Women who are overweight or obese before becoming pregnant generally spend a longer time in labor than thinner women do, a new study suggests.

Past research has shown that obese women undergo cesarean sections at about twice as often as normal-weight women, but the reasons are not fully clear. The new findings suggest that overweight women’s typically slower labor is one factor.

The study, of 612 women seen in North Carolina prenatal clinics, found that those with a pre-pregnancy body mass index (BMI) that fell into the overweight or obese category typically spent more than an hour longer in one of the stages of labor.

The findings suggest that a woman’s pre-pregnancy weight should be a factor in the decision on whether to do a C-section, according to the study’s lead author, Dr. Anjel Vahratian of the University of Michigan at Ann Arbor.

“overweight women might just have a generally longer labor,” she told Reuters Health. Therefore, if a heavier woman’s labor seems to be progressing slowly, Vahratian said, her obstetrician may want to “let her go a bit longer” before opting to do a C-section.

For their study, which is published in the journal Obstetrics & Gynecology, Vahratian and her colleagues focused on women’s progression during the “active” part of the first stage of labor, when the cervix dilates from 4 to 10 centimeters.

They found that while women with a healthy pre-pregnancy BMI typically spent just over 6 hours in this part of labor, overweight and obese women took 7.5 to 8 hours to progress through it.

Pre-pregnancy BMI was linked to longer labor even when other factors were considered - including the baby’s birth size, the mother’s weight gain during pregnancy, and epidural use during labor.

The reason for the slower labor progression is uncertain, but may have to do with excess fat hindering the widening of the birth canal, Vahratian and her colleagues speculate.

The findings add to the list of pregnancy and labor problems for which overweight women are at greater risk. Besides their higher rate of C-section, heavy women are more likely to develop gestational diabetes or high blood pressure during pregnancy, and are more likely to deliver abnormally large infants.

It’s important, Vahratian said, for women of childbearing age to maintain a healthy diet and regularly exercise, so that they can have the “best possible nutritional status” going into pregnancy.

In the U.S., about half of all women of reproductive age are overweight or obese, Vahratian and her colleagues note in their report. In addition, she said, many women - thin and heavy alike-gain much more weight during pregnancy than is recommended. And those excess pounds can be hard to shed later.

Experts advise that normal-weight women gain 25 to 35 pounds over the course of pregnancy, while those who are overweight should put on 15 to 25 pounds. Women should not attempt to lose weight while they’re pregnant.

SOURCE: Obstetrics & Gynecology, November 2004.