A medical breakthrough could offer new treatment for the 100,000 women each year who suffer a miscarriage.
Australian researchers say they have identified low concentrations in early pregnancy of a protein that could be a potential factor in miscarriage.
One in five pregnancies results in the loss of the foetus but the reasons are a mystery. Recurrent miscarriage, defined as the loss of three or more pregnancies, affects 1-2 per cent of women and is linked with genetic, hormonal and other abnormalities but, until recently, most cases were unexplained.
In a letter published in The Lancet, Stephen Tong and colleagues from Monash University, Melbourne and the University of New South Wales, Sydney, say they have found a protein which plays an important part in the immune system and is produced in the placenta. The concentration of the protein, called Macrophage inhibitory cytokine 1 (MIC1), increases during pregnancy. The researchers measured the levels in 300 women and found they were around a third lower in the 100 women who went on to miscarry than in 200 women who had normal births.
Mr Tong, an obstetrician, said it was "tempting to speculate" on the basis of the findings that "changed production of MIC1 in the placenta is part of the mechanism initiating spontaneous pregnancy loss."
He added: "If a causal link between low MIC1 and miscarriage is confirmed then MIC1, or its synthetic analogues [a drug based on the protein], might be useful in the prevention of miscarriage."
Experts welcomed the news but warned that the exact role of the protein in miscarriage had still to be proved. In a commentary in The Lancet, two specialists, Galit Sarig and Benjamin Brenner, from the Rambam Medical Centre in Haifa, Israel, said the "crucial question" is whether the decrease in levels of MIC1 is the cause or effect of the mechanism leading to pregnancy loss.
"If [Mr] Tong and colleagues' results can be confirmed, the way is open to develop novel therapies to prevent pregnancy loss," they write. That prospect was welcomed yesterday by the Miscarriage Association, a charity providing support to women who lose their pregnancies.
Its national director, Ruth Bender Atik, said that "there are oceans that remain unknown".
"Most women find the experience very distressing. For many people it feels like a bereavement." Women who miscarried often felt they had let their families down, she said.
She added: "Some people have this awful British way of saying it [the developing foetus] is not a baby but just a bunch of cells. Unsurprisingly, women don't find that very helpful. It is not a trauma for everyone but it can cause distress."
Eileen Elston, 37, a chartered accountant, had three miscarriages in two years. The last happened in early 2002, a couple of months after her marriage. "It is a loss of the future and of future plans. It is emotionally draining and frustrating," she said.
Doctors said she had a blood clotting disorder which is one of the few known causes of recurrent miscarriage. Also called "sticky blood" it causes difficulties in getting oxygen and nourishment to the baby.
In about half of all cases, early miscarriages are caused by genetic abnormalities. Infections and fevers may also trigger a loss. In later pregnancy, it may be caused by anatomical problems such as a weak cervix or an irregular shaped uterus.