Taking high doses of vitamins C and E during pregnancy may not change a woman's risk of preterm delivery, according to a new study.

The findings suggest that despite evidence linking vitamin C deficiency to preterm birth, supplements of the antioxidant are not an effective means of prevention -- at least in women at average risk of premature delivery.

Together with results from another recent trial, the findings argue against further study of vitamins C and E in these women, the researchers report in the journal Obstetrics & Gynecology.

Some past studies have found a connection between vitamin C deficiency and an elevated risk of preterm birth, including those caused by what's known as premature rupture of membranes -- where a woman's water breaks before the pregnancy has reached full-term and labor has begun.

In addition, vitamin C has been thought to play a role in maintaining the placenta and the membranes that surround the fetus during pregnancy, possibly by limiting so-called oxidative stress.

So researchers have been studying whether vitamins C and E, both antioxidants, might help lower the risk of not only preterm birth but also preeclampsia -- a pregnancy complication marked by a sudden increase in blood pressure and a buildup of protein in the urine.

This latest study is a new analysis from a U.S. government-funded clinical trial published earlier this year showing no benefit of vitamins C and E in lowering preeclampsia risk.

For the trial, researchers randomly assigned 10,154 pregnant women to take either a combination of vitamins C and E or inactive placebo pills beginning somewhere between the 9th and 16th week of pregnancy. All of the women had uncomplicated pregnancies and were not at elevated risk of preterm delivery.

Women in the vitamin group took 1,000 mg of vitamin C and 400 IU of vitamin E per day -- much higher than the 85 mg of vitamin C and 22 IU (or about 15 mg) of vitamin E generally recommended during pregnancy.

Overall, 7 percent of women in both the vitamin and placebo groups had a preterm birth.

The finding echoes that of another recent clinical trial where women on the same doses of vitamins C and E had a similar preterm delivery rate as those given placebo pills, note the researchers, led by Dr. John C. Hauth of the University of Alabama at Birmingham.

And in that trial, women on the vitamins had a higher rate of preterm birth caused by premature rupture of membranes -- just under 5 percent, versus roughly 2 percent in the placebo group.

The lack of benefits in this latest study is not surprising, according to Drs. Isaac Manyonda and Vikram Talaulikar of St. George's University of London in the UK, who were not involved in the study.

This is because the very idea that these vitamins, given at the stated doses, could function as antioxidants is fundamentally flawed, the researchers told Reuters Health in an email.

They pointed out that animal studies in which vitamin C has shown potentially disease-preventing antioxidant activity have used very high doses of the vitamin -- what in humans would amount to as much as 20 grams per day. The human body cannot absorb such levels of vitamin C taken orally.

Moreover, the current findings are in line with a number of clinical trials that have failed to show a benefit of vitamins C and E in lowering the risk of preeclampsia.

There are now eight major international studies that have been completed and published over the past ten years, Manyonda and Talaulikar noted, and none have shown a benefit for vitamins C and E in the prevention or amelioration of preeclampsia.

Along with the lack of effect on preterm births, Hauth's team also found no benefits as far as a number of other pregnancy or newborn complications -- including miscarriage, stillbirth and low birthweight.

Our results, the researchers write, taken in context with similar trials regarding vitamin C and E supplementation, do not support either the clinical use for prevention of spontaneous preterm birth or its neonatal sequelae or further trials of this treatment in similar populations at low risk.

SOURCE: link.reuters.com/typ86n Obstetrics & Gynecology, September 2010.