Exploring New Risk Factors for Postpartum Hemorrhage: Time to Consider Gestational Age?

Predelivery assessment of postpartum hemorrhage risk is now required for all birthing centers by the Joint Commission, based on the 2021 revised guidelines for perinatal care.¹  Integral to this requirement is the use of a postpartum hemorrhage risk assessment tool for all patients upon admission for labor and delivery. Identifying women at heightened risk for postpartum hemorrhage before delivery is critically important, as it allows the anesthesia, obstetric, and nursing teams to prepare by ordering a type and screen or crossmatched blood products, securing appropriate vascular access, providing particularly close monitoring of the patient for excessive bleeding, or even transferring the patient to a higher level of care. Many risk factors for postpartum hemorrhage have been established,²  and contemporary protocols for hemorrhage risk assessment by organizations including the California Maternal Quality Care Collaborative (Stanford, California), American College of Obstetricians and Gynecologists (Washington, D.C.), and Association of Women’s Heath, Obstetric and Neonatal Nurses (Washington, D.C.), incorporate such risk factors in order to classify patients as being at low, medium, or high risk for hemorrhage.^3–5  However, postpartum hemorrhage often occurs in the absence of recognized risk factors.

In this issue of Anesthesiology, Butwick et al. evaluate the association between gestational age and postpartum hemorrhage risk in a population-based retrospective cohort study of women delivering in California and Sweden.⁶  The authors included women undergoing livebirth delivery in California between 2011 and 2015 and in Sweden between 2014 and 2017. Their primary outcome was postpartum hemorrhage and was based on International Classification of Diseases, Ninth Revision codes for California births and blood loss greater than 1,000 ml for Swedish births. They found disparate incidences of hemorrhage between the two groups, 3.2% among women in California and 7.1% among women in Sweden. However, in both cohorts, patients delivering between 41 and 42 weeks of gestation were at increased risk for postpartum hemorrhage; the odds of hemorrhage for deliveries between 41 and 42 weeks was 2.04 in California and 1.62 in Sweden, compared to deliveries between 37 and 38 weeks. In California, women delivering between 22 to 27 weeks also had a higher risk for postpartum hemorrhage. While the risk estimate for women delivering at earlier gestational ages in Sweden was also elevated, the CIs were wide.

These findings by Butwick et al. have important implications, given that the effect size observed was much stronger than in previous studies that showed conflicting associations between gestational age and the risk of postpartum hemorrhage with either null or modest effects. Among two cohorts of Canadian women, there was no association between increased gestational age and risk of hemorrhage, while both U.S. and French cohorts had a moderately increased risk of hemorrhage at over 41 weeks’ gestation, an approximately 1.2-fold increase.^7–9  The reason for this difference as compared with previous studies is unclear but may be due to variations in the definition of postpartum hemorrhage, differences in the way postpartum hemorrhage is identified, improved detection of hemorrhage in conjunction with postpartum hemorrhage protocol implementation, or variable obstetrical practice patterns. Nonetheless, what is clear is that in recent years, women delivering between 41 and 42 weeks in both California and Sweden are at markedly elevated risk for postpartum hemorrhage, suggesting that gestational age truly does play a role in hemorrhage risk.

Postpartum hemorrhage often occurs in the absence of established risk factors. A large retrospective study examining deliveries in the United Sates found that 40% of women experiencing severe atonic hemorrhage requiring transfusion had no known risk factors for hemorrhage.²  Conversely, existing risk factors lack specificity; an analysis of the California Maternal Quality Care Collaborative risk assessment tool found that of the patients in the highest risk group for postpartum hemorrhage, only 7.3% experienced severe hemorrhage.¹⁰  A comparison of the California Maternal Quality Care Collaborative, Association of Women’s Heath, Obstetric and Neonatal Nurses, and New York Safety Bundle for Obstetric Hemorrhage risk assessment tools revealed moderate prediction of severe postpartum hemorrhage with variable sensitivity and specificity.¹¹  The limited predictive ability of existing scoring systems underscores the need for research to identify additional risk factors for postpartum hemorrhage, as Butwick et al. have done. Their findings suggest that gestational age should be included in hemorrhage risk assessment tools going forward. Indeed, the authors found that the association between a gestational age of 41 to 42 weeks and postpartum hemorrhage was stronger than that of the association between several established risk factors for hemorrhage that are currently used in national protocols, including polyhydramnios and macrosomia.

Enhanced assessment of postpartum hemorrhage risk is a national priority for maternal safety; recently established Maternal Levels of Care, as designated by the American College of Obstetricians and Gynecologists and the Society for Maternal-Fetal Medicine (Washington, D.C.), have prioritized that deliveries occur with risk-appropriate maternal care.¹²  Level III and IV maternal care centers are equipped with expert multidisciplinary teams to manage postpartum hemorrhage, including obstetric anesthesiologists, interventional radiologists, intensivists, maternal fetal medicine specialists, hematologists, and perfusionists. Patients at the highest risk of hemorrhage benefit from established plans for delivery at maternal level III or IV centers. For example, delivery of patients with the highest risk for postpartum hemorrhage may warrant surgical subspecialty backup, perfusion for cell salvage, interventional radiology, use of hybrid operating rooms, major transfusion preparedness, coagulopathy monitoring, quantitation of blood loss, and critical care needs. Enhanced postpartum hemorrhage risk assessment will facilitate important mobilization of resources and hemorrhage preparedness. As studies like this one from Butwick et al. elaborate risk factors for postpartum hemorrhage, utilization of maternal levels of care (including delivery of low-risk patients at level I centers) will be facilitated. While advanced gestational age alone will likely not be an indication for triaging patients to high-risk centers, it is possible that gestational age, in combination with several other postpartum hemorrhage risk factors, may be enough to suggest the need for a higher level of maternal care. Better risk assessment tools will allow for clearer thresholds among centers to determine when transfer to a higher level of care is warranted, which ultimately will enhance effective regionalized maternal care, communication between centers, and maternal safety.

Postpartum hemorrhage can often be predicted, but the low specificity and sensitivity of existing risk stratification tools suggest the need to further refine our understanding of patients at risk for this important cause of preventable maternal morbidity and mortality. Butwick et al. provide an important step in delineating how gestational age may influence hemorrhage risk in U.S. and Swedish cohorts. Adding advanced gestational age to existing hemorrhage risk assessment tools is likely warranted, particularly if other large-scale studies show similarly strong associations. Indeed, given the strength of associations seen in the current study, it may in fact be time to revise such tools now. Work to identify additional risk factors and allocate appropriate weight to each factor should continue until unanticipated postpartum hemorrhage is a thing of the past.