Using Longitudinally Linked Data to Measure Severe Maternal Morbidity Beyond the Birth Hospitalization in California.

Most studies of severe maternal morbidity (SMM) include only cases that occur during birth hospitalizations. We examined the increase in cases when including SMM during antenatal and postpartum (within 42 days of discharge) hospitalizations, using longitudinally linked data from 1,010,250 births in California from September 1, 2016, to December 31, 2018. For total SMM, expanding the definition resulted in 22.8% more cases; for nontransfusion SMM, 45.1% more cases were added. Sepsis accounted for 55.5% of the additional cases. The increase varied for specific indicators, for example, less than 2% for amniotic fluid embolism, 7.0% for transfusion, 112.9% for sepsis, and 155.6% for acute myocardial infarction. These findings reiterate the importance of considering SMM beyond just the birth hospitalization and facilitating access to longitudinally linked data to facilitate a more complete understanding of SMM.