NEW YORK (AP) -- A new female sex hormone promises to save thousands of babies who now are born prematurely, a drug firm said Monday.

It can work in either of two ways to save babies who are born too soon from premature labor.  It sometimes stops the premature labor contractions, thus giving the infant more time to develop in the womb.

Or it can make the premature birth easier so the babies are not fatally injured before being born.

The hormone is Relaxin, a natural sex hormone playing a vital role in the birth process.  Warner-Chilcott Laboratories of New York announced success in making supplies available for the first time, under the name Releasin, to help reduce the loss of life from premature labor.

In normal pregnancy, Relaxin stretches pelvic ligaments and softens tissues of the womb and birth canal at the time of delivery.  It also makes labor contractions less severe.  Lack of Relaxin is believed responsible for premature labor and stillbirths.

In many cases of premature labor, there are strong contractions but the birth canal is not ready and the baby is fatally injured.

In some cases when the hormone halted the labor, the normal delivery later on was quick and nearly painless, said Dr. George H. Mangun, director of research of the laboratories.

In clinical trials, Relaxin was credited with saving seven out of 10 babies who otherwise probably would have been stillborn.

The hormone drug is still scarce, and expensive.  Costs of treating one case in a hospital are estimated at from $150 to $175.

Some 200,000 babies are stillborn each year from premature labor.  At present there will be enough Relaxin to treat only some 18,000 women annually, but this could mean saving 12,000 to 14,000 infants, Dr. Mangun said.

The hormone is obtained from the ovaries of pregnant sows, involving elaborate work to separate out purified Relaxin.  From 500 sows, only enough is obtained to treat 6 to 8 women.  More efficient sources of supply are being sought.

The hormone appears most effective when given to women threatened with premature labor in the 29th to 32nd weeks or pregnancy.

In time, when supplies increase, it might be used to induce delivery at desired times, and possibly make birth easier.