Until now the first signs of the condition were the symptoms: large increases in blood pressure, headaches, blurred or altered vision, abdominal or shoulder pain, nausea and vomiting, confusion, shortness of breath and excessive swelling of the hands and feet.

The only treatment is careful monitoring, and early induction of birth if the symptoms become dangerous. Doctors balance the interests of the baby — which does better the longer it stays in the womb — with the threat to the mother’s life. As soon as she has given birth, the symptoms subside.

A team led by Thomas Rademacher, of University College London, has found that testing for the presence of a protein called inositol phosphoglycan P-type in the urine gives a reliable indication that the condition is developing.

They compared the levels of the protein in the urine of 27 women who developed pre-eclampsia with 47 who did not. They found that the women who developed pre-eclampsia had levels of the protein several times greater than those who did not. The increases were detectable before symptoms appeared, up to seven weeks earlier in some cases, the team reports in Hypertension.

It is not known whether the protein is responsible for triggering the condition, but this seems possible. If so, the discovery could open the way to developing more effective treatments.

Professor Rademacher said: “Being able to predict the onset of this disease has been the single greatest challenge in obstetric medicine.

“Pre-eclampsia is the most common of the serious complications that can occur during pregnancy and affects millions of women and children. It is a particular problem in the developing world, where treatment is less readily available.

“Our research has identified that the presence of inositol phosphoglycan P-type is a reliable indicator of whether a pregnant woman will develop PE. Now a reliable diagnostic test has been developed, this paves the way for identifying new treatments.”

Pre-eclampsia affects about 5 per cent of pregnancies. It can occur any time during pregnancy, but normally appears in the last three months.

Worldwide, the condition affects more than seven million pregnancies a year and causes 22,000 maternal deaths. More rarely, it can develop into eclampsia, which affects 700,000 pregnancies a year, leading to 43,000 maternal deaths.

“PE is presently only curable by delivery,” Professor Rademacher said. “In many cases clinicians must deliver a baby in order to save its mother’s life, even if this means the baby is born prematurely.”

PE arises when the placenta releases a toxin, causing the mother’s blood pressure to rise sharply. If it develops into eclampsia it can cause seizures and she may lapse into a coma.

65,000 Number of women who die each year from eclampsia and pre-eclampsia

Source: University College London