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 - 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 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 farther: 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? 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 - what's usually considered bed rest.

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 prove 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.

Plus, 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 medical factors.

The American College of Obstetricians and Gynecologists says bed rest shouldn't be "routinely recommended" for prevention of preterm birth. And women face a tough decision when their doctors do bring it up.

Sandy Lutton spent the last 18 weeks of her pregnancy lying flat in bed, forbidden even to prop up with her laptop, hoping it would prevent her twins from being born too soon.

"I'm not going to sugarcoat it, it was stressful," said the McLean, Va., woman, whose twins, now 2, were born healthy. "I had a lot of time to sit and worry."

Her first son had been born on time, but she'd lost a second baby due to a weak cervix. With the twins, her doctors stitched her cervix closed and recommended strict bed rest, while making it clear there was no proof it would make a difference. Lutton bombarded them with questions, but eventually decided she had to try. Ultrasound exams showed her cervix stayed fine until doctors removed the stitch and delivered her babies; she even cheated with a little extra movement on exam days.

Amid the uncertainty, March of Dimes medical adviser Dr. Siobhan Dolan said women shouldn't hesitate to ask their doctors about the pros and cons of restricting activity.

"Sometimes you feel less regret if you did something even though it didn't change the outcome," acknowledged Dolan, who herself has prescribed bed rest less often in recent years.