Now a new randomized double-blinded trial, published online in The American Journal of Obstetrics & Gynecology, reports that progesterone in its injected form does not work for a much larger risk group: the 10 percent of women with a cervix shorter than 30 millimeters. These women have about the same risk of having a preterm birth — around 25 percent — as those who have already had one.
"The study emphasizes that because progesterone works for one group at risk for preterm birth doesn't mean that it can automatically be extended to others," said the lead author of the study, Dr. William A. Grobman, a professor of obstetrics and gynecology at Northwestern.
Researchers randomly assigned 657 women to a weekly injection of a form of progesterone or to a placebo shot. All were pregnant for the first time with a single baby, and each had a cervical length of less than 30 millimeters at 16 to 22 weeks' gestation. The women were followed through discharge from the hospital.
The scientists were unable to find any significant differences in birth outcomes between the two groups. Of those who took the progesterone, 25.1 percent delivered prematurely (at less than 37 weeks) and so did 24.2 percent of those on the placebo.
Image: Pregnant belly, via Shutterstock