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Systems Factors in Obstetric Care: The Role of Daily Obstetric Volume

Snowden, Jonathan M. PhD; Darney, Blair G. PhD, MPH; Cheng, Yvonne W. MD, PhD; McConnell, K. John PhD; Caughey, Aaron B. MD, PhD

doi: 10.1097/AOG.0b013e3182a2dd93
Original Research

OBJECTIVE: To evaluate whether relatively high-volume days are associated with measures of obstetric care in California hospitals.

METHODS: This is a population-based retrospective cohort study of linked data from birth certificates and antepartum and postpartum hospital discharge records for California births in 2006. Birth asphyxia and nulliparous, term, singleton, vertex cesarean delivery rates were analyzed as markers of quality of obstetric care. Rates were compared between hospital-specific relatively high-volume days (days when the number of births exceeded the 75th percentile of daily volume for that hospital) and low-volume or average-volume days. Analyses were stratified by weekend and weekday and overall hospital obstetric volume. Multivariable logistic regression was used to control for confounders.

RESULTS: On weekends, relatively high-volume days were significantly associated with an elevated risk of asphyxia (27 out of 10,000 compared with 17 out of 10,000; P=.013), whereas no association was present on weekdays (13 out of 10,000 on high-volume days and 15 out of 10,000 on low-volume or average-volume days; P=.182). The cesarean delivery rate among the nulliparous, term, singleton, vertex population was significantly lower on high-volume weekend days (22.0% compared with 23.6% on low-volume or average-volume weekend days; P=.009), whereas no association was present on weekdays (27.1% on high-volume days and 27.6% on low-volume or average-volume days; P=.092).

CONCLUSION: Delivery on relatively high-volume weekend days is a risk factor for birth asphyxia in California. High-volume weekend days also are associated with a lower rate of cesarean delivery in nulliparous women with singleton, vertex presentation pregnancies at term.


Delivery on high-volume weekend days is associated with increased risk of birth asphyxia and a decreased nulliparous, term, singleton, vertex cesarean delivery rate.

Departments of Obstetrics and Gynecology, Medical Informatics and Clinical Epidemiology, and Emergency Medicine, and the Center for Health Systems Effectiveness, Oregon Health and Science University, Portland, Oregon; and the Department of Obstetrics, Gynecology, and Reproductive Sciences, University of California, San Francisco, California.

Corresponding author: Jonathan M. Snowden, PhD, Department of Obstetrics and Gynecology, Oregon Health and Science University, 3181 SW Sam Jackson Park Road, Mail Code L466, Portland, OR 97209; e-mail:

Dr. Snowden and Dr. Caughey are supported by grant R40 MC 25694-01-00 from the Maternal and Child Health Research Program, Maternal and Child Health Bureau (Title V, Social Security Act), Health Resources and Services Administration, Department of Health and Human Services. Dr. Cheng is supported by the University of California San Francisco Women's Reproductive Health Research Career Development Award, National Institutes of Health, the Eunice Kennedy Shriver National Institute of Child Health and Human Development (K12 HD001262). Dr. Darney is supported by an AHRQ postdoctoral award (T32 HS017582).

Financial Disclosure The authors did not report any potential conflicts of interest.

The importance of systems factors in obstetric and perinatal outcomes is well-documented. Previous research has analyzed hospital obstetric volume,1–5 institutional characteristics (eg, academic affiliation),6 and timing of delivery (hour, day, and month).7–13 Although results vary, each of these factors has been associated with both the practice of obstetrics and the frequency of adverse birth outcomes. The mechanisms underlying these associations are not well-understood, but possible explanations include staffing, presence of continuous on-site anesthesia, and readiness to handle fluctuations in patient load.14 When confronted with a higher-than-usual number of deliveries, a labor and delivery unit may operate differently. For example, resources may be stretched thin, resulting in longer intervals between provider consultations and delays in response to emergent conditions. Hospital-specific relative daily volume, a measure of fluctuation in patient load based on the hospital baseline (or “normal” patient load), therefore may be key to furthering our understanding of the role of systems factors in obstetrics.

We hypothesized that relatively higher-volume days would be associated with higher rates of adverse outcomes and examined this hypothesis using a retrospective cohort of all births occurring in California in 2006. High relative daily obstetric volume was defined as delivery on a day with more than the hospital-specific 75th percentile for daily births. We analyzed two outcomes. One was asphyxia, a commonly studied marker of quality in studies of hospital-level factors on obstetric care.2,7,10,11,14,15 The other outcome was the rate of cesarean delivery among nulliparous women at term carrying a singleton, vertex presenting fetus, an increasingly accepted marker for quality of obstetric care.16,17

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We conducted a retrospective cohort study using linked birth and newborn death certificates and hospital discharge data for births occurring in California in 2006. Data linkage of California Patient Discharge Data, Birth Certificate Data, and Death Certificate Data was performed by the state Office of Statewide Health Planning and Development Healthcare Information Resource Center, under the California Health and Human Services Agency. The linked dataset includes health information from maternal records for antepartum hospital admissions during the 9 months before delivery and postpartum admissions up to 1 year after delivery. Also included are birth records and records for all neonate admissions during the first year of life. Linkage of maternal and neonate records was achieved using the record linkage number, a unique encrypted alphanumeric code specific to each mother and newborn pair. The reporting of births and deaths in California is nearly 100% comprehensive, and California Health and Human Services Agency personnel code the data according to uniform specifications, perform rigorous quality checks, and review the birth cohort file before release. Human subject approval was obtained from the Committee on Human Research at the University of California, San Francisco, the California Office of Statewide Health Planning and Development Committee for the Protection of Human Subjects, and from the Institutional Review Board at Oregon Health and Science University.

We excluded births missing information regarding hospital of delivery (n=4,352), births in facilities with 50 or fewer deliveries in 2006 (n=119), births in hospitals with a large (more than 10%) proportion of missing birth dates (n=11 hospitals, N=8,883 births), and all other records missing birth dates (n=9,456). Analyses were conducted using Stata 12 and R 2.13.1.

Our exposure of interest was hospital-specific relative daily volume. We defined high relative daily volume in reference to each individual hospital's distribution of daily deliveries. This hospital-specific distribution was calculated by summing the number of births in that hospital on each day in 2006. A hospital-day was defined as high-volume (relative to that hospital's typical delivery load) if the number of births on that day exceeded that hospital's 75th percentile for daily births. Thus, although approximately one-quarter of each hospital's days were designated high-volume, each hospital had a unique definition of a high-volume day, based on its overall annual delivery volume. These high-volume days indexed unusually busy days in a labor and delivery unit compared with the norm for that hospital. Days that were not designated as high-volume by this protocol were the combined average-volume and low-volume days (low-volume or average-volume). Although final analyses excluded multifetal gestations, stillbirths, and congenital anomalies as determined by diagnosis codes on the birth certificate and the newborn's medical record (International Classification of Diseases, 9th Revision, Clinical Modification codes 740–759.9), these births counted toward the definition of a day as high-volume, or low-volume or average-volume.

We stratified analyses based on overall hospital volume (ie, the total number of deliveries in 2006), divided into the following four previously published volume categories14: 1,199 deliveries or fewer (category 1); between 1,200 and 2,399 deliveries (category 2); between 2,400 and 3,599 deliveries (category 3); and 3,600 or more deliveries in 2006 (category 4). These categories were chosen to separate the hospitals by size while maintaining a sufficient number of births and unique hospitals in each stratum. In addition to hospital obstetric volume, day of week was an a priori stratification variable because weekdays are likely to have both higher daily volume and lower rates of adverse outcomes.10,18,19 Weekdays included Monday through Friday and weekend days included Saturday and Sunday, from midnight to midnight.

The outcomes analyzed were birth asphyxia and the rate of cesarean delivery among nulliparous women at term carrying a singleton fetus presenting by the vertex. Cases of neonatal asphyxia were identified using the International Classification of Diseases, 9th Revision, Clinical Modification codes 768.5, 768.6, 768.7, and 768.9. We included all grades of asphyxia severity (mild or moderate, severe, unspecified severity, and hypoxic-ischemic encephalopathy) to capture data for neonates with lower-grade hypoxia that did not reach the level of hypoxic-ischemic encephalopathy. The cesarean delivery analyses relied on route of delivery and parity as recorded on the birth certificate. Outcome rates were compared between relative high-volume and low-volume or average-volume days, overall and stratified by hospital volume category. We substratified by day of the week (weekend compared with weekday) because day of the week is a key confounder. In this unadjusted analysis, we compared groups using the χ2 or Fisher exact test when appropriate.

We also examined adjusted associations between relative daily volume, overall volume, weekend, and outcomes using separate multivariable logistic regression models for asphyxia and cesarean delivery. We fit two models for each outcome. The first included main effects of relative high-volume day, weekend, and overall hospital volume (with highest-volume hospitals as the referent category). We also tested the interaction of relative daily volume with weekend in a second regression model. Because of the challenges of interpreting odds ratios (ORs) for interaction terms in nonlinear models, we calculated the average marginal effect for high-volume day, weekend, and the joint effect of the two when there was significant interaction (P<.05).20 The regression models controlled for confounding by maternal race or ethnicity, advanced maternal age (age 35 years or older), maternal education (12 years or more compared with less than 12 years), and parity (nulliparous compared with parous, for the outcome of asphyxia). Because maternal age distribution is strongly predictive of nulliparous, term, singleton, vertex cesarean delivery rates, we also controlled for maternal age as a continuous variable and with indicator variables (5-year bands) in a sensitivity analysis.17 The models accounted for clustering at the hospital level and calculated robust standard errors.

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After excluding 4,311 stillbirths, 16,400 multifetal gestations, and 33,270 anomalies, there were 462,322 births occurring in the 257 California hospitals in this analysis. Overall, 32.4% of births occurred on high-volume days. This proportion exceeds 25% because a greater number of births occurred on high-volume days as compared with low-volume or average-volume days. A higher proportion of weekday births occurred on relatively high-volume days (37.6% compared with 14.1% for weekend births; Table 1). Characteristics of women delivering on relatively high-volume days did not differ appreciably from those of women delivering on low-volume or average-volume days. Category 1, the smallest hospitals, contained a large number of low-volume hospitals (90), with the fewest births (n=49,494; Table 2). The higher-volume categories had progressively fewer hospitals, and the number of births was similar between categories 2, 3, and 4 (between 130,000 and 150,000).

Table 1

Table 1

Table 2

Table 2

The distribution of daily volume differed substantially between volume categories. Daily volume increased with overall hospital volume, such that the smallest hospitals had many days with no deliveries, whereas the largest hospitals had up to 35 births in a day. The average 75th percentile cutoff of daily deliveries increased across overall hospital volume categories, from 3 deliveries in category 1 to 17 deliveries in category 4.

Unadjusted bivariate analyses suggested that delivery on a high-volume weekend day was associated with both asphyxia and cesarean delivery in the nulliparous, term, singleton, vertex population (Table 2). Although there was no association between high-volume day and asphyxia for weekday deliveries (13 out of 10,000 on high-volume days compared with 15 out of 10,000 on low-volume or average-volume days; P=0.182), on weekends high-volume days were significantly associated with an increased risk of asphyxia (27 out of 10,000 compared with 17 out of 10,000; P=.013). The elevated risk for relatively high-volume weekend days was diluted when stratified by overall hospital obstetric volume; differences were confined to lower-volume and medium-volume hospitals and were not uniform across categories. There was a slightly lower rate of nulliparous, term, singleton, vertex cesarean delivery on high-volume weekend days (22.0% compared with 23.6% on low-volume or average-volume weekend days; P=.009). This decrease in cesarean delivery on weekend days was observed to a stronger degree in higher-volume hospitals, with rates lower by more than 3% in category 3 and 4 hospitals.

Multivariable logistic regression models examined these associations controlling for confounding by maternal factors. In the main effects model (model 1), weekend was positively associated with asphyxia (OR 1.27, 95% confidence interval [CI] 1.07–1.51; Table 3), as was the lowest category of overall obstetric volume, compared with the highest-volume hospital category (category 1: OR 2.31, 95% CI 1.51–3.52). Delivery on a high-volume day had no significant main effect on asphyxia in model 1. There was a modest inverse association between high-volume day and nulliparous, term, singleton, vertex cesarean delivery in the main terms regression model (OR 0.97, 95% CI 0.95–1.00, P=.038). Results were unchanged in sensitivity analyses using alternative adjustments for maternal age.

Table 3

Table 3

In the interaction model for asphyxia (model 2), the effect of a relatively high-volume day was modified by weekend delivery (P for interaction term=.012), as the bivariate analysis (Table 2) suggests. The positive interaction term coefficient indicated higher asphyxia rates on high-volume weekend days. The average marginal effect indicated that asphyxia rates were elevated on high-volume weekend days by a factor of more than 50%, as compared with low-volume or average-volume weekdays (although CIs overlapped; Table 4). The results of model 2 indicated that there was no significant interaction between high-volume days and weekend for cesarean delivery in the nulliparous, term, singleton, vertex population (P=.151).

Table 4

Table 4

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Our findings suggest that hospital-specific high daily volume has variable associations with the outcomes studied. These associations were complex and not uniform across hospitals of different sizes. Although there was no overall association between delivery on a relatively high-volume day and risk of asphyxia, delivery on a relatively high-volume weekend day was associated with a significantly higher risk of asphyxia. This elevated asphyxia risk appeared to be confined to medium-volume and small-volume hospitals, where asphyxia rates are higher overall.

For nulliparous, term, singleton, vertex cesarean deliveries, a lower rate was observed on high-volume weekend days. This difference was only present in hospitals with more than 2,400 annual deliveries. It is notable that for both outcomes, high daily volume was a significant predictor only on weekend days. This finding is logical given the differences that characterize weekends as compared with weekdays in a labor and delivery unit (eg, fewer deliveries, differences in provider staffing, physicians' desire for personal and family time).8,19 More surprising to us was the finding that the daily volume-related differences were observed in higher-volume hospitals for one outcome (cesarean delivery) and in lower-volume hospitals for another (asphyxia). We speculate that this reflects the extent to which each outcome is influenced by systems factors in hospitals of different sizes.

In the case of asphyxia, there is likely to be some background rate that is unpreventable. In this population, asphyxia rates were inversely related to overall hospital obstetric volume. The higher asphyxia rates in the lower-volume hospitals likely means that a greater proportion of these asphyxia cases are within the control of systems and provider factors. In contrast, higher-volume hospitals have more variability in staffing models (eg, around-the-clock obstetric and anesthesia coverage, including the presence of laborists, compared with on-call coverage) as compared with lower-volume hospitals that generally cannot afford such flexible models of care.21 Therefore, it seems likely that nulliparous, term, singleton, vertex cesarean delivery rates, which are sensitive to provider decision-making, would be more sensitive to differences in daily volume in higher-volume hospitals where staffing models are more variable. Although there is a broad goal to lower the cesarean delivery rate, cesarean delivery is the standard of care in some circumstances, and neither high nor low rates are universally preferred.16 We hypothesize that the lower nulliparous, term, singleton, vertex cesarean delivery rate on high-volume weekend days was a result of decreased availability of resources, less individual attention to each patient, and consequently a higher threshold for intervention (although we did not have data to study these factors). If less intensive patient monitoring allows a greater proportion of labors to proceed without intervention, without harming quality of care, this could inform efforts to reduce the cesarean delivery rate. Analyses of daily volume also must consider the interplay between obstetric outcomes. In our findings, they appeared to be independent, with the possible exception of weekend days in category 3 hospitals where there was a lower nulliparous, term, singleton, vertex delivery rate and a borderline significantly higher asphyxia rate.

These findings build on previous work regarding systems factors in obstetric care and elucidate what is likely one component of the mechanism between these systems factors (eg, weekend delivery and overall low hospital volume) and obstetric outcomes.2,7,11,14,19 Relative daily hospital obstetric volume is analogous to the concept of crowding, which has been studied in emergency department settings and more recently in pediatric hospitalizations.22–25 Emergency department crowding is increasingly known to have significant implications for key health outcomes (such as mortality) and clinical processes of care.26 Future work should analyze additional outcomes that may be related to relative daily obstetric volume, analyze associations stratified by staffing models, and explore alternative definitions of the exposure. For example, a small proportion of births occurring in the early hours of the morning may reflect daily volume on the previous day, and Friday night and Monday morning may reflect weekend conditions. Additionally, although we were not able to delineate planned cesarean deliveries in our database, future work might benefit from examining these births separately when defining weekday daily volume.

The limitations of this study should be taken into account when interpreting these results. Asphyxia and nulliparous, term, singleton, vertex cesarean delivery rate are only two indicators of quality of obstetric care. Although we argue that they are valid markers of quality of obstetric care, there is no consensus regarding what is the optimal marker.27–29 We did not have any data regarding staffing models, which would have allowed us to analyze potential mechanisms underlying these associations. We used a basic definition of nulliparous, term, singleton, vertex cesarean delivery. There are more nuanced definitions (eg, that of the Joint Commission) that require data that were not available in our database. The absolute magnitude of the observed associations was small (eg, the risk difference for asphyxia was approximately 0.1% on weekend days). Our definition of asphyxia included a clinically heterogeneous mix of cases identified in patient discharge data, which has variable reliability. For cesarean delivery rates, one must consider whether reductions of 1.5–3 percentage points are clinically meaningful. Our results are not likely generalizable to all settings obstetric practice because staffing models may differ by state based on varying regulations. Finally, our definition of relatively high-volume day used an arbitrary cutoff with a binary exposure definition, and results may differ with a different cutoff or with further stratification (eg, quartiles). All cutoffs are arbitrary to some extent, and our understanding of relative daily volume would be improved by examining alternative definitions, as is seen in the literature regarding emergency department crowding.22,30

This study provides evidence that relative high daily volume is associated with changes in obstetric practice and outcomes on weekends in California hospitals. Specifically, higher asphyxia rates and lower nulliparous, term, singleton, vertex cesarean delivery rates were observed on these days. By focusing on a measure of hospital-specific relative daily volume in addition to the standard measure of overall hospital volume, these findings add nuance to our current understanding of relationships between volume and outcomes in obstetrics. It will be important for future research to help determine the specific characteristics of the various hospitals that contribute to this finding (eg, nursing staffing, physician staffing, physical space on labor and delivery units, ancillary services in the hospital such as availability of anesthesia). However, it will be even more important to determine whether changes to these factors can reduce perinatal asphyxia and maintain optimal obstetric care during times of high volume on labor and delivery.

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