Compared with the 1960s, U.S. women have in recent years spent two to three hours longer in labor, according to researchers at the U.S. National Institutes of Health.

The extra time is spent in the first stage of labor -- the longest part of the labor process, before the "pushing" stage.

New moms, themselves, are different now than 50 years ago; on average, they're older and weigh more, and their newborns are bigger too.

"But even when we take these changing demographics into account, labor is still longer," lead researcher Dr. Katherine Laughon, of the National Institute of Child Health and Human Development, said in an interview.

"We weren't able to fully address the potential reasons with this study," Laughon said.

One partial explanation may be epidural pain relief, which is far more common now than 50 years ago. Epidurals are known to slow labor down by about 40 to 90 minutes, Laughon said.

And many women may gladly trade the extra time in labor for the pain relief.

WHAT'S "NORMAL"?

From there, the explanations get more speculative.

Laughon noted that many more women now have labor inductions or planned cesarean sections versus decades ago. So women who actually go into spontaneous labor these days may differ somehow from their counterparts of 50 years ago.

But whatever the underlying reasons, Laughon said the findings suggest that doctors may need to rethink the definition of "normal" labor -- a concept that's based on what was the norm for women a half-century ago.

For example, doctors have traditionally considered labor to be abnormal if there's no change in the cervix after two hours in the "active" part of the first stage of labor.

At that point, they may intervene by either giving the medication oxytocin, which stimulates contractions, or doing a C-section.

But based on these new findings, doctors may be able to wait longer before taking those measures -- "as long as the mom and baby are healthy," Laughon said.

EPIDURALS MUCH MORE LIKELY

The findings, reported online in the American Journal of Obstetrics & Gynecology, are based on two government studies done decades apart.

One included about 39,500 U.S. women who delivered a full-term "singleton" between 1959 and 1966. The other tracked more than 98,000 women who had a full-term singleton between 2002 and 2008.

All of the women had a spontaneous labor -- that is, no labor induction.

Not surprisingly, contemporary women were far more likely to get an epidural -- 55 percent did, versus just four percent of their counterparts 50 years ago. And their doctors were much more likely to end up doing a cesarean to deliver the baby, or to use oxytocin.

Twelve percent of women in the recent study had a C-section, compared with three percent in the 1960s. Meanwhile, 31 percent were given oxytocin, versus 12 percent 50 years ago.

When it came to length of labor, first-time moms in recent years typically spent 2.6 hours longer in the first stage, compared with their counterparts in the 1960s.

The difference between generations was a bit smaller -- two hours -- when the researchers focused on women who'd given birth before.

Though the reasons for the longer labor times are not fully clear, Laughon said the bottom line is that there may be a new "normal."

"I think we need to revisit the definitions of 'abnormal' labor, and the timing of the interventions that we use," she said.

SOURCE: bit.ly/HnLTxW American Journal of Obstetrics & Gynecology, online March 12, 2012.

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