The discovery means doctors may be able to identify the condition and rectify it early on.

It occurs late on in pregnancy and causes blood pressure to rise and tissue to retain abnormal amounts of water.

If left untreated it can cause eclampsia, or convulsions, which can prove deadly.

The high blood pressure associated with pre-eclampsia often forces doctors to deliver babies early, as sustained high blood pressure can cause oxygen deficiency in the child.

Comparisons

Researchers at the US National Institute of Child Health and Human Development compared 134 women who developed pre-eclampsia with139 similar women who did not.

They found that in women who got pre-eclampsia, levels of a hormone that regulates the size of blood vessels drop weeks before the condition became evident.

This has allowed doctors to get an early warning of the condition using a urine test.

In the past, efforts to prevent the disease using aspirin have failed.

Treatment

Now doctors hope to focus efforts on restoring the body's proper balance of the hormone.

Publishing their results in the Journal of the American Medical Association, they said they could not be certain that the hormone - prostacyclin - was the only element involved, but it was a critical factor.