“It’s time to re-examine what normal and abnormal is, and revise our guidelines based on modern obstetric population,” said Dr. Yvonne W. Cheng, the lead author of the study and an associate professor of obstetrics and gynecology at the University of California, San Francisco.

The research is part of a growing body of evidence suggesting that a normal second stage of labor is now longer than it was decades ago. In 2010, a study of more than 62,000 women found it was as long as 3.6 hours for first-time mothers after an epidural, and 2.8 hours for women who did not get one.

A 2012 summary of a joint meeting of ACOG and the National Institutes of Health concluded that adequate time for each labor stage “appears to be longer than traditionally estimated.” For the second stage, it is closer to four hours for first-time mothers who had epidurals, and three hours for those who did not.

But this latest study is the first to suggest such an extended second stage may be ordinary.

“One of the messages of this study is, sit on your hands a little longer, don’t rush into an instrumental vaginal delivery or a cesarean, because really everything could be fine,” said Dr. Barbara Leighton, a professor of anesthesiology at Washington University in St. Louis School of Medicine, who has researched the effects of epidurals on labor.

But while Dr. Leighton supports revising ACOG recommendations, she believes that the current study did not prove that longer labor is caused by epidural anesthesia. Women who request anesthesia may be predisposed to longer labor for other reasons, she said.

Dr. Jeffrey Ecker, the chairman of the committee on obstetrics practice for ACOG, said today’s clinicians are “increasingly recognizing there can be healthy outcomes and vaginal deliveries of healthy babies when the second stage extends beyond how it’s traditionally been defined.”

He added, “Often what’s best and most appropriate — and most difficult — during labor is patience.” He would not say whether a revision of guidelines is in the works.