Progesterone linked to decline in preterm births. (Women with Previous Preterm Birth).

Recipient of progesterone shots also had fewer babies born weighing less than 2,500 g.

SAN FRANCISCO -- Physicians finally have a proven tool that may prevent some preterm births in women with a history of preterm delivery: weekly shots of hydroxyprogesterone caproate.

The treatment showed promise in several small trials 30 years ago but was neglected for decades until investigators for the National Institutes of Health chose it as the most promising agent to test in a randomized, placebo-controlled trial at 19 medical centers in the Maternal-Fetal Medicine Units Network.

All patients in the current trial had at least one previous preterm delivery at a mean of 31 weeks' gestation.

Investigators stopped the trial early when it became clear that hydroxyprogesterone caproate decreased preterm births by a third, compared with placebo, and the researchers deemed it unethical not to treat all the subjects with the hormone. In addition, significantly fewer babies of subjects in the treated group were born weighing less than 2,500 g, Dr. Paul J. Meis reported during the annual meeting of the Society for Maternal-Fetal Medicine.

The results suggest that five or six women with a history of spontaneous preterm birth would need to be treated with hydroxyprogesterone caproate to prevent one birth before 37 weeks' gestation. Twelve such women would need to be treated to prevent one birth before 32 weeks' gestation, said Dr. Meis, who is a professor of ob.gyn. at Wake Forest University, Winston-Salem, N.C.

"It's the first hopeful treatment we've had for preventing preterm birth," he said.