The Association Between Hospital Obstetric Volume and Perinatal Outcomes in California

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Although evidence confirms the benefits of regionalization of high-risk obstetrics to high-volume hospitals with specialized care, a similar consensus has not been reached on the role of regionalization and hospital volume in low-risk or general obstetric practice. Neonatal mortality, birth asphyxia, and intrapartum fetal death are frequently used as indicators/markers of quality of obstetric care. This retrospective, cohort study was undertaken to analyze the relationship between obstetric volume and perinatal outcomes in California.

The study analyzed linked birth/infant death certificates with hospital discharge diagnoses for births occurring in California in 2006. Maternity hospitals were divided into 4 categories based on obstetric volume, defined as the total number of deliveries occurring in the hospital during 2006. Hospitals with fewer than 50 deliveries were excluded. The lowest-volume category (category 1) included hospitals with 1200 deliveries or less, with a monthly average of less than 100 deliveries. Category 2 included smaller-to-intermediate facilities with 1200 to 2399 deliveries, category 3 hospitals had 2400 to 3599 deliveries, and high-volume category 4 comprised hospitals with 3600 deliveries or greater. The analysis was also stratified by teaching hospital status and whether rural or nonrural. Rural hospitals were divided into categories by volume: 50 to 599 deliveries (category R1), 600 to 1699 deliveries (R2), and 1700 deliveries or greater (R3). Frequency of neonatal death and asphyxia was determined among live births within these volume categories, separately for rural and nonrural hospitals. Rates were also calculated for the lower-risk population of births at full-term gestation that had birth weights greater than 2500 g. Multivariable logistic regression was used to calculate the odds of asphyxia associated with medium- and lower-volume maternity units. Regression models were stratified by rural geography.

The study included 268 hospitals that performed a total of 527,617 births in 2006. More hospitals were in category 1. Categories 2 and 3 included fewer hospitals but more deliveries, and category 4 had a small number of hospitals that delivered up to 7900 babies. Rural geography was highly correlated with low volume; almost 50% of the lowest-volume hospitals were rural, but none of the highest-volume hospitals were rural. Because no overlap was noted between rural hospitals and teaching hospitals, results were stratified by geography, urban and suburban versus rural. For the nonrural hospitals, prevalence of preterm birth and low birth weight increased with increasing hospital volume. Hospitals in categories 3 and 4 had a patient mix comprising fewer white patients compared with hospitals with lower volume. In rural hospitals, middle- and higher-volume hospitals had a majority of Hispanic patients with lower educational attainment compared with patients at the smallest rural hospitals, which had mostly white patients. Asphyxia decreased with increasing hospital volume among all births, from a high of 20/10,000 live births in category 1 to 11/10,000 in category 4 (P < 0.001). When restricted to the lower-risk subpopulation, the trend was similar, and the differences were significant, decreasing from 18/10,000 live births at lower-volume hospitals to 9/10,000 in category 4 hospitals. An increased rate of neonatal death was apparent in medium- and higher-volume hospitals as compared with category 1, but the difference was not present when restricted to term births with birth weight greater than 2500 g, where the risk was 2 to 3 per 10,000 live births (P = 0.376). At rural hospitals, rates of asphyxia decreased across categories of increasing rural hospital volume (40/10,000 live births in category R1 hospitals to 8/10,000 in R3 hospitals, P < 0.001); the decrease attenuated only slightly when restricted to term deliveries with birth weight greater than 2500 g. The association between hospital volume and asphyxia was assessed using multivariable logistic regression, restricted to the lower-risk subpopulation, and adjusted for maternal race/ethnicity, education, advanced maternal age, teaching hospital (in the nonrural model), and intrahospital clustering. With categories 4 and R3 as the reference, significantly elevated odds of neonatal asphyxia were seen among births at lower-volume hospitals, in both the rural and nonrural analysis. Odds of asphyxia were approximately doubled at category 1 nonrural hospitals (adjusted odds ratio [aOR], 2.10) and category 2 hospitals (aOR, 1.92). The aOR for asphyxia in category 3 hospitals did not differ from that in the highest-volume category. Category R1 hospitals had elevated aORs of asphyxia, whereas the aOR was not significantly different in R2 and R3 categories.

In this large cohort, an increased prevalence of birth asphyxia was found in lower-volume hospitals in rural and nonrural settings. This finding was observed both overall and for term deliveries with birth weight greater than 2500 g. The distinction between rural and nonrural hospitals should be considered when evaluating policy implications of these results because solutions may differ based on the geographic setting of the hospital and the patients served.

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