During the same study period, increases in the U.S. cesarean delivery rate and in the proportion of pregnant women with chronic conditions, postpartum hemorrhage, obesity, multiple births, and advanced maternal age have been documented. Against the backdrop of increases in numbers of women with high-risk conditions, a call for returning the “M” to maternal-fetal medicine has been made.2 Given this context, a more nuanced review of cases is needed to identify modifiable risks and develop best practices to deal with risks that might not be modifiable. Approximately 5,600 women died during a delivery or postpartum hospitalization during the period covered in this report. Although deaths are the most tragic events in obstetrics, our estimates suggest that severe maternal morbidity is on the order of 100 times more common. On an annual basis, with approximately 4,000,000 births in the United States, 129 episodes of severe maternal morbidity will affect approximately 52,000 women. Expanding the focus to what lies below the tip of the iceberg is needed, and we offer a comprehensive construct as a starting point for investigation. The information provided in this report has the potential to be used for clinical reviews, development of quality-of-care indicators, and identifying future research priorities in obstetrics.

The results of our study should be considered in light of the following limitations. First, although our identification of severe complications was based on an algorithm that uses ICD-9-CM codes and several data-driven criteria such as in-hospital mortality, transfer from or to another health care facility, and length of hospital stay, we were unable to fully assess the severity of these conditions. Hospital-based administrative databases that make up the Nationwide Inpatient Sample are primarily used for billing, and hence are subject to errors of omission and commission by medical coders as well as changes over time in coding practices. For example, we found that the average number of ICD-9-CM codes for all delivery and all postpartum hospitalizations increased from 3.6 and 2.9 to 4.6 and 4.5 between 1998–1999 and 2008–2009, respectively. However, the increasing number of postpartum hospitalizations in our study suggests that at least some increase in severe morbidity is the result of the increase in the number of hospitalizations with true morbidity rather than the result of the increase in the number of ICD-9-CM codes per admission. Moreover, administrative data do not provide details regarding sociodemographic or clinical obstetrical risk factors for severe maternal morbidity. Although identifying women according to the indications of severe complications may result in misclassification, the error likely will affect sensitivity rather than specificity as has been demonstrated in validation studies of hospital discharge data. However, the number of cases identified, especially at hospital and local levels, will be small and using medical records to review the clinical courses will serve to validate the proposed indicators and identify points of intervention for preventable complications.

In conclusion, we present an overview of trends in severe maternal morbidity, update previous reports, and propose a new standard for monitoring severe maternal morbidity that remains open to emerging issues in obstetrical care and management. Our findings suggest a substantial increase in severe complications for delivery and postpartum hospitalizations from 1998–1999 to 2008–2009, particularly as indicated by the growing rates for blood transfusions, acute renal failure, shock, acute myocardial infarction, respiratory distress syndrome, aneurysms, and cardiac surgery during delivery hospitalizations.