Taking high doses of vitamins C and E during pregnancy does not prevent preeclampsia, and the practice may pose risks for babies and pregnant women, according to two recent studies. In both trials, women took supplements containing 1,000 mg of vitamin C and 400 international units of vitamin E or a placebo. (For comparison, standard prenatal vitamins typically contain 100 to 250 mg of vitamin C and 100 international units of vitamin E.)

Low plasma levels of the antioxidant vitamins E and C have been measured in pregnant women with preeclampsia, supporting the theory that antioxidants may reduce the risk of preeclampsia in pregnant women and perinatal complications in their infants. In 1999 a small study of 283 British women found that vitamins C and E cut the risk of preeclampsia in half, suggesting a simple way to prevent this potentially fatal condition.

In a more recent study, reported in the Lancet, the same investigators recruited 2,400 women who were at risk for preeclampsia from a wide range of clinical conditions, including hypertension or previous preeclampsia. The women took vitamins C and E or a placebo daily, starting in the second trimester. The results differed strikingly from those of the earlier study-the incidence of preeclampsia was similar in both groups (15% in the treatment group versus 16% in the placebo group), but women who took the supplements gave birth to more low-birth-weight infants than did those in the placebo group (28% versus 24%). Those in the supplement group also required more medical treatments, such as antihypertensives and steroids.

The New England Journal of Medicine reported the results of a trial in Australia involving 1,877 healthy, nulliparous women who started vitamin C and E supplements or a placebo in the second trimester. The rates of preeclampsia (6% of supplement users versus 5% of those who received the placebo), infant deaths, and low-birth-weight infants did not differ significantly between the groups, but women in the supplement group were 1.5 times more likely to be hospitalized for hypertension, for reasons unknown to the authors.

Both teams of authors believe that high doses of vitamins C and E are contraindicated in pregnancy because they confer little benefit and may even be detrimental. But the usual prenatal multivitamins do not increase the risk of low-birth-weight infants or cause any harm.

Poston L, et al. Lancet 2006;367(9517): 1145-54;Rumbold AR, et al. N Engl J Med 2006;354(17):1796-806.