Role of Maternal Age in Increasing Severe Maternal Morbidity Rates in the United States

OBJECTIVE:To evaluate a commonly proposed explanation for increasing rates of severe maternal morbidity (SMM) in the United States: shifts in the birthing population to older maternal ages, a known risk factor for SMM.METHODS:We conducted a cross-sectional analysis comparing delivery hospitalizations from two time points (2008-2009 to 2017-2018) using hospital discharge data from the National Inpatient Sample. We used demographic decomposition techniques to evaluate whether increasing rates of SMM and nontransfusion SMM were explained by population-level increases in maternal age or changes in age-specific rates. Analyses were stratified by race and ethnicity.RESULTS:Rates of SMM and nontransfusion SMM significantly increased in the United States between 2008 and 2018 from 135.6 to 170.5 and 58.8 to 67.9 per 10,000 delivery hospitalizations, respectively, with increases observed for nearly all racial and ethnic groups. Over this same period, the proportion of births to people younger than age 25 years decreased and births to people of advanced maternal age (35 years and older) increased, with the largest increases occurring among people identified as non-Hispanic American Indian/Alaskan Native (9.8-13.0%), non-Hispanic Black (10.7-14.4%), and Hispanic (12.1-17.1%). Decomposition analyses indicated that the changing maternal age distribution had little effect on SMM trends. Rather, increases in SMM and nontransfusion SMM were primarily driven by increases in age-specific SMM rates, including rising rates among younger people. Contributions of maternal age shifts were minimal for all racial and ethnic groups except among non-Hispanic Black people, for which 17-34% of the rise in SMM was due to increasing maternal age.CONCLUSION:Except among certain racial groups, increases in U.S. population-level SMM rates over the past decade were due to increases in age-specific rates rather than shifts to older maternal age among the birthing population. Increasing SMM rates across the maternal age spectrum could indicate worsening prepregnancy health status of the birthing population.