Pre-eclampsia, a condition marked by high blood pressure, headaches, blurred vision and protein in the urine, affects 10-15 per cent of women pregnant for the first time.

Normally it can be controlled, but every year in Britain it causes the deaths of up to ten women and around a thousand babies. Worldwide, it is believed to be responsible for more than 200,000 deaths among women.

This week a team from the University of Vermont College of Medicine, in Burlington, Vermont, will report results from a study of the use of Viagra in pregnant rats with induced high blood pressure. The results, to be presented at the congress of the International Union of Physiological Sciences in San Diego on Tuesday, show that Viagra eliminated deaths among foetuses of rats with high blood pressure.

It also increased the weight of the unborn rats to normal, compared with the untreated group, which were 20 per cent below average.

During pregnancy, arteries that supply the womb with blood enlarge, allowing blood flow to increase more than tenfold. This ensures that the foetus is adequately nourished and is provided with sufficient blood flow during labour.

All mammals show a similar adaptation, includings rats, sheep, pigs and human beings, so there is reason to suppose that what works in rats may work in people. Viagra works by inhibiting an enzyme, PDE-5, that prevents the expansion of arteries, so, if pre- eclampsia is the result of the failure of the arteries to expand in pregnancy, starving the womb of blood and increasing blood pressure, Viagra may provide a treatment.

Although Viagra did not reduce blood pressure in the rats, according to a team led by George Osol, it did reverse the symptoms of the condition. The diameters of the arteries on the treated animals did not quite match that in normal rats, but it was significantly larger than in those with high blood pressure, “suggesting that Viagra may have increased blood flow to the uterus and placenta”. In addition, the foetuses were of normal weight and survived much better than in those of rats with high blood pressure. In the untreated rats, 11 per cent of the foetuses either died or were re- absorbed; in the treated ones, none was lost.

Professor Osol said that the results “are exciting because they suggest that Viagra (or other drugs that inhibit PDE-5) may have beneficial effects in hypertensive pregnancy and, possibly, pre-eclampsia”.

He said it was interesting that the effects were achieved without any reduction in blood pressure. This meant that it might avoid some of the complications of giving drugs to reduce blood pressure, which can reduce the vital supplies of blood to the womb and put the baby at risk.

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