WASHINGTON – 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 worsen 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 moms-to-be is told to cut her activity 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 OB-GYNs.

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, like they are for other unproven treatments.

So why is rest prescribed so often?

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