The researchers also found that babies born in the later group were slightly bigger and healthier, with higher measures of overall health, called Apgar scores, at one and five minutes after birth.

After controlling for a variety of factors, the researchers found that labor was, on average, 2.6 hours longer in the 2002-8 group for women having their first baby, and a little under two hours longer for women who had previously given birth.

Medical intervention may be the cause, the study’s authors said.

“Some of the difference is due to change in obstetric practice, and that should drive us to look at labor management,” said a co-author, Dr. D. Ware Branch, a professor of obstetrics and gynecology at the University of Utah.

Other experts were uncertain that medical intervention was making labor longer. “There are a lot of unmeasurables in these patients,” said Dr. Christian M. Pettker, an assistant professor of obstetrics and gynecology at Yale. “It could be inherent properties of the women beyond what they’re measuring.”

In any case, Dr. Pettker said, a longer labor may not be bad.

“Adverse outcomes are certainly lower than they were in the ’50s, and neonatal survival is extraordinarily better even in full-term infants than it was in the ’50s and ’60s,” he said. “So it’s not necessarily so that this longer pattern is worse.”