New research is raising fresh concern that an age-old treatment for troubled pregnancies — bed rest — doesn't seem to prevent premature birth, and might even increase that risk.

Doctors have known for years that there's no good evidence that bed rest offers any benefit for certain pregnancy complications, and it can cause side effects in the mother, not to mention emotional and financial strain. Yet estimates suggest nearly 1 in 5 mothers-to-be is told to cut her activity — ranging from quitting work to actually staying in bed all day — at some point during pregnancy.

Now, spurred by the latest study, some specialists are issuing a call for strict studies to finally settle the controversy — and until then, for doctors not to assume that a prescription to take it easy can't hurt.

"Bed rest is misperceived as an inexpensive, innocuous, logical recommendation," Dr. Joseph Biggio Jr. of the University of Alabama at Birmingham wrote in the latest issue of Obstetrics & Gynecology, a journal read by thousands of practitioners.

In a separate review of past studies that failed to support bed rest, a trio of obstetricians and ethicists at the University of North Carolina, Chapel Hill, went a step further. They said it's not ethical to keep prescribing bed rest unless the women are enrolled in a research study, as they are for other unproven treatments.

So why is rest prescribed so often? There aren't a lot of good treatments to prevent prematurity and other problems.

"Patients want you to do something, and physicians want to do something," explained Dr. Catherine Spong, a maternal-fetal medicine specialist at the National Institutes of Health who co-authored the latest research.

Spong and colleagues took a closer look at a study of treatments for women at risk of premature birth because of an increasingly diagnosed complication called a short cervix.

Bed rest is a broad term that doesn't just mean staying in bed all the time — and during that treatment study, doctors were free to decide if the participants also should restrict their activities, essentially offering a real-world test of the effects. The prescriptions ranged from no sexual activity, to partial or complete work restrictions, to complete restriction of non-work activity as well.

Nearly 40 percent of the 646 pregnant women enrolled in the study were prescribed some type of activity restriction in the second or third trimester. Most were told to restrict all three types of activity — sexual, work and non-work.

The surprise: Some 37 percent of women who took the precautions had a premature baby, compared with just 17 percent of the women who didn't scale back, the researchers report in Obstetrics & Gynecology.

"The data suggests that bed rest does not prevent preterm birth in this high-risk population, but it doesn't definitively answer that question," cautioned Spong. She wants to see a more strict study — the kind that randomly assigns women to rest or not — to determine if there's a difference.

The study also raises questions about harm, although the women who rested merely may have been at higher risk for a preemie. They were older and had somewhat more serious cervical complications. But the researchers pointed to other studies that link bed rest to increased stress and anxiety, both of which are associated with prematurity and smaller babies.

Bed rest is well known to increase a mother's risk of a dangerous blood clot, as well as side effects including bone and muscle loss.

Interestingly, the new study found those who rested were more likely to have private insurance, raising questions about the influence of social rather than medicalfactors.