It’s one of the great frustrations of obstetric medicine: humans have been reproducing for hundreds of thousands of years, and yet doctors still haven't unraveled the mystery of why some women give birth well before their babies have fully developed in the womb.

Despite researchers' and physicians' best efforts, the rate of preterm births—defined as babies born before 37 weeks of gestation—climbed 30 percent from 1981 through 2006. Since then, it has fallen slightly; in 2008, the most recent year for which figures are available, 12.3 percent of women delivered babies before 37 weeks, the time when a fetus's lungs typically reach maturity. That's down from 12.8 percent in 2006. Though the causes of this overall increase are not fully known, the high rate of multiple births, the fact that more women are having children later in life, and an uptick in the number of induced labors and voluntary scheduled C-sections are partly to blame.

A new study published online in Ultrasound in Obstetrics and Gynecology offers ob/gyns some new options in caring for mothers-to-be. The research, conducted by the National Institutes of Health in conjunction with a pharmaceutical company, shows that a progesterone gel can help women at risk of premature birth extend their pregnancies and boost the health of their newborns. "This is a major breakthrough in maternal and child health," said Roberto Romero, chief of the perinatology research branch of the National Institute of Child Health and Human Development and a co-author of the paper. "The prevention of preterm birth has been the most difficult challenge facing obstetrics and prenatal care and has been a goal of NICHD of the last 50 years." The study followed 458 pregnant women who had been diagnosed via intravaginal ultrasound during their second trimester as having a short cervix, a major predictor of preterm delivery. Of those women, 235 were treated with a vaginal progesterone gel developed by Columbia Laboratories, a pharmaceutical company that collaborated on the study with the NIH, and 223 volunteers were treated with a placebo gel. In the progesterone group, 8.9 percent delivered babies prior to 33 weeks of gestation vs. 16 percent in the placebo group. By 37 weeks of gestation, 30 percent of women in the progesterone group had given birth, as compared to 33 percent in the placebo group. The babies born to the progesterone group were also less likely to suffer complications such as respiratory distress syndrome, a breathing problem caused by underdeveloped lungs.

While previous studies have shown that progesterone could prolong a high-risk pregnancy, this is the first major study to also demonstrate improved outcomes for newborns. Some obstetricians are beginning to call for all pregnant women to undergo ultrasound screening during the second trimester to check for a shortening of the cervix. "This is an important study since prior to this our best way of identifying those at highest risk for pre-term birth were those with a prior history [of preterm birth],” said Ronald Wapner, director of the division of maternal fetal medicine at Columbia University Medical Center, who was not involved in the research. “By screening patients with vaginal ultrasound to look at cervical length we can identify and treat at-risk pregnancies before they have a preterm baby.” In December 2010, a group of researchers publishing in the same journal reported that universal cervical-length screening of pregnant women would be cost-effective and may prevent 22 cases of neonatal death for every 100,000 women screened. The current standard of practice for ultrasound examination in the second and third trimesters is to examine the uterine cervix, as recommended by the Guidelines of the American Institute of Ultrasound in Medicine. However, the guidelines do not require measuring cervical length, perhaps because there was little that could be done about it. "I believe that the guidelines need to change to implement universal screening of cervical length in the midtrimester of pregnancy,” says Romero. "Women with a short cervix could then be offered vaginal progesterone to prevent preterm birth."