Women take longer to give birth today than did women 50 years ago, according to an analysis of nearly 140,000 deliveries conducted by researchers at the National Institutes of Health. The researchers could not identify all of the factors that accounted for the increase, but concluded that the change is likely due to changes in delivery room practice. The study authors called for further research to determine whether modern delivery practices are contributing to the increase in labor¹ duration.

The researchers compared data on deliveries in the early 1960s to data gathered in the early 2000s. They found that the first stage of labor had increased by 2.6 hours for first-time mothers. For women who had previously² given birth, this early stage of labor took two hours longer in recent years than for women in the 1960s. The first stage of labor is the stage during which the cervix（子宫颈） dilates³（扩大） , before active pushing begins.

Infants born in the contemporary group also were born five days earlier, on average, than were those born in the 1960s, and tended to weigh more. The women in the contemporary group tended to weigh more than did those who delivered in the 1960s. For the contemporary group, the average body mass index before pregnancy⁴ was 24.9, compared with 23 for the earlier generation. Body mass index is a measure of body fat based on height and weight. At the time they gave birth, the mothers in the contemporary group were about four years older, on average, than those in the group who gave birth in the 1960s.

"Older mothers tend to take longer to give birth than do younger mothers," said the study's lead author, S. Katherine Laughon, M.D., of the Epidemiology Branch of the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD). "But when we take maternal⁵ age into account, it doesn't completely explain the difference in labor times."

Among the change in delivery practice the researchers found was an increase in the use of epidural anesthesia, the injection of pain killers⁶ into the spinal⁷ fluid, to decrease the pain of labor. For the contemporary group, epidural injections were used in more than half of recent deliveries, compared with 4 percent of deliveries in the 1960s. The study authors noted⁸ that epidural anesthesia is known to increase delivery time, but said it doesn't account for all of the increase.

Doctors in the early 2000s also administered the hormone⁹ oxytocin（催产素） more frequently (in 31 percent of deliveries, compared with 12 percent in the 1960s), the researchers found. Oxytocin is given to speed up labor, often when contractions¹⁰ seem to have slowed. Its use should be expected to shorten labor times, Dr. Laughon explained.

"Without it, labor might even be longer in current obstetrics（产科学） than what we found," she said.

Their analysis was published online in the American Journal of Obstetrics and Gynecology.