Progesterone product may reduce premature births. (Full-Length Pregnancy).

A therapy first tried in the 1960s can both extend a pregnancy in a woman who is at risk of giving birth prematurely and reduce a newborn's risk of complications, a new study finds. The drug, 17-alpha-hydroxy-progesterone caproate (17P), is a natural metabolic product of the female hormone progesterone. In previous small-scale tests, it showed mixed results.

For the new study, researchers enrolled 463 pregnant women who had delivered a previous child prematurely--after an average of 31 weeks. A full-term pregnancy lasts 40 weeks, and a delivery is considered premature if it occurs before 37 weeks. Being born even a few weeks ahead of schedule can slow a child's development.

The researchers randomly assigned two-thirds of the women to receive weekly injections of 17P beginning in the second trimester and the other women to get inert shots.

Of the women receiving the drug, 36 percent gave birth prematurely, compared with 55 percent of those receiving the placebo shots, says Paul J. Meis of Wake Forest University School of Medicine in Winston-Salem, N.C. Also, babies of mothers getting 17P weighed more and averaged fewer complications, such as brain hemorrhages and serious intestinal problems, than babies of the placebo group did, Meis and his colleagues report in the June 12 New England Journal of Medicine. It's still unclear how supplements of this first cousin of progesterone can keep a risky pregnancy on an even keel, Meis says.

"This is a very exciting study," says Peter S. Bernstein of the Albert Einstein College of Medicine in New York. Currently, physicians have no way to stop premature labor once it starts, so a drug to avoid it may represent "one magic bullet," he says. However, only further study will reveal which at-risk women are most likely to benefit from 17P injections, Bernstein says.

Study coauthor Alan M. Peaceman of Northwestern University School of Medicine in Chicago agrees that researchers now need to "hone down the population of women" whom 17P will benefit.

Toward that end, Meis' team plans to test the drug on women pregnant with twins, a group prone to premature births. Meanwhile, Meis and Peaceman both say that they would be willing to prescribe 17P for women with a history of premature births. However, although the drug is approved for fertility treatment, it isn't commercially available.

Others are less sure about giving 17P. Jeffrey C. King of New York Medical College in New York says that although the new research is well done, it needs to be replicated. "I would hate to see [17P] blindly adopted by lots of practitioners because they have nothing else to offer patients," he says.

"Obstetrics has a somewhat dark history of rapidly adopting technologies and treatments that are subsequently shown to be not effective and in some eases dangerous," says King.

Consider the synthetic estrogen called diethylstilbestrol (DES). It was prescribed from 1940 to 1971 to prevent complications in pregnancies but turned out to increase cancer risk. Meis acknowledges that this outcome may have discouraged research into 17P over the past 3 decades, even though the hormones differ chemically.