Institutional prevalence of class III obesity modifies risk of adverse obstetrical outcomes

Women with prepregnancy class III obesity (body mass index ≥40 kg/m²) are at an increased risk of perinatal complications and adverse obstetrical outcomes. Estimates of the magnitude of risk that these women face vary widely, which may reflect differences in institutional experience caring for women with obesity.

We sought to characterize the relationship between institutional prevalence of prepregnancy class III obesity and the risk of adverse perinatal outcomes among these women, hypothesizing that higher-prevalence institutions would have lower rates of adverse maternal and perinatal outcomes among this population.

We conducted a retrospective cohort study using chart-abstracted data on births in Washington state from Jan. 1, 2012, to Dec. 31, 2017. The analysis was restricted to hospitals that delivered at least 1 patient per month with prepregnancy class III obesity. Institutional prevalence of prepregnancy class III obesity was calculated, and hospitals were classified as either high or low prevalence. We included nulliparous women with vertex-presenting singleton pregnancies at ≥37 weeks of gestation. We excluded births with missing initial body mass index. The primary outcome was the incidence of cesarean delivery. Secondary outcomes were induction of labor, postpartum complications, postpartum readmission, and neonatal intensive care unit admissions. We compared outcomes between women with prepregnancy class III and all obesity at high- and low-prevalence hospitals using the χ² test or the Fishers exact test as appropriate. Binary logistic regression was performed to compare outcomes at high- and low-prevalence hospitals. A hospital-adjusted multivariable regression model that controlled for baseline institutional rates of each outcome and compared outcomes between high- and low-prevalence hospitals was developed. A final multivariable logistic regression that controlled for both baseline institutional variation as well as potential clinical confounders was performed.

A total of 20,556 women at 6 hospitals were eligible for inclusion; the prevalence of prepregnancy class III obesity was 6.2% and 2.1% in high- and low-prevalence hospitals, respectively. Obese women, including those with class III obesity in a high-prevalence hospital, were more likely to be Latina and less likely to be of advanced maternal age and carry private insurance. After adjusting for the institutional cesarean delivery rate, women with prepregnancy class III obesity had significantly increased odds of cesarean delivery (odds ratio, 1.53, 95% confidence interval, 1.12-2.10); however, after adjusting for significant covariates, the association no longer achieved significance (odds ratio, 1.68, 95% confidence interval, 0.97-2.94). The hospital-adjusted odds of postpartum readmission were significantly increased for women with prepregnancy class III obesity when delivering in low-prevalence institutions (odds ratio, 6.61, 95% confidence interval, 1.93-22.56), and the association was further strengthened after controlling for significant covariates (odds ratio, 15.20, 95% confidence interval, 2.32-99.53). None of the models demonstrated significantly different odds of induction of labor, postpartum complications, or neonatal intensive care unit admission by institutional prevalence of prepregnancy class III obesity.

Even after controlling for underlying hospital and subject characteristics, women with prepregnancy class III obesity had significantly increased odds of postpartum readmission, and a trend toward increased odds of cesarean delivery, when delivering in institutions with less experience caring for women with obesity.