Pre-pregnancy obesity increases the likelihood that women with low-risk pregnancies who have not had prior cesarean deliveries will have C-sections, even after controlling for social and medical risk factors, a study led by a Boston University School of Public Health (SPH) researcher has found.
The study, published online in the journal Birth, offers the strongest evidence yet identifying pre-pregnancy obesity as an independent risk factor for cesarean delivery. The research team analyzed 2012 birth certificate data for more than 2.2 million new mothers, in 38 states, who had not had prior cesareans. Overall, 46.4 percent of mothers had BMIs (body mass index) in the overweight (25.1 percent) or obese (21.3 percent) categories.
Rates of cesareans were 1.6 to 2 times higher for obese women than for those in the normal weight category, the study found, with higher rates among the very obese. The sample was limited to women who had a singleton birth at full term.
“The likelihood of a primary cesarean increased consistently across categories of obesity in a population of low-risk women who would otherwise be likely candidates for a vaginal birth, even after controlling for demographic and medical risk factors,” says lead author Eugene Declercq, SPH professor of community health sciences.
Prior research has shown a link between obesity and cesarean delivery, but those studies were limited by several factors, including relying on surveys samples or using older data, the research team says. Rates of both obesity and cesarean deliveries have increased in recent years.
The authors say there was no clear reason why the primary cesarean rate would be higher in obese women, noting that because of the added risks associated with surgery in obese patients, physicians often try to avoid cesareans. The team controlled for maternal age, hypertension, and other medical and social risk factors.
Among the possible reasons they raised is that obese patients may be more closely monitored because of a greater risk of stillbirth, which could trigger more interventions. In addition, labor management could be influenced by maternal weight, as practitioners may initiate surgery earlier on obese patients to avoid an emergency procedure or complication.
The rate of cesareans for these lower-risk women in the normal weight range was 14.6 percent, according to the study. That increased to 22.9 percent for women in the lower obesity range, and 35.1 percent for women at the high end of the range. Among first-time mothers, more than two in five with pre-pregnancy obesity had cesareans.
The greatest difference was found in younger mothers, where the primary C-section rate for those in the highest obesity category was 2.8 times that of normal-weight mothers.
Declercq says that while the once-rapid growth of pre-pregnancy obesity in the US has slowed, “the consequences of having hundreds of thousands of United States mothers begin their pregnancy obese remains.”
He says that while campaigns to prevent maternal obesity may be “challenging,” they are “necessary steps in mitigating the negative effects of obesity on maternal and infant health.”
Besides Declercq, authors on the study include: Candice Belanoff, SPH clinical assistant professor of community health sciences; Ronald Iverson, vice chair of obstetrics in BU’s School of Medicine; Marian MacDorman, a research professor at the University of Maryland’s Population Research Center; and Michelle Osterman, a statistician in the Division of Vital Statistics, Reproductive Statistics Branch, at the National Center for Health Statistics.