Data Source and Sample
We conducted a retrospective cross-sectional analysis using hospital records from January 1, 2002 through December 31, 2017 that were contained in the Nationwide Inpatient Sample (NIS). The NIS is made available by the Healthcare Cost and Utilization Project (HCUP) and is currently the largest all-payer, publicly available inpatient database in the United States. (Healthcare Cost and Utilization Project (HCUP)) The two-staged cluster sampling design ensures that hospitalizations in the NIS are representative of the population on important factors including month of admission, primary reason for hospitalization, hospital size, location, ownership, and teaching status. Hospitalization-level weights are provided with each annual database which allow national estimates to be generated. In 2017, the NIS contained approximately seven million inpatient hospitalizations each year (35 million when weighted) from 47 participating states. HCUP transitioned from ICD-9-CM to ICD-10-CM format on October 1, 2015.
Our study sample included pregnancy-related hospitalizations among women aged 15 to 49 years, identified using an HCUP-created variable “NEOMAT” which captures maternal diagnosis records with diagnosis and procedure codes for pregnancy and delivery in the ICD-9-CM era. In the ICD-10-CM period, diagnoses codes ‘Oxx.x’ were used to identify pregnancy and delivery related hospitalizations. To assess the study’s primary exposure, we first scanned ICD-9-CM codes (the principal diagnosis and up to 29 secondary diagnoses) in each woman’s discharge record for an indication of gestational diabetes mellitus (GDM) (ICD-9-CM: 648.8x, ICD-10-CM: O24.xx or O99.81) and/or pre-existing (i.e., diagnosis of type 1 or 2 diabetes prior to the pregnancy) (ICD-9-CM:249.xx, 250.xx, 648.0x; ICD-10-CM: E10.xx, E11.xx, E13.xx). The outcome of interest was stillbirth (ICD-9-CM: 656.4x, V27.1x, V27.3x, V27.4x, V27.6x, V27.7x, 768.0, 768.1, 779.9, 632; ICD-10-CM: O36.4x, P95.xx, P96.9, Z37.1, Z37.3, Z37.4, Z37.6, Z37.7).
Individual-level sociodemographic and behavioral characteristics were also extracted from the NIS databases. Maternal age in years was classified into three categories: 15-24, 25–34, and 35-49 years. Self-reported maternal race/ethnicity was first based on ethnicity (Hispanic or non-Hispanic [NH]), and the NH group, further subdivided by race (White, Black, or other). As the ‘other’ racial group is not clearly defined in the dataset, our analyses were limited to NH-White, NH-Black and Hispanics pregnant women. Median household income, which served as a proxy for socio-economic status, was estimated using the patient’s zip code and subsequently grouped into quartiles. We classified the primary payer for hospital admission into following categories: Medicare, Medicaid, private (commercial carriers, private health maintenance organization [HMOs], and preferred provider organization [PPOs]); and self-pay. We also considered several hospital characteristics including teaching status (teaching vs. non-teaching), location (urban vs. rural), and US region (Northeast, Midwest, South, or West).
Information for this study was obtained from a retrospective secondary de-identified data source. Therefore, patient involvement and core outcome set requirements are not applicable to this analysis.