Weekly injections for pregnant women of a drug derived from the hormone progesterone could have prevented nearly 10,000 premature births in 2002, U.S. researchers reported on Monday.

And that just includes pregnancies involving single births, not the multiple pregnancies that often lead to preterm births, the team of researchers reports in the February issue of the journal Obstetrics & Gynecology.

The researchers based their projection on a study using a hormone derivative known as 17 alpha-hydroxyprogesterone caproate, or “17P” for short.

Although researchers are not quite sure how it prevents early delivery, some studies had suggested it may help. The 2003 U.S. government study of 450 pregnant women showed it reduced preterm births - before 37 weeks - by one third.

The women were considered at high risk of preterm delivery because they had delivered a previous premature baby.

Joann Petrini and colleagues from the March of Dimes charity, the Centers for Disease Control and Prevention, the National Institute of Child Health and Human Development and other centers estimated that in 2002, about 30,000 women who had premature babies may have been helped by 17P.

If one third of them carried to term, that would translate to 10,000 prevented premature births, the researchers said.

“Prematurity is a serious problem that affects 1 in 8 babies in the United States, and 17P offers promise in an area where there have been few successes,” Petrini said in a statement.

The March of Dimes says the rate of premature birth in the United States has risen 29 percent since 1981, with more than 470,000 babies born prematurely each year.

Premature babies are at increased risk of infection, short-term illness and death, as well as health problems later in life.