High doses of vitamin supplements may raise the risk of pre-eclampsia in pregnant women rather than protecting against it, research suggests.
The condition can be fatal to mother and child
Up to 25,000 British women every year are affected by pre-eclampsia, which causes blood pressure to rise to levels which threaten mother and baby.
Vitamin C and E were thought to cut the risk.
But a Lancet study by the charity Tommy's found women at high-risk should not take large doses of the vitamins.
Pre-eclampsia has been linked to the production of toxic molecules called free radicals by the placenta.
A previous small-scale study carried out by the same charity suggested vitamin C and E could tackle this risk by blocking the damage caused by free radicals.
However, the latest study, funded by the Wellcome Trust, found the reverse appeared to be true.
Some 2,400 expectant mothers with high blood pressure, kidney problems, clotting disorders or diabetes were given either extra vitamin supplements or a placebo.
The team discovered that pre-eclampsia appeared about a week earlier among those who received the vitamins - and they were also 15% more likely to deliver low birth-weight babies.
Researcher Professor Lucilla Poston said: "Our findings of an increase in low birth weight and an increased need for treatment for pre-eclampsia suggest that these high doses of vitamins C and E do not work in preventing pre-eclampsia in this high-risk group."
Her colleague Professor Andrew Shennan stressed there was no evidence that taking pregnancy-specific multivitamin preparations would produce the same results.
He stressed that it was important that pregnant women, and those trying for a baby continued to take folic acid supplements to reduce the risk of neural tube defects, such as spina bifida.
Michael Rich, of the charity Action on Pre-eclampsia, said: "Thousands of women throughout the UK and, indeed the world, will be massively disappointed to hear the news that the vitamins in pre-eclampsia trial has shown no benefit.
"We had high hopes for the use of high dose vitamins C and E in reducing the risk of developing pre-eclampsia.
"However, a good thing that has come out of the trial is that it will prevent the use of high dose vitamin C and E slipping into clinical practice.
"Careful management has long been seen as the best way to deal with pre-eclampsia - that remains the same."