BOSTON -- More and more women are giving birth to preemies, and now, there finally may be the first effective treatment to stop babies from being born too soon.

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Doctors Discuss Study

Injections of the common hormone progesterone worked so well in women at high risk of giving birth early, that a large study was stopped early and the results were made known Thursday.

In the last decade, the number of babies born before their time has increased 27 percent.

Now doctors may have a new weapon.

"It really has the potential to represent an unbelievably important breakthrough," Massachusetts General Hospital Dr. Fredric Frigoletto said.

Researchers gave 306 high-risk women -- those with a history of early delivery -- weekly injections of progesterone. Another 153 women got a placebo. They found the hormone reduced the risk of premature birth by 34 percent.

Experts in the field said that the results are surprising.

"This is very significant, and the intervention is fairly easy to apply and not very expensive," Frigoletto said.

Researchers believe progesterone is safe for use during pregnancy, as it has been safely used to treat infertility for many years.

While there is no known cause for most premature births, the rise could be due to more older women having babies and the epidemic of obesity, experts said.

Babies born prematurely are at increased risk for neurological, hearing and behavioral problems. So prevention is key, and soon doctors may be offering those at risk of premature birth a new option.

"We have 4 million births a year, 10 or 11 percent are preterm. If it stands the test of scrutiny then I think it will become standard of practice," Frigoletto said.

Progesterone injections were toyed with in the '60s and '70s as a means to prevent early birth, but its effectiveness was never really studied.

The results of the study, which was conducted in 19 sites around the country, were announced Thursday at a national meeting of obstetricians in San Francisco.