Daily use of a vaginal gel containing the hormone progesterone in the second half of pregnancy substantially reduces the risk of premature delivery in a specific group of women, according to a new study.

Women with a short cervix — the necklike structure connecting the base of the womb to the vagina — have nearly a 50 percent chance of delivering a baby at least two months early. Between 2 and 3 percent of pregnant women fall into that category, and they are the ones who might benefit from a progesterone gel.

About one in eight infants is born at less than 37 weeks’ gestation and is at increased risk for infection, breathing problems, retardation and death. The normal length of pregnancy is 40 weeks.

“I believe this will change practice. I believe screening for a short cervix and treating it will become routine in pregnancy,” said Roberto Romero, a researcher at the National Institute of Child Health and Human Development, who led the clinical experiment.

The study, published online Wednesday by the journal Ultrasound in Obstetrics and Gynecology, involved 32,000 pregnant women in the United States and nine other countries. They each underwent a vaginal ultrasound as part of their prenatal care. About 2.3 percent had a cervix measuring 10 to 20 millimeters (0.4 to 0.8 inches), considered short.

The women in that group were then randomly assigned to use either a progesterone or a placebo gel — they didn’t know which they had — internally every day from the 20th to the 38th week of pregnancy.

When the study was over, 9 percent of women using the progesterone gel had delivered before 33 weeks, compared with 16 percent using placebo. Five percent of women using progesterone delivered before 28 weeks — an event that puts newborns at particularly high risk for problems — compared with 10 percent using placebo.

More important, the progesterone gel reduced the rate of complications to the babies.

In the women taking progesterone, 3 percent had babies with respiratory distress syndrome (the most common complication) and 8 percent had babies who suffered a serious illness or died. Among the women taking placebo, the rates were 8 percent and 14 percent.

While delivery at less than 37 weeks gestation is the most common definition of “preterm,” few babies born close to that cutoff have serious problems. Complications become more common at less than 35 weeks.

Curiously, when birth at 37 weeks is used as the endpoint, progesterone gel and placebo show no difference even though it is highly beneficial with a 35-week endpoint. What that suggests, Romero said, is that progesterone gel is pushing women with short cervixes out of the danger zone and closer to term, although many still deliver somewhat early.

Progesterone is responsible for keeping the cervix long and closed, and it also helps keep the muscles of the uterus inactive during pregnancy. A short cervix may be evidence that the tissue is less responsive to progesterone. The gel helps overcome that by providing more of the hormone.

About 40 percent of women who deliver early have a short cervix. While that condition is the strongest predictor of preterm delivery, it is not the only one. Women with preeclampsia, diabetes and anatomically abnormal uteri are also at risk of delivering early.