The larger a pregnant woman is when she checks in on delivery day, the greater her risk of having a cesarean section, suggests a large new study.

Nearly one of every three births in the U.S. is now delivered by cesarean, a surgery that has been linked to complications for both mom and baby such as infection, bleeding and hysterectomy. This rate is about 50 percent higher than it was in the mid-1990s, according to the U.S. Centers for Disease Control and Prevention.

“As clinicians, we are faced with so many issues when taking care of patients with higher BMI, and one of them is a greater risk for cesarean,” lead researcher Dr. Michelle Kominiarek of Indiana University told Reuters Health.

She added that while previous studies had already linked cesarean delivery and body mass index (BMI) - a measure of weight that takes into account height - none had been large or detailed enough to determine how other factors might alter that risk, such as prior births or cesarean sections.

To get a closer look at the issue, Kominiarek and her colleagues collected data on nearly 125,000 women from the National Institutes of Health’s Consortium on Safe Labor who gave birth between 2002 and 2008. Then they analyzed the circumstances surrounding each birth, as well as the delivery route.

A total of 14 percent of the women studied underwent cesareans, report the researchers in the American Journal of Obstetrics and Gynecology.

They found that for every unit increase in BMI, as measured on arrival for delivery, a woman’s risk of cesarean delivery rose by 4 percent.

The team also discovered that this risk varied depending on whether or not a woman had given birth before or had previously undergone a cesarean section. A one-unit increase in BMI raised the risk of cesarean 5 percent for a woman delivering her first child, 2 percent for women with children and prior cesarean, and 5 percent for women with children but without a prior cesarean

These effects remained after accounting for factors such as maternal age, race and cervical dilation at hospital admission.

Overall, those who had a prior cesarean had about double the risk of having another: more than 50 percent of laboring women with a BMI over 40, which is considered morbidly obese.

Part of the motivation for repeat cesareans is concern over a vaginal birth tearing scars left over from the previous surgery. However, a separate study recently found that these uterine ruptures are not as common as previously thought, occurring in less than one percent of vaginal births after cesarean. (See Reuters Health report, July 21, 2010.)

Other factors associated with the risk of cesarean in the current study included an age of 35 or older, black or Hispanic race, and diabetes.

“The increase in the cesarean rate in this country is a multifaceted issue,” Dr. Hugh Ehrenberg of The Ohio State University, who was not involved in the study, told Reuters Health. “Obesity is certainly a significant cog in that wheel.”

Ehrenberg also pointed to a few weaknesses of the study, including the lack of data on newborn size and the inconsistency of cesarean rates across the study centers - ranging from one in four to one in 10 women.

The latter could represent differences in provider attitudes as a result of varying levels of experience. “If you’re not seeing a lot of obese women at delivery, you may more readily cut somebody because you’re uncomfortable and not because they’ve failed in labor,” said Ehrenberg. “Being really big doesn’t necessarily mean you shouldn’t be allowed to labor.”

Exactly how obesity contributes to cesarean risk still has not been well addressed, added Kominiarek. “What is ultimately the safest delivery route for someone with a high BMI? Is it best to have an elective c-section, or is it just as safe to labor and then have a c-section? It will require more research to answer such questions.”

SOURCE:  American Journal of Obstetrics and Gynecology, online August 5, 2010.