The U.S. maternal mortality ratio continues to climb and reached a rate of 21–22 per 100,000 in 2013 and 2014. Many explanations for this trend have been offered. Although the United States has a higher rural population than many European nations, such factors are present to an even greater degree in Canada, which is even more rural, yet has a maternal mortality ratio of 10 per 100,000 live births.23 Furthermore, our data failed to identify a statistical correlation between statewide maternal mortality and either rural status or poverty (Table 3). Immigration has also been cited as a factor in this trend. However we found lower mortality for Hispanic women who make up the majority of recent immigrants (Fig. 1; Table 2). This finding has been noted previously and has been attributed to unique social factors and family support often available to these women.24 The high U.S. cesarean delivery rate has also been invoked as an explanation for increased mortality, yet our data demonstrate only a weak correlation of mortality with cesarean delivery. Furthermore, previous work has demonstrated that this correlation does not reflect causation—the overwhelming majority of maternal deaths associated with cesarean delivery is a consequence of the indication for the cesarean delivery, not the operation itself.25 Although medical factors such as hypertensive disease, diabetes, tobacco use, and obesity have been shown to be correlated with increased maternal morbidity, statewide population differences in rates of these conditions were not significantly correlated with mortality ratios (Table 3). The 1999 change in maternal mortality coding practices (ICD-9 to ICD-10) might also be invoked as an explanation for this trend in the United States. However, the continued upward trend in mortality more than a decade later and the absence of such a trend in Canada,23 which uses the same coding system, casts doubt on this assumption.
Our data suggest that much of the variation in statewide maternal mortality ratios in the United States is accounted for by social rather than medical or geographic factors—unintended pregnancy, unmarried mother, and non-Hispanic black race (Tables 3 and 4) and provide evidence for a strong contribution of racial disparity to maternal mortality ratio in the United States. Particularly striking is the tight correlation between statewide ethnic composition and maternal mortality (Table 4). A factor derived from factor analysis, which primarily represented ethnic background, accounted for 26% of the differences in statewide mortality. We note that although Washington, DC, has the highest maternal mortality ratio in the nation, non-Hispanic white patients in this district have the lowest mortality ratio in the United States. Excellent care is apparently available but is not reaching all the people.
These data support two conclusions. First, states that may pride themselves on the intrinsic quality, leadership, organization, and funding of obstetric health care in their state based on national maternal mortality ratio rankings must realize that in many instances, such favorable rank simply reflects a different proportion of non-Hispanic black patients in the population rather than intrinsically superior medical care. The converse applies as well.
Second, comparative health care statistics that do not adjust for these important demographic factors are of little significance in judging the intrinsic quality of available health care in an individual state. Most importantly, these data strongly suggest that racial disparities in health care availability, access, or utilization by underserved populations are important issues faced by states in seeking to decrease maternal mortality. Ethnic genetic differences may also be involved. In addition, the potential role of unconscious (implicit) bias in this significant racial disparity must be considered.25
Finally, available publications consistently document relatively good maternal outcomes for select groups of otherwise healthy older women undertaking pregnancy.26,27 Such data, coupled with the national age-related mortality ratios presented in Table 1, suggest that many older mothers in the United States are not healthy. The mortality ratio in women 45 years of age or older surpasses those in many low-resource nations. Again, these numbers are small, suggesting caution in interpretation of these data. However, careful health screening and preconception counseling are recommended before undertaking pregnancy in such women, especially among those who plan to conceive after assisted reproductive technologies for whom such screening should always be possible.
There are several limitations to this study. First, it is recognized that there exists significant underreporting of maternal mortality in the United States when data are obtained based on ICD cause-of-death codes.28,29 Actual maternal mortality rates are therefore likely to be higher than those reported here. However, no data exist to document either differential accuracy of coding based on ethnic background or among different states. Thus, errors so introduced are likely to be random rather than systematic and similar for all states and would not significantly alter our fundamental conclusions. In addition, our data sets do not allow a precise determination of the causes of death, although such data have been extensively reported in other recent series from the United States.30–32
We conclude that the increased mortality ratios seen in the United States in recent years reflect significant social as well as medical challenges and are closely related to lack of access to health care in the non-Hispanic black population. Our results provide evidence for the strong contribution of racial disparity to maternal mortality ratio in the United States and to interstate differences in maternal mortality ratio and suggest that addressing issues related to health care disparity and access for this population will play an important role in national attempts to reverse this mortality trend.30–32
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