Researchers say that pregnant women at risk of the potentially fatal condition pre-eclampsia appear to benefit from doses of vitamins C and E.
The study, published in the Lancet, found that taking the supplements halved the chance of developing pre-eclampsia.
The condition involves the pregnant mother with a high-blood pressure, and can interfere with the feeding of the unborn child through the placenta.
Between five and ten per cent of women develop pre-eclampsia. The risk is higher if the condition runs in the family, it is the first pregnancy, and if the mother is over 40 years old or has diabetes or existing high blood pressure. If left untreated, it can cause internal bleeding, or even seizures (eclampsia).
The study, carried out at hospitals in London, looked at 346 women known to be at high-risk of the condition. Half were given 1000mg of vitamin C, and 400 IU of vitamin E a day, and compared with the other women, who received a placebo instead. Pre-eclampsia happened in 17% of the placebo group, compared with only 8% of those in the vitamin group.
Overall, the levels of two chemicals which relate to pre-eclampsia were reduced by 21% in the group. Although the report's authors said that they did not know why the vitamins should have such a pronounced effect, they suggested that, as powerful antioxidants, they may be mopping up free radicals which can damage the body and cause high blood pressure.
Professor Lucilla Poston, who co-ordinated the study, said: "We hope it may lead to an effective treatments which will prevent the onset of this devastating illness and its consequences."