The study authors called for further research to determine whether modern delivery practices are contributing to the increase in labor 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 previously 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 dilates, 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 pregnancy 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 maternal 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 killers into the spinal 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 noted 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 hormone oxytocin 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 contractions 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 than what we found,' she said.

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

The study compared data from nearly 40,000 deliveries between 1959 and 1966 with records of almost 100,000 deliveries that took place in 2002 through 2008. Data from the recent deliveries were collected through the NICHD-supported Consortium on Safe Labor.

Dr. Laughon conducted the study with D. Ware Branch, M.D., of Intermountain Healthcare and the University of Utah, in Salt Lake City; Jun Zhang, Ph.D., M.D., with the NICHD at the time of the study and now with the Shanghai Jiaotong University School of Medicine, China; and Julie Beaver, M.S., formerly with the NICHD.

Other differences between the two groups reflect changes in later stage delivery practices. For example, in 1960s-era deliveries the use of episiotomy (surgical incision to enlarge the vaginal opening during delivery), and the use of forceps, surgical instruments used to extract the baby from the birth canal, was notably more common.

In current practice, doctors may intervene when labor fails to progress. This could happen if the dilation of the cervix slows or the active phase of labor stops for several hours, Dr. Laughon explained. In these cases, intervention can include administering oxytocin or performing a cesarean delivery.

In fact, the study found that the rate of cesarean delivery was four times higher today than it was 50 years ago (12 percent vs. 3 percent).

The authors note that while their study does not identify all the factors contributing to longer delivery times, the findings do indicate that current delivery practices may need to be re-evaluated.

The women in the contemporary cohort had an average pre-pregnancy BMI of 24.9. A BMI of 25 is considered overweight. Overweight and obesity raise the risk of pregnancy complications for mother and baby. Women who are overweight or obese and who would like to become pregnant should speak with their health care provider about losing weight before becoming pregnant.

About the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD): The NICHD sponsors research on development, before and after birth; maternal, child, and family health; reproductive biology and population issues; and medical rehabilitation. For more information, visit the Institute's website at http://www.nichd.nih.gov/.

About the National Institutes of Health (NIH):
NIH, the nation's medical research agency, includes 27 Institutes and Centers and is a component of the U.S. Department of Health and Human Services. NIH is the primary federal agency conducting and supporting basic, clinical, and translational medical research, and is investigating the causes, treatments, and cures for both common and rare diseases. For more information about NIH and its programs, visit www.nih.gov.