Despite being commonly prescribed, activity restriction does not appear to lower the risk for preterm birth in pregnant women with a short cervix, and may even have the opposite effect, according to a new prospective study.

Researchers found women who walked fewer than 3500 steps per day delivered at an earlier gestational age than women who moved more (34.9 vs 37.7 weeks, P = .04) and were more likely to deliver before 34 weeks (47.3% versus 17.7%, P = .03).

“I think the information in this study is information all obstetricians should know,” said Celeste A. Green, MD, assistant professor of maternal-fetal medicine at Emory University School of Medicine, Atlanta, who was not involved in the research.

“The biggest takeaway for me is that it gives me objective targets and evidence-based recommendations to educate patients with short cervix when they inevitably ask, ‘Should I be on bed rest?’” she said.

The work adds to earlier research linking lower activity levels to a higher risk for preterm delivery, including in women with a short cervix.

Activity restriction in pregnancy dates back hundreds of years to the Tudor period in England, when expectant mothers would be confined to their home or room, explained Anthony Sciscione, director of maternal-fetal medicine at CristianaCare in Newark and lead author of the new study.

“There are no studies that have shown a benefit” to the practice “despite its common use even today,” Sciscione said. But several studies have suggested bed rest can have negative effects, he added, including bone loss, thromboembolism, maternal de-conditioning, and adverse psychosocial effects on the patient and their family.

For the AWARE study, in Obstetrics & Gynecology, Sciscione and his colleagues recruited 120 participants from two randomized trials of preterm birth prevention in women with a short cervix (defined as < 20 mm in one study and < 30 mm in the other).

The women were enrolled from 16 0/7 to 23 6/7 weeks of gestation and told to wear a Fitbit wrist accelerometer, which calculated their physical activity until delivery.

More than a quarter of the participants said at enrollment they had been placed on some type of activity restriction by their practitioner, and this proportion rose to 36% by the end of the study. There was no significant difference in median number of steps per day between patients who were put on activity restriction and those who were not. But 51% of the restricted group reported modifying their work schedules compared with 29% of the unrestricted group.

Fifty-five women of the 117 with complete Fitbit data took fewer than a median of 3500 steps a day, whereas 62 women took more. The researchers say they used this cut-off because a recent study found pregnant women took on average only 5246 steps per day.

Latency from time of enrollment to delivery — the primary outcome — was not different between the groups (hazard ratio, 0.95; 95% CI, 0.88-1.03), although women who took fewer steps delivered at a significantly earlier gestational age.

Sciscione stressed there was no difference in mean cervical length between the groups with higher and lower activity levels, arguing against reverse causation. There was also no difference in the recommendations for activity restriction based on cervical length.

“Bottomline, do not prescribe and rather strongly discourage activity restriction in pregnancy,” Sciscione said. “Decreasing activity may actually increase preterm birth and its antecedent adverse outcomes.”

Green cautioned that “although there is a lot of evidence demonstrating that bed rest/sedentary lifestyles are ill-advised, we should be careful not to apply the results of this study to every condition which makes a pregnancy high risk.”

“We physicians should be careful about recommending bed rest when it may in fact do more harm than good to our patients,” she concluded. “I do advise that if patients have a history of preterm delivery, they establish prenatal care as early as possible — a preconception visit is even better!”

The study did receive commercial funding. Sciscione and Green reported having no relevant financial disclosures.