Severe Maternal Morbidity in the United States

Maternal morbidity includes physical and psychologic conditions that result from or are aggravated by pregnancy and have an adverse effect on a woman’s health. The most severe complications of pregnancy, generally referred to as severe maternal morbidity (SMM), affect more than 65,000 women in the United States every year. Based on recent trends, this burden has been steadily increasing.¹

Rises in SMM are likely driven by a combination of factors, including increases in maternal age,²  pre-pregnancy obesity,^{3, 4} pre-existing chronic medical conditions,^5,6 and cesarean delivery.^{2, 7}  The consequences of the increasing SMM prevalence are wide-ranging and include higher health service use, higher direct medical costs, extended hospitalization stays, and long-term rehabilitation.¹ The review of SMM cases provides an opportunity to identify points of intervention for quality improvements in maternal care. Tracking SMM will help monitor the effectiveness of such interventions.

How does CDC define severe maternal morbidity?

To define SMM, CDC uses delivery hospitalization data and International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) diagnosis and procedure codes that indicate a potentially life-threatening maternal condition or complication.¹ Delivery hospitalizations are identified using a published algorithm⁸ that incorporates diagnostic codes for an outcome of delivery, diagnosis-related group delivery codes, and procedure codes for selected delivery-related procedures. Delivery hospitalizations with SMM are identified using another published algorithm¹ that includes 25 specific ICD-9-CM diagnosis and procedure codes in the table below and captures indicators of organ-system failure that likely represent specific well-defined severe events. Irrespective of the pregnancy outcome, women who have any ICD-9-CM code that indicates such a potentially severe event are designated as having SMM.  

Severe Maternal Morbidity Indicators and Corresponding ICD-9-CM Codes

Note: ICD-9-CM, International Classification of Diseases, Ninth Revision, Clinical Modification

What is the data source?

The source of data for national SMM estimates is the Nationwide Inpatient Sample (NIS), the largest all-payer hospital inpatient care database in the United States.⁹ This is one of a family of databases and software tools developed as part of the Healthcare Cost and Utilization Project sponsored by the Agency for Healthcare Research and Quality.¹⁰ NIS is a stratified sample of approximately 20% of all community hospitals in the United States, with hospitals selected using five characteristics: rural or urban location, number of beds, region, teaching status, and ownership.⁹ The database includes all discharges from the selected hospitals and, for the most recent year (2011), provided information from 1,045 hospitals in 46 US states.⁹ Each record is weighted to account for the complex sampling design, and nationwide estimates can be derived when analytic weights are applied during analysis.¹

Trends in severe maternal morbidity

Trends in SMM between 1998 and 2011 are shown in 2-year increments in the figure below. We observe a clinically and statistically significant increase in SMM between 1998–1999 and 2010–2011 (test for trend p=0.014). For every 10,000 delivery hospitalizations during the most recent 2-year period (2010–2011), there were 163 delivery hospitalizations with at least one SMM indicator. This represents a 26.1% increase in the US SMM rate from the previous 2-year period (2008–2009). 

Blood transfusion was the most common indicator of SMM during 1998–2011. For the most recent 2-year period (2010–2011), blood transfusion was a SMM indicator for 117 of 10,000 delivery hospitalization; the 2nd–5th most frequent indicators of SMM during delivery hospitalizations were disseminated intravascular coagulation (32 per 10,000 delivery hospitalizations), heart failure during a procedure or surgery (18 per 10,000 delivery hospitalizations), hysterectomy (9 per 10,000 delivery hospitalizations), and operations on the heart of or pericardium (7 per 10,000 delivery hospitalizations).

Trends in Severe Maternal Morbidity in the United States: 1998-2011

This graph shows the rates and standard errors of severe maternal morbidity during delivery hospitalizations in the United States in 2-year increments from 1998-2011.

Severe maternal morbidity rate (standard errors)

• 1998-1999, 73.82 (1.65)
• 2000-2001, 78.59 (1.86)
• 2002-2003, 91.24 (2.23)
• 2004-2005, 106.33 (3.74)
• 2006-2007, 116.86 (2.93)
• 2008-2009, 129.08 (3.34)
• 2010-2011, 162.76 (1.02)

The red line indicates a significant increasing trend in severe maternal morbidity between 1998-1999 and 2010-2011 (test for trend p=0.014).

Note: The severe maternal morbidity rate is estimated as the number delivery hospitalizations with at least one severe maternal morbidity indicator per 10,000 delivery hospitalizations; rate standard errors are shown as positive error bars with caps.

References

1. Severe maternal morbidity among delivery and postpartum hospitalizations in the United States. Obstet Gynecol. 2012;120(5):1029-1036.
2. Births: Final Data for 2011 [PDF - 1.63MB]. Hyattsville, MD: National Center for Health Statistics; 2013. Accessed September 5, 2013.
3. Maternal morbid obesity and the risk of adverse pregnancy outcome. Obstet Gynecol. 2004;103(2):219-224.
4. Maternal body mass index and the risk of preeclampsia: A systematic overview. Epidemiology. 2003;14(3):368-374.
5. Maternal morbidity and risk of death at delivery hospitalization. Obstet Gynecol. 2013;122(3):627-633.
6. Near-miss maternal mortality: cardiac dysfunction as the principal cause of obstetric intensive care unit admissions. Obstet Gynecol. 2012;119(2 Pt 1):250-255.
7. Indications contributing to the increasing cesarean delivery rate. Obstet Gynecol. 2011;118(1):29-38.
8. An Enhanced Method for Identifying Obstetric Deliveries: Implications for Estimating Maternal Morbidity. Matern Child Health J. 2008;12(4):9.
9. Agency for Healthcare Research and Quality. Overview of the National Inpatient Sample Web site. Accessed September 5, 2013.
10. Agency for Healthcare Research and Quality. Overview of the Healthcare Cost and Utilization Project. Accessed September 5, 2013.