"The phenomenon of preterm birth is complex and there are probably multiple pathways involved so, ultimately, we will probably have to have multiple solutions," said Dr. John Thorp, a co-author on the twin study and a professor of obstetrics and gynecology at the University of North Carolina at Chapel Hill.

Preterm birth is defined as birth that occurs before the 37th week of pregnancy. About 12 percent -- or one in eight -- of babies born in the United States are born preterm, according to the March of Dimes. The earlier a baby is born, the more likely it is that the baby will have health or developmental problems.

In the first study, researchers randomly assigned 661 women who were pregnant with twins to receive weekly injections of 17 alpha-hydroxyprogesterone caproate (17P) or a placebo injection. The injections began at 16 to 20 weeks of gestation and ended at 35 weeks.

The researchers found that 17P was not effective in preventing preterm birth. Delivery or fetal death before 35 weeks' gestation occurred in 41.5 percent of the pregnancies in the 17P group and in 37.3 percent of the pregnancies receiving placebo. Serious adverse events to the baby occurred in 20 percent of the 17P group and 18 percent of the placebo group, according to the study.

"This therapy that we hoped would be a magic bullet in the prevention of preterm birth was not effective for twins," Thorp said.

In the second study, researchers compared the use of vaginal progesterone to a placebo in women with a short cervix. A short cervix develops sometime during early to mid-pregnancy, according to Dr. Robert Welch, chairman and program director of obstetrics and gynecology at St. John's Providence Hospital in Southfield, Mich. Experts aren't sure exactly what causes a woman to develop a short cervix, but it's not something that can be predicted ahead of time, he noted.

For this study, British researchers measured the cervical length of nearly 25,000 pregnant women. They found 413 (1.7 percent) had a short cervix. From that group, they randomized 250 of the women to receive either 200 milligrams of vaginal progesterone, administered nightly, or a placebo.

In this population, researchers found a significant benefit from the progesterone therapy. Delivery before 34 weeks of gestation occurred in 19.2 percent of those receiving progesterone vs. 34.4 percent of those on placebo.

"This study certainly gives hope to women diagnosed with short cervix," said Welch, who added that progesterone is a fairly inexpensive treatment, generally costing less than $100 for therapy throughout pregnancy. And, he said, it appears to be safe to use in pregnancy.

As to why one study showed a benefit while the other didn't, Thorp said it's likely that there are likely numerous pathways or mechanisms that lead to preterm birth, and there will likely have to be numerous therapies to prevent each type of preterm birth. It could also be that one study used injectable progesterone, while the other was administered vaginally, or it might be that twins or triplets need higher doses of progesterone.

"There are a lot of things we try to prevent preterm birth, but few are effective. In women with a history of preterm delivery, preliminary studies have shown progesterone can reduce preterm birth and there are no birth defects associated with it. We don't have anything else to offer women with a history of preterm birth. And we haven't had really had anything to offer women with a short cervix up to this point. Unfortunately, it looks like with twins, we still don't have a lot to offer," Welch said.

More information

To learn more about preterm labor and birth, visit the March of Dimes