Prepregnancy Obesity and Primary Cesareans among Otherwise Low-Risk Mothers in 38 U.S. States in 2012

The United States has recently experienced increases in both its rate of obesity and its cesarean rate. Our objective was to use a new item measuring prepregnancy body mass index (BMI) on the U.S. Standard Certificate of Live Birth to examine at a population level the relationship between maternal obesity and primary cesarean delivery for women at otherwise low risk for cesarean delivery.

By 2012, 38 states with 86 percent of United States births had adopted the U.S. Standard Certificate. The sample was limited to the 2,233,144 women who had a singleton, vertex, term (37-41 weeks) birth in 2012 and no prior cesarean. We modeled the likelihood of a primary cesarean by BMI category, controlling for maternal socio-demographic and medical characteristics.

Overall, 46.4 percent of otherwise low-risk mothers had a prepregnancy BMI in the overweight (25.1%) or obese (21.3%) categories, with the obese category distributed as follows: obese I (BMI 30.0-34.9, 12.4%); obese II (BMI 35.0-39.9, 5.5%); and obese III (BMI 40+, 3.5%). Obesity rates were highest among American Indian and Alaska Native (32.5%) and non-Hispanic black mothers (30.5%). After adjustment for demographic and medical risks, the adjusted risk ratios (95% confidence intervals) of cesarean for low-risk primiparas were: 1.61 (1.60-1.63) for obese I, 1.86 (1.83-1.88) for obese II, and 2.21 (2.18-2.25) for obese III mothers compared with mothers in the normal weight category.

A relationship between prepregnancy obesity and primary cesarean delivery among relatively low-risk mothers remained even after controlling for social and medical risk factors.