Scientists are claiming a breakthrough which could lead to a test and cure for the pregnancy disorder pre-eclampsia.
The condition can be fatal to mother and child
The US National Institutes of Health team found women with pre-eclampsia had high levels of two proteins several months before developing the condition.
The research, published in the New England Journal of Medicine, was compiled after analysing blood tests from 4,500 pregnant women.
Experts said a cure and early test was still likely to be a long time away.
Pre-eclampsia, caused by a defect in the placenta, which supplies nutrients and oxygen to the foetus, happens towards the end of pregnancy and causes high blood pressure and kidney problems.
Up to one in 10 pregnant women can get pre-eclampsia, with one in 50 suffering severe problems.
It is responsible for the deaths of about five women and up to 600 babies each year in the UK with the only cure being delivery of the baby.
Scientists still do not fully understand what causes the placenta defect, although it is known that family history, being over 40, and a first time pregnancy can increase the risk.
The researchers in this study found the 120 women they tested who developed pre-eclampsia later in pregnancy had raised levels of the soluble form of the two proteins - endoglin and fms-like tyrosine kinase 1 - in their bloodstreams compared to those who did not get pre-eclampsia.
Raised levels of the proteins could be seen up to three months before the disorder developed.
Researchers said it was unclear exactly how the imbalance caused pre-eclampsia, but they suggested it could deprive blood vessels of essential nutrients, causing them to sicken and die.
National Institutes of Health director Elias Zerhouni said: "The finding appears to be an important step in developing a cure for pre-eclampsia.
"It may also provide the basis for predicting whether or not a woman will develop the disorder."
Maggie Blott, of the Royal College of Obstetricians and Gynaecology, said: "If we had a reliable screening tests and especially a cure it could make a huge difference to many women.
"Pre-eclampsia often develops very quickly so knowing a woman was likely to get it would be a great help.
"However, I think we are still a long way from such a situation."