Women giving birth today spend two to three hours longer in labor than women giving birth half a century ago, according to researchers at U.S. National Institutes of Health.

Researchers explain that the extra time is spent in the first stage of labor, the longest part of the labor process before the “pushing” stage, which had increased by 2.6 hours for first time mothers and two hours for women who have previously given birth compared to women giving birth in the 1960s.

The study was based on data collected through the NICHD-supported Consortium on Safe Labor, and compared about 39,500 deliveries between 1959 and 1966 with records of more than 98,000 deliveries that took place in 2002 through 2008.

All of the women in the study had a spontaneous labor that was not induced.

The findings also show that infants today are on average born five days earlier and weighed more compared to those born in the 1960s, and new moms are now older and have a higher body mass index compared to moms 50 years ago.

Researchers said the longer births could be explained by the fact that contemporary mothers were on average about four years older than those who gave birth in 1960s.

"Older mothers tend to take longer to give birth than do younger mothers," lead author Dr. Katherine Laughon, of the National Institute of Child Health and Human Development said in a statement released on Friday. "But when we take maternal age into account, it doesn't completely explain the difference in labor times."

However researchers noted that labor was still longer even when factors like age and weight were taken into account.

Researchers said that while the reason for the extra time is still unknown, it could partially be explained by more common epidural anesthesia compared to 50 years ago, which is known to slow labor down by about 40 to 90 minutes. Epidural injections were used in more than half of contemporary births compared to only four percent in the 1960 group.

Laughon said that the delivery process may be even longer if doctors had not started administering the hormone oxytocin, which stimulates contractions, more frequently in 2000s. The findings show that oxytocin was administered in 31 percent of deliveries, compared with 12 percent in the 1960s.

Researchers recommend that doctors reassess the definition of "normal" labor which is based on the norm for women 50 years ago. Doctors label a birth abnormal when there is no change in the cervix after two hours in the “active” part of the first stage of labor, which they may then intervene by either giving oxytocin medication which stimulates contractions or by doing a C-section.

Just as doctors have recently begun administering oxytocin and epidural injections more frequently, C-section procedures have also increased from 3 percent in 1960 to 12 percent in 2000.

Laughon told Reuters that, based on the latest findings, doctors should consider waiting longer before taking those measures -- "as long as the mom and baby are healthy".

The findings were published online in the American Journal of Obstetrics & Gynecology.