Having a short cervix increases a pregnant woman’s chance of delivering prematurely, and a new study suggests going on bed rest does nothing to allay that risk.

Researchers found that women were more than twice as likely to give birth before 37 weeks when doctors told them to restrict some combination of their work, non-work and sexual activities.

“Way back when… people would have problems in pregnancy and we would have no interventions available, so we just told them to rest,” said Dr. Nathan Fox, an obstetrician/gynecologist at Maternal Fetal Medicine Associates and the Icahn School of Medicine at Mount Sinai in New York.

The new study, he told Reuters Health, “demonstrates that bed rest is something that is still prevalent, but unfortunately remains an intervention that is not proven to be effective and is potentially harmful.”

Although the findings don’t support bed rest for women with a short cervix, it’s unclear why restricting activity would increase the chance of premature birth, researchers said.

One possibility is that women who were put on bed rest had certain qualities that made them more likely to deliver early in the first place, but that were unknown to the researchers.

Or, the stress and anxiety that can come with being bedridden may actually increase certain risks, Dr. William Grobman from Northwestern University in Chicago and his colleagues wrote in Obstetrics & Gynecology.

“The point is, even if we didn’t imagine that there was any increased risk, we’re still incredibly far from showing any decrease in risk,” Grobman told Reuters Health.

Restricting activities may be recommended to any woman who doctors perceive to be at risk of premature birth, whether because of a short cervix or being pregnant with twins or triplets, for example.

However, Grobman said, “There’s not good evidence from good studies that activity restriction significantly improves outcomes.”

The new data come from a study that was originally designed to measure the effects of progesterone injections on the risk of premature birth among pregnant women with a short cervix.

For the analysis, 646 of the women answered weekly questions about whether doctors had placed them on any type of activity restriction. Of those, 39 percent were put on bed rest or given milder activity constraints during their pregnancies, at an average of 24 weeks.

The researchers tracked women through their pregnancies and found 37 percent of activity-restricted women gave birth prematurely, compared to 17 percent of those who were allowed to stay active and keep working.

That pattern held after Grobman’s team took into account women’s age and race, their medical history and any issues found on ultrasounds.

Bed rest can cause side effects, the researchers noted, such as bone loss, blood clots and loss of conditioning.

“Strict bed rest is really problematic,” said Fox, who wasn’t involved in the new research.

Grobman said any pregnant woman who is told to restrict her activity or stay in bed should discuss with her doctor whether there’s data to support that recommendation, given her condition.

“Without evidence of benefit and other potential risks, we really have to ask why we would be prescribing this,” he said.

Fox recommended that women who are inclined to reduce their activity because of pregnancy concerns not go overboard.

“If someone decides they want to stop jogging because they have a short cervix, I don’t have a problem with that,” he said. “If someone decides to go on strict bed rest, I do have a problem with that.”

SOURCE: Obstetrics & Gynecology, online May 8, 2013.