'Crucial' new recurrent miscarriage insight
Fertility scientists say they have made a "crucial breakthrough" in understanding why some women have repeated miscarriages.
There has been debate about whether giving steroids would help women who have lost multiple pregnancies.
University of Warwick researchers say they have now shown how low steroid levels lead to some miscarriages.
Experts said identifying the right women for treatment would be key, as steroids may make the problem worse.
More than one in seven pregnancies end in miscarriage.
Many women will successfully have a baby at the next attempt, but recurrent miscarriage - losing three or more pregnancies in a row - affects one in 100 in the UK.
Prof Siobhan Quenby from the University of Warwick said: "It causes incredible psychological distress and anguish.
"The routine advice in the UK is if blood tests identify no cause then there's no treatment, that's terribly unacceptable to patients."
The scientific debate centred around a part of the immune system called NK cells, which appear in higher levels in the wombs of some women who miscarry.
There were suggestions steroids could help these women and yet it was not clear how NK cells could cause a miscarriage and they were known to be important for an embryo to implant in the womb.
The idea was not fully tested in large clinical trials.
Now researchers publishing in The Journal of Clinical Endocrinology and Metabolism, believe that the NK cells are merely a marker of something more serious happening in the womb lining.
Tests suggest that low steroid levels make the womb itself less likely to accept an embryo and damage the way it nourishes a foetus that does implant. These processes in turn lead to higher NK cell levels.
Prof Quenby said: "This work is really exciting because after years of controversy and doubt, we have a crucial breakthrough."
She said up to one in three women had high NK levels and is now calling for a clinical trial to test whether steroids would help them.
In the meantime she said women should not be buying steroids in an attempt to treat themselves, as too much could also lead to miscarriage.
"It is really important women do not go out and take steroids, they might be in the category when it will do more harm."
Nick Macklon, a professor of gynaecology and obstetrics at the University of Southampton, said the field was lacking understanding of what was happening in miscarriage.
He told the BBC: "This is a crucial breakthrough in the understanding of recurrent miscarriage, it's the gateway to the clinical trial.
"But what this shows is that steroids shouldn't be given to all, we need to be sure that is the problem in women before they're given."