BOSTON - Though medical authorities have dropped their reservations about giving women epidurals early in labor, some doctors and pregnant women still worry the painkiller's numbing effect will prolong labor and prompt a Caesarean section.

A study reported in the New England Journal of Medicine may finally dispel those fears.

"Women often feel guilty or weak when they request an epidural early in labor. I hope this study will help women see that there is no shame in asking for an epidural," said lead author Dr. Cynthia Wong of Northwestern University in Chicago. "The message for women and their obstetricians and gynecologists is that there is no reason why women who want an epidural should not get it when they first request it."

More than 3.5 million women go into labor each year in this country. Epidural use has greatly expanded over the past decade, to more than 1.5 million women annually.

Epidurals deliver numbing medicine through a skinny plastic tube that is threaded into the back, close to spinal nerves, mostly bypassing the mother's bloodstream. More recent techniques, sometimes called "walking epidurals," provide lighter doses, allow women to push, and even enable them to walk throughout labor.

Doctors have welcomed epidurals as an alternative to "systemic" pain medicine through the bloodstream, which can leave a woman feeling nauseated and doped-up and even enter the baby's body.

The Northwestern University study tested a type of low-dose pain relief known as a "combined spinal epidural." A small dose of pain reliever is first injected into the spinal fluid, and the epidural is later fed through the same hole into a space a bit farther from the spine.

In this study, 728 women in first-time labor were divided into two groups. One group received the spinal shot and then got epidurals when the cervix was dilated to about two centimeters. The other group initially received pain-relieving medicine directly into their bloodstreams, and put off epidurals until four centimeters if they could tolerate the pain.

In the end, the C-section rate was statistically a tie: 18 percent in the early epidural group and 21 percent in the delayed group.

The early epidural group also delivered 90 minutes sooner than the other women, who averaged eight hours in labor. However, Dr. Gary Hankins, an obstetrician at the University of Texas Medical Branch at Galveston who helped shape the medical guidelines, said it is not clear if that finding would apply broadly.

"There's really no reason to withhold the treatment if a woman is in terrible pain," said Dr. David Birnbach, a University of Miami anesthesiologist who is a member of the College of Obstetricians.