Vaginal progesterone has been shown to reduce risk of premature birth in women at high risk of delivery before 33 weeks — but new research published in The Lancet suggests this treatment, which is widely recommended in the U.S., U.K., and Europe, is ineffective.

Researchers at the University of Edinburgh initially set out to investigate the safety of progesterone administration. They were under the guise that the therapy worked, and their main focus was to examine any longer term effects the treatment had children. "Despite recommendations for progesterone use there are few data on long-term benefit or safety for the baby beyond the neonatal period," the study authors wrote. "The objective of the study was to determine whether vaginal progesterone prophylaxis given to reduce the risk of preterm birth affects neonatal and childhood outcomes."

Half of the more than 1,200 women enrolled in the study were given progesterone and the others were given a placebo or "dummy pill." The study, which is reportedly the first to examine the effects of progesterone in babies born after pregnant women undergo therapy, mainly focused on the women who were considered to have a greater risk for preterm birth. Women considered high risk were women who either previously delivered a baby early or lost a baby late in pregnancy.

Researchers found that while progesterone appeared to be safe, the therapy did not reduce the risk of premature delivery. It also did not have a significant effect on childhood outcomes compared to the placebo group. This means in addition to being negatively associated with reduced risk of preterm birth, progesterone therapy did not offer long-term benefits or harm in two-year-old children. What's more, adverse effects in mothers and children were reported in 70 of 610 patients (11 percent) in the placebo group compared to 59 of 616 patients (10 percent) in the progesterone group.

"Although neurodevelopmental impairments were similarly distributed in each group, somatic impairments in renal, gastrointestinal, and respiratory systems though of low frequency, were more common in the progesterone group," researchers found.

Preterm birth, or a birth that takes place more than three weeks before the baby is due, is the most common cause of death among infants worldwide, according to the Centers for Disease Control and Prevention (CDC). In 2014, preterm birth affected about one of every 10 infants born in the U.S., and children born prematurely were more likely to suffer from neurodevelopment and cognitive impairments. These effects may increase with the degree of prematurity.

According to the CDC, preventing premature birth remains a challenge. However, women can reduce their risk for delivering a baby early if they quit smoking and avoid alcohol, as well as illicit drugs during pregnancy.

Source: Norman J, Marlow N, Messow C, et al. Vaginal Progesterone Prophylaxis for Preterm Birth (the OPPTIMUM study): A Multicentre, Randomized, Double-Blind Trial. The Lancet. 2016.