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.