Nifedipine, the muscle relaxant generally used to prevent premature birth, is not all that effective.

When a pregnant woman goes into early labor, her obstetrician may give her drugs to quiet the woman's uterus and prevent premature birth.

New research shows, however, that Nfedipine works no better than a placebo at maintaining pregnancy after the initial bout of preterm labor is halted, say scientists at the Stanford University School of Medicine, Lucile Packard Children's Hospital and Santa Clara Valley Medical Center. The new trial is the first-ever placebo-controlled test of nifedipine, a muscle relaxant originally developed to lower blood pressure, and its effect on premature delivery with prolonged treatment.

"Medication use should be minimized in pregnancy unless it's clearly indicated," said Deirdre Lyell, assistant professor of obstetrics and gynecology at Stanford and the study's lead author. Serious side effects of nifedipine in pregnancy are rare, Lyell said, but even a low risk isn't worthwhile if the drug has no benefit. "We all want to prevent preterm birth, but prolonged treatment with nifedipine doesn't appear to be an answer."

The findings will appear in the December issue of the journal Obstetrics and Gynecology.

Preterm births, defined as deliveries before 37 weeks of pregnancy, are on the rise in the United States. Pregnancy normally lasts 40 weeks. A report released earlier in November by the March of Dimes gives the United States a "D" grade for its rate of preterm births, which increased between 1981 and 2005 from 9.4 to 12.7 percent of all births. Smoking, lack of insurance and early intervention by physicians were cited as major contributing factors.