Objective
The impact of hospital obstetric volume specifically on maternal outcomes remains under studied. We examined the impact of hospital obstetric volume on maternal outcomes in low-risk women who delivered non–low-birthweight infants at term.
Study Design
We conducted a retrospective cohort study of term singleton, non–low-birthweight live births from 2007-2008 in California. Deliveries were categorized by hospital obstetric volume categories and separately for nonrural hospitals (category 1: 50-1199 deliveries per year; category 2: 1200-2399; category 3: 2400-3599, and category 4: ≥3600) and rural hospitals (category R1: 50-599 births per year; category R2: 600-1699; category R3: ≥1700). Maternal outcomes were compared with the use of the chi-square test and multivariable logistic regression.
Results
There were 736,643 births in 267 hospitals that met study criteria. After adjustment for confounders, there were higher rates of postpartum hemorrhage in the lowest-volume rural hospitals (category R1 adjusted odds ratio, 3.06; 95% confidence interval, 1.51–6.23). Rates of chorioamnionitis, endometritis, severe perineal lacerations, and wound infection did not differ between volume categories. Longer lengths of stay were observed after maternal complications (eg, chorioamnionitis) in the lowest-volume hospitals (16.9% prolonged length of stay in category 1 hospitals vs 10.5% in category 4 hospitals; adjusted odds ratio, 1.91; 95% confidence interval, 1.01–3.61).
Conclusion
After confounder adjustment, few maternal outcomes differed by hospital obstetric volume. However, elevated odds of postpartum hemorrhage in low-volume rural hospitals raises the possibility that maternal outcomes may differ by hospital volume and geography. Further research is needed on maternal outcomes in hospitals of different obstetric volumes.
A wide range of factors likely affect obstetric outcomes for low-risk women who deliver in hospitals. For example, hospital-level factors that include ownership, obstetric provider and nursing staffing, and delivery volume are known increasingly to affect perinatal outcomes. Hospital obstetric volume is one hospital factor that has received research attention in recent years, with some studies demonstrating increased rates of adverse perinatal outcomes at the extremes of annual delivery volume. Several of these studies were conducted outside of the United States, some focused on one or two outcomes (some exclusively neonatal), and some did not stratify by maternal risk profile.
The impact of hospital volume on neonatal outcomes has been well studied in very low-birthweight infants, which led to evidence-based recommendations for neonatal regionalization and designated levels of neonatal intensive care. Perinatal regionalization gained widespread attention with the 1976 publication of the landmark March of Dimes report Toward Improving the Outcome of Pregnancy and can be defined as “a systematized cohesive regional network in which the complexity of patient needs determines where and by whom care should be provided.” In contrast with neonatal outcomes, maternal outcomes have not yet been studied in the same level of detail, particularly in the larger population of pregnant women who deliver non–low-birthweight infants at term. This evidence gap is especially pressing given the increasing prevalence of severe maternal complications (eg, postpartum hemorrhage) and maternal death in the developed world. Further, the birth setting and obstetric care that specifically optimize maternal outcomes may not be identical to those that optimize outcomes for their neonates. Because a policy of regionalization must take into account both maternal and fetal/neonatal outcomes, it is crucial to advance our understanding of maternal outcomes across different hospital settings and characteristics that include obstetric volumes. It was our aim to help fill this gap regarding hospital volume for a variety of maternal outcomes in low-risk women.
To address this gap, we analyzed the association between annual hospital obstetric volume and various maternal complications of hospitalization. We conducted a retrospective cohort study of births in the state of California from 2007-2008, analyzing linked vital statistics data and hospital discharge data. We categorized hospitals by annual delivery volume and analyzed rates of maternal outcomes across categories to provide evidence on the impact of hospital obstetric volume, specifically on maternal outcomes. We restricted our sample to the population of women who delivered a non–low-birthweight infant at term without certain preexisting medical conditions to inform the dialogue on optimal birth setting for low-risk women. We hypothesized that obstetric complications would be more frequent in the highest- and lowest-volume hospitals.
Materials and Methods
This was a retrospective cohort study of California deliveries in 2007-2008, with the use of linked vital statistics/patient discharge data. The California Patient Discharge Data, Vital Statistics Birth Certificate Data, and Vital Statistics Death Certificate data are linked and maintained by the Office of Statewide health Planning and Development (OSHPD), Healthcare Information Resource Center, under the California Health and Human Services Agency. The dataset contains patient discharge data (diagnosis and procedure codes) for antepartum admissions in the 9 months before delivery, maternal and infant admissions in the year after delivery, and data from the US Standard Certificate of Live Birth. Maternal and infant records were linked with the record linkage number, which is a unique encrypted alphanumeric code that is specific to each mother/baby pair. Reporting of births in California is almost 100% comprehensive, with California Health and Human Services Agency personnel coding the data according to uniform specifications, performing rigorous quality checks, and reviewing the birth cohort file before it is released. We obtained human subjects approval from the Committee on Human Research at the University of California, San Francisco, the California OSHPD Committee for the Protection of Human Subjects, and the Institutional Review Board at Oregon Health & Science University. The linked dataset did not contain potential patient privacy/identification information, so informed consent was exempted.
Considerations of hospital volume and obstetric regionalization depend on hospital geography. Rural labor and delivery units face a unique set of issues in assuring patient safety, and regionalization/high obstetric volume may not be feasible for such hospitals. Rural hospitals account not only for a substantial proportion of hospitals in our study (27%) but also for a small fraction of births (8%). This reflects the marked difference in the distribution of delivery volume for rural vs nonrural hospitals: volumes are lower in rural hospitals (often by a factor of 3-5), with key demographic and health-related differences between populations of the women who are served. To reflect these distinct distributions and to assess for effect modification by rurality, we conducted a geography-stratified analysis with a separate definition of ‘low rural volume.’ Maternity hospital rurality was defined based on OSHPD rural hospital designations, the presence of a California Association of Rural Health Clinics member clinic, and/or a rural zip code according to the Rural-Urban Commuting Area-2 codes.
Annual hospital obstetric volume was categorized with the use of previously published volume categories. For nonrural maternity hospitals, the categories were: 50-1199 deliveries per year (category 1), 1200-2399 deliveries (category 2), 2400-3599 deliveries (category 3), and ≥3600 deliveries per year (category 4). Rural hospitals were divided into separate, previously published volume categories: 50-599 births per year (category R1), 600-1699 births (category R2), and ≥1700 births per year (category R3). Further, we compared outcomes between rural and nonrural hospitals with the lowest obstetric volume (≤1000 annual deliveries) to help tease out the effect of low absolute volume vs geography.
To define a population of relatively low-risk deliveries, we restricted analyses to women who carried a singleton, vertex-presenting fetus at term. We excluded low birthweight infants (birthweight, <2500 g) and fetuses with chromosomal or anatomic anomalies (defined by the birth certificate and International Classification of Diseases, 9th Revision , Clinical Modification [ICD-9] codes 740-759.9). Women with preexisting diabetes mellitus and chronic hypertension (as defined by ICD-9 codes) were also excluded, as were women with a previous cesarean delivery. All exclusions were conducted after we calculated hospital obstetric volume.
We compared rates of outcomes across hospital volume categories separately for rural and nonrural hospitals. The following maternal quality outcomes were analyzed as outcomes: chorioamnionitis, endometritis, postpartum hemorrhage (overall and stratified by spontaneous vaginal delivery, operative vaginal delivery, and cesarean delivery), transfusion of blood products, severe perineal lacerations (3rd and 4th degree) in spontaneous vaginal deliveries, wound infection in cesarean deliveries, and the cesarean delivery rate in the nulliparous, term, singleton, vertex population. The final outcome was prolonged length of stay (LOS; as indicated in the discharge data), which was defined as maternal LOS of >3 days for vaginal deliveries and >5 days for cesarean deliveries. The ICD-9 codes listed in Table 1 were used to define chorioamnionitis, endometritis, postpartum hemorrhage, blood transfusion, severe perineal lacerations, and wound infection. Mode of delivery and parity were derived from birth certificate data. Maternal death was not recorded in the database.
| Outcome | Codes |
|---|---|
| Chorioamnionitis | 658.4, 658.40, 658.41, 658.43, 762.7 |
| Endometritis | 670, 670.00, 670.02, 670.04, 670.1, 670.10, 670.12, 670.14, 672, 672.00, 672.02, 672.04 |
| Postpartum hemorrhage | 285.1, 666, 666.0, 666.00, 666.02, 666.04, 666.1, 666.10, 666.12, 666.14, 666.2, 666.20, 666.22, 666.24, 666.3, 666.30, 666.32, 666.34 |
| Transfusion of blood products | 99.00, 99.01, 99.02, 99.03, 99.04, 99.05, 99.06, 99.07, 99.08, 99.09 |
| Severe perineal lacerations (3rd or 4th degree) | 664.2, 664.20, 664.21, 664.24, 664.3, 664.30, 664.31, 664.34, 664.60, 664.61, 664.64 |
| Wound infection | 674.3, 674.30, 674.32, 674.34 |
Unadjusted comparisons between volume categories were calculated with the χ 2 test. We used multivariable logistic regression models to assess the association between obstetric volume category and outcomes, controlling for confounding. We adjusted for the following potential confounders: advanced maternal age (≥35 vs <35 years old), maternal education (≥12 vs <12 years), maternal race/ethnicity, maternal public insurance status, prenatal care initiation (first trimester vs later), teaching hospital, and where appropriate, parity (nulliparous vs multiparous). Covariates were selected for inclusion in the model based on a priori subject matter knowledge. Models adjusted for hospital-level clustering of outcomes with the clustered Huber/White variance estimator and calculated robust standard errors.
Finally, we were interested in whether the management of maternal complications differed among hospital volume categories. Therefore, we calculated the rate of prolonged LOS after maternal complications (chorioamnionitis, endometritis, and postpartum hemorrhage). We compared the rates of prolonged LOS after maternal complications across volume categories in nonrural hospitals (cell sizes were too small in rural hospitals). We estimated adjusted associations using multivariable logistic regression, as described earlier. In all analyses, statistical significance was indicated by a probability value of < .05 and/or 95% confidence intervals (CIs).
Power calculations revealed substantial power to detect modest differences (odds ratio [OR], 1.3) in even the rarest outcome (blood transfusion) between volume categories in nonrural hospitals (power, 98%). For outcomes that were stratified by mode of delivery, power was in some cases lower (eg, 77% power to detect an OR of 1.5 for wound infection after cesarean delivery), although still substantial for some outcomes (eg, >99% for postpartum hemorrhage in cesarean deliveries). Power was lower, but still substantial, in the rural analyses (eg, 82% power to detect an OR of 1.5 for transfusion). For rural analysis that was stratified by mode of delivery, we were under-powered for some outcomes (eg, 24% power to detect an OR of 1.5 for wound infection) but not for others (eg, 83% power to detect an OR of 1.5 for postpartum hemorrhage after cesarean delivery). Power was sufficient for prolonged LOS after maternal complications in nonrural hospitals (eg, 82% power to detect an OR of 1.3 for postpartum hemorrhage), but insufficient in rural hospitals (eg, 12% power). We therefore restricted the LOS analyses to nonrural hospitals.
Results
There was a total of 267 maternity hospitals in California that met study criteria, with a total of 736,643 deliveries. Two hundred eleven hospitals with 678,622 deliveries were located in nonrural locations, and 56 hospitals were located in rural locations, with 58,021 deliveries ( Table 2 ). The larger hospitals in categories 3 and 4 cared for more black and Asian American women compared with the smallest hospitals (categories 1 and 2), which cared for a larger share of white women (categories 1 and 2: 30% white; categories 3 and 4: 25% white). In rural hospitals, the higher-volume hospitals (category R3) cared for predominantly Hispanic women with lower educational attainment (73% Hispanic; 22% education ≥12 years), compared with the lowest-volume rural hospitals in category R1 whose patient populations were majority white (55% white; 38% education ≥12 years).
| Characteristic | Nonrural, % | Rural, % | |||||||
|---|---|---|---|---|---|---|---|---|---|
| Overall a | Hospital volume category | Overall f | Hospital volume category | ||||||
| 1 (50-1199 births per year) b | 2 (1200-2399 births per year) c | 3 (2400-3599 births per year) d | 4 (≥3600 births per year) e | R1 (50-599 births per year) g | R2 (600-1699 births per year) h | R3 (≥1700 births per year) i | |||
| Nulliparous | 46.8 | 44.4 | 47.3 | 45.6 | 47.6 | 43.0 | 45.4 | 43.0 | 41.1 |
| Advanced maternal age | 15.1 | 14.9 | 15.6 | 13.4 | 16.0 | 8.5 | 9.1 | 9.0 | 7.7 |
| Public insurance | 49.2 | 56.1 | 45.5 | 50.4 | 49.4 | 69.2 | 64.1 | 70.4 | 71.7 |
| Education ≥12 y | 47.2 | 43.3 | 48.2 | 44.4 | 49.0 | 30.2 | 38.1 | 32.7 | 22.1 |
| Prenatal care initiated in 1st trimester | 83.6 | 80.2 | 82.0 | 83.2 | 85.5 | 70.5 | 73.7 | 68.2 | 69.7 |
| Race/ethnicity | |||||||||
| White | 26.3 | 28.1 | 30.5 | 24.0 | 25.1 | 33.2 | 55.0 | 29.3 | 19.2 |
| Black | 4.9 | 4.0 | 4.1 | 5.3 | 5.2 | 2.4 | 2.4 | 0.8 | 3.6 |
| Hispanic | 54.0 | 55.6 | 53.1 | 56.6 | 52.5 | 58.7 | 34.2 | 65.2 | 72.9 |
| Asian-American | 12.9 | 10.2 | 10.2 | 12.1 | 15.3 | 2.7 | 3.1 | 2.0 | 3.0 |
| Other | 2.1 | 2.1 | 2.1 | 1.9 | 2.0 | 2.9 | 5.3 | 2.7 | 1.3 |
a n = 678,622 births/n = 211 hospitals
b n = 48,450 births/n = 49 hospitals
c n = 164,372 births/n = 69 hospitals
d n = 183,573 births/n = 48 hospitals
e n = 282,722 births/n = 45 hospitals
f n = 58,021 births/n = 56 hospitals
g n = 17,742 births/n = 35 hospitals
h n = 17,699 births/n = 14 hospitals
In unadjusted analyses, hospital obstetric volume was associated with multiple outcomes. Rates of chorioamnionitis, endometritis, postpartum hemorrhage, and severe lacerations differed significantly for nonrural hospitals ( P < .001; Table 3 ). The magnitude of these differences was small (frequently <1%) and generally favored lower-volume hospitals when a pattern was apparent (eg, in chorioamnionitis and postpartum hemorrhage after cesarean delivery).
| Variable | Nonrural hospital volume category, % | P value | |||
|---|---|---|---|---|---|
| 1 (50-1199 births per year) | 2 (1200-2399 births per year) | 3 (2400-3599 births per year) | 4 (≥3600 births per year) | ||
| Chorioamnionitis | 1.8 | 2.0 | 2.5 | 2.2 | < .001 |
| Endometritis | 0.5 | 0.5 | 0.8 | 0.7 | < .001 |
| Postpartum hemorrhage | |||||
| Overall | 2.9 | 2.7 | 2.9 | 2.8 | < .001 |
| Spontaneous vaginal delivery | 2.9 | 2.6 | 2.7 | 2.7 | < .087 |
| Operative vaginal delivery | 3.7 | 3.3 | 3.7 | 3.8 | .160 |
| Cesarean delivery | 2.6 | 2.8 | 4.1 | 3.1 | < .001 |
| Blood transfusion | 0.5 | 0.6 | 0.6 | 0.5 | < .001 |
| Severe perineal lacerations (spontaneous vaginal delivery) | 2.3 | 2.4 | 2.6 | 2.8 | < .001 |
| Wound infection (cesarean delivery) | 0.7 | 0.6 | 0.7 | 0.6 | .268 |
| Prolonged length of stay a | 2.8 | 2.7 | 2.3 | 2.5 | < .001 |
| Nulliparous, term, singleton, vertex cesarean delivery | 28.1 | 26.8 | 25.1 | 27.8 | < .001 |
a Prolonged length of stay: >3 days for vaginal deliveries; >5 days for cesarean deliveries.
Significant differences were also observed in unadjusted analysis across rural hospital volume categories ( Table 4 ). Postpartum hemorrhage decreased with increasing obstetric volume. The overall hemorrhage rates decreased from 4.5% in category R1 hospitals to 1.7% in category R3 hospitals; for operative vaginal deliveries, the decrease was 6.2-2.6% (both P < .001).
| Variable | Rural hospital volume category, % | P value | ||
|---|---|---|---|---|
| R1 (50-599 births per year) | R2 (600-1699 births per year) | R3 (≥1700 births per year) | ||
| Chorioamnionitis | 0.7 | 0.8 | 0.7 | .561 |
| Endometritis | 0.5 | 0.5 | 0.4 | .312 |
| Postpartum hemorrhage | ||||
| Overall | 4.5 | 3.3 | 1.7 | < .001 |
| Spontaneous vaginal delivery | 4.4 | 3.3 | 1.7 | < .001 |
| Operative vaginal delivery | 6.2 | 5.2 | 2.6 | < .001 |
| Cesarean delivery | 4.1 | 2.6 | 2.0 | < .001 |
| Blood transfusion | 0.7 | 0.6 | 0.6 | .442 |
| Severe perineal lacerations (spontaneous vaginal delivery) | 2.1 | 1.6 | 2.3 | < .001 |
| Wound infection (cesarean delivery) | 0.6 | 0.9 | 0.6 | .260 |
| Prolonged length of stay a | 1.8 | 1.6 | 1.2 | < .001 |
| Nulliparous, term, singleton, vertex cesarean delivery | 24.9 | 26.4 | 24.0 | .003 |
a Prolonged length of stay: >3 days for vaginal deliveries; >5 days for cesarean deliveries.
After adjustment for confounders, there were few significant differences between volume categories in nonrural hospitals ( Table 5 ). The odds of blood transfusion were significantly higher in category 2 hospitals (OR compared with category 4, 1.30; 95% CI, 1.03–1.66). For all other outcomes, 95% CIs included 1, and effect sizes were generally small (mostly 0.8-1.4). In rural hospitals, odds of postpartum hemorrhage remained elevated in low-volume hospitals after adjustment for confounders in the regression models ( Table 6 ). The overall odds of postpartum hemorrhage was 3-fold higher in the lowest-volume hospitals compared with the highest-volume (category R1 OR, 3.06; 95% CI, 1.51–6.23). Adjusted odds of postpartum hemorrhage were 2-fold higher in medium-volume rural hospitals compared with highest-volume (category R2 OR, 1.95; 95% CI, 1.00–3.81). These increased risks of postpartum hemorrhage were also observed in models that were stratified by mode of delivery. There were no other differences for the other outcomes between rural volume categories.
| Variable | Nonrural hospital volume category, odds ratio (95% confidence interval) | |||
|---|---|---|---|---|
| 1 (50-1199 births per year) | 2 (1200-2399 births per year) | 3 (2400-3599 births per year) | 4 (≥3600 births per year) | |
| Chorioamnionitis | 1.13 (0.74–1.74) | 1.13 (0.79–1.60) | 1.36 (0.94–1.95) | Reference |
| Endometritis | 1.07 (0.73–1.55) | 0.94 (0.70–1.26) | 1.40 (1.00–1.96) | Reference |
| Postpartum hemorrhage | ||||
| Overall | 1.29 (0.92–1.81) | 1.14 (0.86–1.52) | 1.22 (0.86–1.74) | Reference |
| Spontaneous vaginal delivery | 1.32 (0.95–1.83) | 1.15 (0.88–1.52) | 1.15 (0.83–1.57) | Reference |
| Operative vaginal delivery | 1.16 (0.81–1.67) | 0.94 (0.67–1.32) | 1.09 (0.74–1.61) | Reference |
| Cesarean delivery | 1.22 (0.76–1.97) | 1.15 (0.75–1.76) | 1.62 (0.92–2.83) | Reference |
| Blood transfusion | 1.02 (0.76–1.36) | 1.30 (1.03–1.66) | 1.19 (0.94–1.51) | Reference |
| Severe perineal lacerations (spontaneous vaginal delivery) | 1.01 (0.83–1.23) | 0.94 (0.82–1.09) | 1.04 (0.87–1.24) | Reference |
| Wound infection (cesarean delivery) | 1.41 (0.87–2.29) | 1.03 (0.75–1.42) | 1.28 (0.88–1.84) | Reference |
| Prolonged length of stay b | 1.41 (0.94–2.13) | 1.27 (0.97–1.66) | 1.06 (0.79–1.42) | Reference |
| Nulliparous, term, singleton, vertex cesarean delivery | 1.02 (0.85–1.22) | 0.93 (0.81–1.06) | 0.88 (0.78–1.00) | Reference |
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