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Giving pregnant women epidural spinal anesthesia to ease their pain during the late stages of labor and delivery may not prolong the birthing process, a new experiment suggests.

"Many obstetric providers believe that the numbness and weakness in a woman's legs from epidural medications may affect a woman's ability to push out a baby," said senior study author Philip Hess, an anesthesiology researcher at Harvard Medical School and Beth Israel Deaconess Medical Center in Boston.

"Our study focused on this question, and we found that this is not true," Hess said by email. "Continuing epidural medication while pushing had no effect whatsoever on the time it took a woman to deliver her baby, or any other measured variable."

For the experiment, researchers randomly assigned 400 first-time mothers at a maternity hospital in Nanjing, China, to either continue or discontinue epidurals started earlier in labor when it came time to push.

The doctors and mothers didn't know whether pain medication was continued during the second stage of labor, which starts when the cervix is fully dilated and women can begin to push.

That's because all of the women retained the catheters that had been inserted to deliver pain medication earlier in labor. Half of them received pain medicine through the catheters while they pushed, and the rest got saline solutions that looked identical but contained no drugs.

With pain relief, the average time it took women to deliver babies once their cervix fully dilated was 52 minutes, compared to 51 minutes with saline, a difference that was too small to rule out the possibility that it was due to chance, researchers report in Obstetrics & Gynecology.

There also wasn't a meaningful difference in the proportion of women who had what's known as spontaneous vaginal deliveries, when women don't need interventions such as drugs to induce labor, or forceps or vacuums to help remove babies, or ­Caesarean-section deliveries.

With an epidural, 193 women, or 97 percent, had spontaneous vaginal deliveries, compared to 198, or 99 percent, with saline.

Not surprisingly, women reported higher pain levels and less satisfaction with their pain relief when they didn't get continued epidural medication during delivery.

Doctors stopped epidurals during the second stage of labor in 38 cases when delivery wasn't progressing quickly enough. It turned out that 17 of these patients were receiving saline, and 21 were getting epidural pain medicines.

One limitation of the study is that it was done at a single hospital: Labor and delivery or epidural protocols might differ elsewhere, researchers note. The results in Chinese first-time mothers also might not reflect what would happen for women with different racial or ethnic backgrounds or for women who had already had at least one baby.

Still, epidurals used today are much different from those of a generation ago, and the findings suggest it may be time for doctors to rethink their approach to pain relief during delivery, Hess said.

"Today's epidural pain relief can be targeted to providing good pain relief with minimal weakness," Hess said. "So much so that in some centers like my own, women are strong enough to be able to walk" even as their pain is being eased.