Any baby born before 37 completed weeks of pregnancy is considered a preterm (or premature) birth. Among those babies born preterm, approximately 83% are born between 32 and 36 weeks of gestation, 10% are born between 28 and 31 weeks of gestation, and 6% are born at less than 28 weeks gestation. All preterm babies are at risk for serious health problems, and the earlier they are born, the greater their risk of long-term disabilities and death.

Women who have had a preterm birth in the past are at a greatly increased risk for preterm birth in subsequent pregnancies. Other risk factors for preterm birth include a previous low birth weight delivery (less than 5 ½ pounds), multiple births, smoking, unplanned pregnancy, infections, and poor nutrition.

Despite the magnitude of this problem, there exist no truly effective treatments to prevent preterm deliveries. Recently, the results of several small trials have suggested that 17-alpha-hydroxyprogesterone caproate (17P), a type of progesterone, may reduce the incidence of preterm birth. Therefore, a group of researchers set out to conduct a larger study of 17P in women who were at high risk of preterm birth.

This study, published in the June 12, 2003 issue of the New England Journal of Medicine , found that weekly treatment with 17P between the 16th and 20th weeks of pregnancy significantly reduced the rate of preterm birth in high-risk women. It also reduced the likelihood of several complications for their infants.

About the study

The researchers conducted a double-blind, placebo-controlled trial involving 463 pregnant women with a documented history of spontaneous preterm birth. All of the women enrolled in the study were between the 16th and 20th weeks of their pregnancy and had a history of preterm birth. All of the women also underwent an ultrasound to confirm the stage of their pregnancy and to identify any possible fetal anomalies.

The women were then randomized to receive either weekly injections of 250 mg (milligrams) of 17P or weekly injections of a placebo (castor oil). These injections were continued until either the completion of the 36th week of pregnancy or birth. All of the women participating in the study continued to receive appropriate prenatal care from their chosen physicians. After delivery,obstetrical nurses documented the date, birth weight, neonatal course, and any complications for each birth.

The findings

The base-line characteristics for all 463 women in the study were similar. These characteristics included duration of current pregnancy, marital status, body-mass index, educational level, smoking status, and substance use during pregnancy.

The researchers found that treatment with 17P significantly reduced the incidence of delivery at less than 37 weeks of gestation (36.3% in the progesterone group vs 54.9% in the placebo group), delivery at less than 35 weeks of gestation (20.6% in the progesterone group vs 30.7% in the placebo group), and delivery at less than 32 weeks gestation (11.4% in the progesterone group vs 19.6% in the placebo group).

They also found that there was a significant reduction in the risk of low birth weight (below 5 ½ lbs.) for infants in the progesterone group. Infants of women treated with 17P also had significantly lower rates of necrotizing enterocolitis (an inflammation causing injury to the bowel), intraventricular hemorrhage (bleeding into the normal fluid spaces within the brain), and need for supplemental oxygen.

How does this affect you?

The study concluded that weekly injections of 17P beginning in the 16th through the 20th week of pregnancy and continuing to the 36th week of pregnancy or birth resulted in a substantial reduction in the rate of recurrent preterm birth among high-risk women. It also reduced the likelihood of several neonatal complications.

However, it is still too early to recommend widespread use of 17P. Until more is known about the hormone and how works, it is best to proceed cautiously. Thirty years ago, physicians routinely used another hormone—diethylstilbesterole (DES)—for the same purpose. It was later discovered that DES exposure led to an increased risk of a rare vaginal cancer in girls born to these women. Even worse, the drug was never shown to effectively reduce the risk of preterm birth. While 17P appears to work, and there is currently no reason to suspect that it will cause such serious problems down the road, the experience with DES serves as a reminder of the dangers inherit in the use of any medications, particularly during pregnancy.

In addition, although 17P significantly reduced the rate of preterm birth among those women in the study who received it, the overall rate of preterm birth was still quite high (36.3%) in this high-risk group. Therefore, more research is needed to identify both other causes of preterm birth and other methods of preventing it.