Lead author Dr. S. Katherine Laughon of the Epidemiology Branch of the Eunice Kennedy Shriver National Institute of Child Health and Human Development analyzed nearly 140,000 deliveries from the early 1960s to data gathered in the early 2000s.
The researchers could not identify all of the factors that accounted for the labor time, but 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," Laughon said in a statement. "But when we take maternal age into account, it doesn't completely explain the difference in labor times."
The women in the contemporary group tended to weigh more -- average body mass index before pregnancy was 24.9 -- than did those who delivered in the 1960s, with a BMI of 23.
The researchers found 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.
Epidurals are known to increase delivery time but this too doesn't account for all of the increase, the study said.
The findings were published online in the American Journal of Obstetrics and Gynecology.