women of reproductive age.
In half of cases, no obvious cause will be found and in a third of those, abnormally high numbers of NK cells are found in the womb lining.
The cells should make up no more than 5% of all those in the lining.
Dr Siobhan Quenby and colleagues at the Liverpool Women's Hospital believe the NK cells might attack the growing embryo and stop it from implanting properly in the womb, leading to miscarriage.
The cells have steroid hormone receptors on their surface, which led the researchers to question whether giving susceptible women steroid drugs might help prevent miscarriage.
Natural killer cells
They recruited 110 women who had severe recurrent miscarriage. They had experienced an average of six miscarriages with no known cause.
The researchers took samples of the women's womb linings and tested them to identify the quantity of NK cells.
Any women whose endometrial cells contained more than the normal range were offered 20mg of the oral steroid prednisolone for 21 days from the start of their menstrual cycle.
Out of a possible 33 women, 29 agreed to take the drug.
When the researchers took a second endometrial sample after the 21 days of prednisolone treatment, they found the steroid had significantly reduced the percentage of NK cells in the womb lining.
Dr Quenby told the 21st annual conference of the European Society of Human Reproduction and Embryology in Copenhagen, Denmark: "The women had an average of 14% before treatment, with the highest percentage being 72%, and after treatment the average dropped to 9%."
Now that they have shown the treatment can lower NK cell count, the researchers plan to see if it will make a difference to miscarriage rate.
Dr Quenby's team has already had some success in using prednisolone to treat women who have undergone recurrent miscarriages.
The first was a woman who had had 19 miscarriages, who went on to have a baby in 2002 after receiving the steroid.
Since then, two more women have had babies, including Ruth Hirst, 42 and from Huddersfield.
Another three women are pregnant and doing well, said Dr Quenby.
Dr Quenby said: "Anecdotally it seems fantastic. But there are many unanswered questions.
"We need to prove it now with a large randomised controlled trial."
She is looking to recruit 700 women to take part in the study. Half would receive the steroid and half would receive a dummy pill.
It should begin in the next few months, she said, but added that they needed more women to come forward - and extra funding.
Dr Adam Balen, a gynaecologist from Leeds and member of the British Fertility Society, said: "This is promising research that requires explanation.
"It is essential to do a placebo study [using a dummy pill] to prove that prednisolone does work."