Background

Using ICD-9 codes underestimates the prevalence of obesity in adults; however, the validity of these codes in studies of pregnancy-related outcomes is not known.

Objectives

To compare classification of maternal obesity based on ICD-9 codes in hospital discharge records versus data from birth certificates in the same women, examine predictors of agreement, and assess how associations between obesity and two birth outcomes differ by source of weight data.

Methods

This population-based study included 2 329 145 California births between 2007 and 2012. We compared data on obesity from childbirth hospital discharge records (ICD-9 codes for obesity) and birth certificates (pre-pregnancy body mass index (BMI) calculated from weight and height) and identified predictors of agreement between the two sources. Logistic regression models assessed whether the two definitions of obesity resulted in different estimates of the associations of obesity with caesarean birth and large-for-gestational age.

Results

Overall, 464 754 women (20.0%) had obesity based on their pre-pregnancy BMI while only 100 002 (4.3%) had an obesity-related ICD-9 code. The sensitivity of ICD-9-based obesity was low at 16.2%; however, obesity codes were highly specific at 98.7%, with a negative predictive value of 82.5% and a positive predictive value of 75.2%. Among women with obesity identified by the birth certificate, those with pre-pregnancy and pregnancy-related complications (eg diabetes and hypertension) were more likely to have an obesity-related diagnosis in their delivery hospital discharge record. Using ICD-9 codes overestimated the association of obesity with caesarean birth and newborn large-for-gestational age.

Conclusions

ICD-9 codes in childbirth discharge records captured only one in five women with pre-pregnancy obesity. Sensitivity varied by maternal characteristics and conditions. This misclassification resulted in bias when examining the association of obesity and pregnancy-related outcomes.

Objective

Independent and joint impacts of maternal race/ethnicity and obesity on adverse birth outcomes, including pre-eclampsia, low birth weight, and macrosomia, were characterized.

Methods

Retrospective cohort study of all 2007 California births was conducted using vital records and claims data. Maternal race/ethnicity and maternal body mass index (BMI) were the key exposures; their independent and joint impact on outcomes using regression models was analyzed.

Results

Racial/ethnic minority women of normal weight generally had higher risk as compared with white women of normal weight (e.g., African-American women, pre-eclampsia adjusted odds ratio [aOR] 1.60, 95% confidence interval [CI]: 1.48-1.74 vs. white women). However, elevated BMI did not usually confer additional risk (e.g., pre-eclampsia aOR comparing African-American women with excess weight with white women with excess weight, 1.17, 95% CI: 0.89-1.54). Obesity was a risk factor for low birth weight only among white women (excess weight aOR, 1.24, 95% CI: 1.04-1.49 vs. white women of normal weight) and not among racial/ethnic minority women (e.g., African-American women, 0.95, 95% CI: 0.83-1.08).

Conclusions

These findings add nuance to our understanding of the interplay between maternal race/ethnicity, BMI, and perinatal outcomes. While the BMI/adverse outcome gradient appears weaker in racial/ethnic minority women, this reflects the overall risk increase in racial/ethnic minority women of all body sizes.