WEDNESDAY, Nov. 26 (HealthDay News) -- A muscle relaxant drug widely used to prevent premature birth when women go into early labor is no more effective than a placebo at maintaining pregnancy, after the initial spell of preterm labor is stopped.

So say U.S. researchers who conducted the first ever placebo-controlled test of nifedipine and its effect on premature delivery with prolonged treatment. The study included 71 women who'd been successfully treated for preterm labor between 24 and 34 weeks of pregnancy. The women were randomly assigned to receive either nifedipine or placebo every six hours until 37 weeks of pregnancy or delivery, whichever came first.

The team found no differences between the two groups of women in terms of whether their pregnancies lasted until 37 weeks; of how long delivery was delayed; or in their babies' gestational age, birth weight and complications of prematurity. About 40 percent of women in both groups reached 37 weeks of pregnancy, with delivery delayed an average of one month.

The study is published in the December issue of the journal Obstetrics & Gynecology.

"Medication use should be minimized in pregnancy unless it's clearly indicated," lead author Dr. Deirdre Lyell, an assistant professor of obstetrics and gynecology at Stanford University School of Medicine, noted in a school news release.

Serious side effects are rare in pregnant women treated with nifedipine, she said, but even a low risk isn't worthwhile if a drug provides no benefit.

"We all want to prevent preterm birth, but prolonged treatment with nifedipine doesn't appear to be the answer," said Lyell, who believes a larger study of nifedipine is warranted.

More information

The U.S. National Institute of Child Health and Human Development has more about preterm labor and birth.