SAN FRANCISCO - In a groundbreaking study, researchers said yesterday the hormone progesterone could help prevent premature births in a surprisingly high number of high-risk pregnancies.

"The evidence of this treatment's effectiveness was so dramatic, the research was stopped early," said Dr. Paul Meis of Wake Forest University Baptist Medical Center.

Progesterone is naturally produced by the ovaries, and it softens the uterus lining into a spongy bed that holds a fertilized egg. Doctors have prescribed it for years to help infertile and menopausal women, but there had been limited research into its effect on premature births.

The study found weekly injections of the hormone reduced the chance of premature births by 34 percent in 306 high-risk women. An additional 153 women were injected with a placebo. All the women previously had given birth prematurely, the single biggest indication of risk.

"The results are so good that it's surprising," said Fredric Frigoletto, chief of obstetrics at Massachusetts General Hospital in Boston. "No intervention that we have ever applied has had any measurable effect. This is very good news."

Meis said progesterone previously was toyed with as a preventive for premature births in the 1960s and 1970s, but no one had completed a serious study on the subject.

"I think it's going to awaken people to an old idea that kind of slipped away," said Alan DeCherney, chairman of the obstetrics and gynecology department at the University of California, Los Angeles.

Emile Papiernik, a French obstetrician, conducted a tiny progesterone study in 1970 that showed promise. But he said he could not interest any pharmaceutical companies or government agencies to finance a more comprehensive experiment.

The latest study is being carried out by 19 hospitals across the country under supervision by the National Institutes of Health. Meis presented the results at the annual meeting of the Society for Maternal-Fetal Medicine.

In 2001, about 476,000 babies were born prematurely in the United States, a 27 percent increase since 1981, according to the March of Dimes. One in eight babies was born before the 37th week of pregnancy, which is considered full term.

Babies born prematurely are at increased risk for neurological, hearing and behavioral problems. The average hospital charge in 2000 for a premature baby was $58,000, compared with $4,300 for a typical newborn, according to the March of Dimes.