Treating high-risk pregnant women with the hormone progesterone cut their rate of premature delivery by 45 percent and helped lower the risk of breathing complications in their babies, U.S. researchers said on Wednesday.

The findings, published online in the journal Ultrasound in Obstetrics and Gynecology, offer a relatively simple way to prevent premature birth in women with a short cervix, a known cause of preterm birth.

"The study published today offers hope to women, families and children," Dr. Roberto Romero, chief of the perinatology research branch of the National Institutes of Health, said in a statement.

"Worldwide, more than 12 million premature babies - 500,000 of them in this country - are born each year, and the results are often tragic. Our clinical study clearly shows that it is possible to identify women at risk and reduce the rate of preterm delivery by nearly half, simply by treating women who have a short cervix with a natural hormone - progesterone," Romero said.

Babies born too early -- before the 33rd week of pregnancy -- have a higher risk of early death and long-term health and developmental problems.

In the United States, 12.8 percent of babies were born preterm in 2008, raising their risk of dying in their first year and having breathing difficulties, cerebral palsy, learning disabilities, blindness and deafness.

The study, done by researchers at the NIH and 44 medical centers around the world, looked at effects of giving progesterone to women with a short cervix, which is the part of the uterus that opens and shortens during labor.

Researchers suspect that women with a short cervix may not have enough of this hormone, and giving it during pregnancy in a gel form might help prolong their pregnancies.

The team studied 458 women with a short cervix who got either a vaginal gel containing progesterone or a placebo between the 19th and 23rd week of pregnancy.

Only 8.9 percent of women who got the gel delivered before the 33rd week of pregnancy, compared with 16.1 percent who were in the placebo group.

The treatment also helped babies. Only 3 percent of babies born to women treated with progesterone had respiratory distress syndrome compared with 7.6 percent of babies in the placebo group.

"We have for a long time known that short cervix is associated with an increased risk of preterm birth," said Dr. Ashley Roman of New York University's Langone Medical Center, who was not involved with the research.

Roman said other studies have shown that progesterone can cut the risk of premature birth in women with this problem. She said the NIH study is important because it shows that the treatment also reduces respiratory problems in newborn babies.

"Not only are fewer babies being delivered preterm, fewer babies have medical problems associated with prematurity," she said in a statement.