Many tasks can be tackled more quickly now than 50 years ago, but delivering a baby naturally it seems is not one of them, according to a U.S. government study.

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, who said the findings suggest doctors may need to rethink the definition of "normal" labor.

The extra time is spent in the first stage of labor - the longest part of the process, before the "pushing" stage, according to findings published in the American Journal of Obstetrics & Gynecology.

Mothers are different as well. 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," said lead researcher Katherine Laughon, at the National Institute of Child Health and Human Development.

Though Laughon said the study wasn't able to fully address the potential reasons for the difference, 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.

The findings were based on two government studies done decades apart.

One, between 1959 and 1966, included about 39,500 women who delivered a full-term baby, while the other tracked more than 98,000 women who had a full-term baby between 2002 and 2008. All of the women had a spontaneous labor - that is, not induced.

When it came to length of labor, first-time mothers in recent years typically spent 2.6 hours longer in the first stage, compared with their counterparts in the 1960s. The difference dropped to two hours with women who had given birth before.

Contemporary women were far more likely to have an epidural - 55 percent, compared to just four percent of counterparts 50 years ago. Twelve percent had a C-section compared with three percent in the 1960s, while 31 percent were given oxytocin, which stimulates contractions, against 12 percent 50 years ago.

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, doctors may need to redefine "normal" labor, a concept that's based on what was the norm for women a half-century ago.

For example, doctors have 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 ocytocin or doing a C-section.

Laughon said the bottom line is that there may be a new "normal," adding: "I think we need to revisit the definitions of 'abnormal' labor, and the timing of the interventions we use."