A new study has shown that nifedipine, a drug which is widely used to prevent premature births, works no better than a placebo at maintaining pregnancy after the initial bout of pre-term labour is halted.

Medication use should be minimized in pregnancy unless it's clearly indicated, said Dr. Deirdre Lyell, assistant professor of obstetrics and gynecology at Stanford University School of Medicine, and the study's lead author.

Working in collaboration with researchers from Lucile Packard Children's Hospital and Santa Clara Valley Medical Center, Lyell recruited 71 women who had been successfully treated for pre-term labour between 24 and 34 weeks of pregnancy.

The women were then randomly assigned to receive doses of nifedipine or placebo every six hours until 37 weeks of pregnancy or until delivery, whichever came first. The researchers hoped that nifedipine would prevent pre-term labour from re-starting.

Lyell and her colleagues evaluated whether subjects' pregnancies lasted to 37 weeks and measured how long delivery was delayed. They also noted the babies' gestational age at delivery, birth weight, and complications of pre-maturity-such as neurological disorders, chronic lung disease, and vision and hearing problems.

She said that there were no differences between nifedipine and placebo for any measurement.

She revealed that about 40 percent of women in both groups reached 37 weeks of pregnancy, with delivery delayed an average of a month, and that babies' average health was the same in both groups.