New research suggests that the length of time women spend in labor partially depends on race, with Asian women tending to have a longer childbirth process and black women a shorter one.

Understanding such differences is important, researchers say, because it may broaden the definition of a “normal” length of labor - and affect doctors’ decisions on whether to intervene with a Cesarean section.

“Physicians intervene based on what they think is ‘normal,’” said senior study author Dr. Aaron B. Caughey, an assistant professor of obstetrics and gynecology at the University of California, San Francisco.

Right now, he explained in an interview, doctors still use the 50-year-old “Friedman labor curve” as a guide for how well a woman’s labor is progressing. This is essentially a “cookie-cutter” approach, Caughey told Reuters Health, and in recent years, studies have been questioning how well this standard definition of normal fits U.S. women.

To investigate whether race affects length of labor, he and his colleagues examined data on more than 27,000 women who’d given birth at their center between 1976 and 2001, and report their findings in the American Journal of Obstetrics & Gynecology.

They found that, on average, women of Asian descent spent the longest time in the second stage of labor - the “pushing stage” that ends in delivery - while African-American women had the shortest second stage.

The differences were clearer among first-time mothers. Compared with white women having their first baby, black women spent an average of 22 minutes less in the second stage of the labor. Hispanic women were in this stage for about 7 minutes less than white women, while Asian women spent 5 minutes more.

First-time Asian-American mothers were also the most likely to spend more than 3 hours in the pushing stage. In contrast, black women - first-time mothers or not - were about half as likely as white women to be in second-stage labor that long.

According to Caughey, it’s likely that other factors, including age, weight and height, also affect a woman’s length of labor. He and his colleagues are currently studying that question.

Though no two women are exactly alike, Caughey noted, guidelines based on race, age and weight, for instance, could help doctors judge whether labor is not progressing well and a C-section or other intervention is necessary.

The rate of C-section in the U.S. now stands at an all-time high of nearly 30 percent, with a common reason for the procedure being “failed progress” of labor. Given that, Caughey said, it’s important to better understand what an “abnormal” labor truly is.

SOURCE: American Journal of Obstetrics & Gynecology, September 2006.