Factors Underlying the Temporal Increase in Maternal Mortality in the United States

Abstract

Objective: To identify the factors underlying the recent increase in maternal mortality ratios (maternal deaths per 100,000 live births) in the United States.

Methods: We carried out a retrospective study with data on maternal deaths and live births in the United States from 1993 to 2014 obtained from the birth and death files of the Centers for Disease Control and Prevention. Underlying causes of death were examined between 1999 and 2014 using International Classification of Diseases, 10th Revision (ICD-10) codes. Poisson regression was used to estimate maternal mortality rate ratios (RRs) and 95% confidence intervals (CIs) after adjusting for the introduction of a separate pregnancy question and the standard pregnancy checkbox on death certificates and adoption of ICD-10.

Results: Maternal mortality ratios increased from 7.55 in 1993, to 9.88 in 1999, and to 21.5 per 100,000 live births in 2014 (RR 2014 compared with 1993 2.84, 95% CI 2.49-3.24; RR 2014 compared with 1999 2.17, 95% CI 1.93-2.45). The increase in maternal deaths from 1999 to 2014 was mainly the result of increases in maternal deaths associated with two new ICD-10 codes (O26.8, ie, primarily renal disease; and O99, ie, other maternal diseases classifiable elsewhere); exclusion of such deaths abolished the increase in mortality (RR 1.09, 95% CI 0.94-1.27). Regression adjustment for improvements in surveillance also abolished the temporal increase in maternal mortality ratios (adjusted maternal mortality ratios 7.55 in 1993, 8.00 per 100,000 live births in 2013; adjusted RR 2013 compared with 1993 1.06, 95% CI 0.90-1.25).

Conclusion: Recent increases in maternal mortality ratios in the United States are likely an artifact of improvements in surveillance and highlight past underestimation of maternal death. Complete ascertainment of maternal death in populations remains a challenge even in countries with good systems for civil registration and vital statistics.

Figure 1

Maternal mortality ratios and 95% confidence intervals (CI) in the United States, 1993–2014, including and excluding late maternal deaths (identified by International Classification of Diseases, 10^th Revision [ICD-10] codes O96 and O97) (A), and maternal mortality ratios and 95% CIs in the United States, 1993–2014, including and excluding late maternal deaths (O96 and O97) and excluding four new ICD-10 codes (O26.8, O96, O97, and O99) (B). The year when the ICD-10 coding system and the standard pregnancy checkbox on death certificates were introduced are shown in A.

Figure 2

Maternal mortality ratios in the United States and California, 1999–2014 (A) and maternal mortality ratios in the United States and California restricted to maternal deaths due to other specified pregnancy-related conditions (O26.8) and other maternal diseases classifiable elsewhere but complicating pregnancy, childbirth, and the puerperium (O99) (B). All rates shown, except the maternal mortality ratios in the United States, are 3-year moving averages.