A lack of key immune system cells may be linked to recurrent miscarriage, say scientists.
Cambridge-based researchers believe that the body normally damps down immune activity during pregnancy, halting attacks on the baby.
When certain immune cells - thought to be responsible for this - were removed from pregnant mice, they miscarried.
The research, published in the journal Nature Immunology, could lead to tests to predict repeat miscarriages.
It could also be eventually harnessed to provide treatments for this, and drugs to prevent organ transplant rejection, say the scientists.
While some immune system cells appear to be responsible for actually carrying out attacks on "foreign" material identified in the body, such as bacteria, others in the background help coordinate attacks, select targets and generally control the level of immune activity.
Marked for destruction
During pregnancy, a woman carries and nourishes an organism which, by normal criteria, would be selected for destruction by the immune system.
Even though it contains the mother's genetic material, it carries equal amounts from the father, and may be strikingly different, for example have a different blood group.
To stop this happening, the immune system is modified during the pregnancy.
The researchers, led by Dr Alexander Betz at the Medical Research Council Laboratory of Molecular Biology, looked for the key changes in the immune system in the first days of pregnancy which might be involved in this process.
They found that numbers of a particular type of "t-cell" in a pregnant mouse's bloodstream were "systematically expanded" during this period.
When numbers of this cell type were artificially reduced in pregnant mice, the pregnancy was lost - clearly signifying their importance in this process.
Mysterious cause
Up to 1% of women in the UK suffer recurrent miscarriages, for no apparent reason.
It has long been suggested that problems with the immune system could be to blame.
Now tests for recurrent miscarriage - and perhaps even treatments to prevent it, may eventually be possible.
Dr Betz said: "The prospects are exciting, however, the work so far has only been in mice.
"The next step is to see if the immune systems of pregnant women behave in the same way.
"Only then can treatments be developed, and eventually trialled on humans."
Professor Maggie Dallman, from Imperial College London, said that other experiments had suggested that this type of cell might play a role not only in protecting the foetus, but also potentially in helping the body tolerate transplant organs.
She said: "There are lots of things that seem to protect the foetus from being destroyed - but this is a very plausible additional mechanism."
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