A cheap and simple treatment can halve the chances of women developing a dangerous complication of pregnancy which kills more than 50,000 mothers and their unborn babies a year worldwide.

Pre-eclampsia, also know as toxaemia, affects 700 women a year in the UK. Characterised by high blood pressure, it may progress to constrictions in blood flow and oxygen to the brain. When this occurs the disease is called eclampsia.

Eclampsia causes convulsions which carry a 2% risk of death, and three to four women and 1,000 babies in the UK die from it each year.

Now a large scale study, published in The Lancet today, involving more than 10,000 women in 33 countries, reports that injections or a drip containing magnesium sulphate - a cheap compound similar to epsom salts - can halve the chances of a woman with pre-eclampsia developing the potentially fatal eclampsia.

The findings of the three-year "Magpie" trial suggest thousands of women's lives could be saved.

Lelia Duley, an obstetric epidemiologist at Oxford University's Institute of Health Sciences, who led the Medical Research Council funded trial, said the findings were exciting. "The treatment could save countless lives across the world if itwere introduced routinely for pregnant women with pre-eclampsia. Importantly, it is a very inexpensive treatment, making it especially suitable for use in low-income countries."

Pre-eclampsia is most common in a first pregnancy, in women with a family history of the condition, and in those carrying twins. In the UK antenatal clinics check women's blood pressure and urine to see if they are at risk.

Jim Neilson, a consultant obstetrician at Liverpool women's hospital, the largest maternity unit in the UK, said that the magnesium sulphate had proved safe for mothers and babies, although a quarter of women had mild, temporary side effects. He said the breakthrough should lead to "rapid change in clinical practice in this and other countries".

The treatment had not led to a rise in rates of caesarean sections, and it had had the bonus of cutting the incidence of haemorrhage from placentas separating from the uterus.

Giving women magnesium sulphate for pre-eclampsia could slash the high maternal death rate in developing countries. However, though the drug costs only £3.45 for the four injections and, seven years ago, was shown to be the best treatment for women suffering convulsions, it is still unavailable in many countries.

· Claire Giles, 32, from west London, took part in the "Magpie" trial when she developed pre-eclampsia during her first pregnancy and became seriously ill.

She was 32 weeks pregnant when she started to swell but did not go to hospital for six weeks. By then she was in great pain.

"I was so swollen my back was like a waterbed," she said. The circumference of the widest part of her legs, she added, measured about 90cm. Doctors asked her to enter the trial. "I felt so ill I think I'd have agreed to anything."

She took part in a "blind" study, not knowing if she was treated with magnesium sulphate, and she had an emergency caesarean section on an ordinary hospital bed because she had become too large for the operating table. But her baby, Matthew, now aged two, was successfully delivered and has stayed healthy