Though women who’ve had a Cesarean section can often deliver vaginally with their next pregnancy, the odds of success may be lower for overweight and obese women, according to researchers.

Their study of nearly 8,600 pregnant women with a history of Cesarean section found that the likelihood of a successful vaginal delivery dipped as a woman’s weight climbed. Overweight and obese women were more likely than their lighter peers to require a Cesarean section after an attempt at vaginal delivery failed.

The findings are published in the October issue of the American Journal of Obstetrics & Gynecology.

At one time, it was thought that when a woman had a Cesarean section, she would have to deliver any subsequent children the same way; the procedure leaves a scar on the uterus, and there is a risk of it rupturing during a vaginal delivery. However, doctors have found that many women can safely go through labor after a Cesarean section - and that a successful vaginal delivery is better for the mother than a repeat cesarean.

Still, recent research suggests that this option may be riskier for certain women, according to the authors of the new study, led by Dr. Peter Baumann of Wayne State University in Detroit. The question of whether this includes overweight women has been unclear, they say.

To investigate, the researchers looked at records for 8,580 German women who gave birth between 1991 and 1997. All had undergone a Cesarean section with their last pregnancy and had never delivered vaginally.

Overall, 78 percent of the women opted for a trial of labor - an attempt at vaginal delivery - while 22 percent elected to have a Cesarean section.

Of the women who underwent a trial of labor, 29 percent ultimately needed a Cesarean section, with the odds being greater among heavier women. When the researchers looked at pregnancy outcomes according to body mass index (BMI) - measure of weight in relation to height - women in the normal-weight category (BMI below 25) had the highest rate of success with vaginal delivery, at 79 percent.

This rate was slightly lower among overweight women (BMI between 25 and 30), at 74 percent, and it continued to dip as BMI category climbed. Sixty-nine percent of obese women had a successful trial of labor, while severely obese women delivered successfully just over half of the time.

In general, the heavier a woman was, the more likely she was to need drugs to induce labor, to have a large baby or to have preeclampsia - a potentially dangerous pregnancy complication marked by high blood pressure, among other signs.

But even when the researchers accounted for these factors, higher BMI still lowered the odds of a successful trial of labor.

Despite this, though, pregnancy outcomes in general were not better with elective Cesarean section, according to Baumann and his colleagues. In both groups, for instance, few newborns had a low Apgar score - a measure of an infant’s heart rate, breathing and other indicators of health minutes after birth.

And regardless of weight, few women who had attempted vaginal delivery suffered a rupture of the uterine scar.

So while heavier women may have lower odds of a successful vaginal delivery, the researchers conclude, repeat cesarean, overall, “is not necessarily associated with better obstetrical outcomes.”

SOURCE: American Journal of Obstetrics & Gynecology, October 2005.