Infant Deaths — United States, 2005–2008

Marian F. MacDorman, PhD, T. J. Mathews, MS

National Center for Health Statistics, CDC

Corresponding author: Marian F. MacDorman, PhD, Division of Vital Statistics, National Center for Health Statistics, CDC. Telephone: 301-458-4356; E-mail: mfm1@cdc.gov.

Introduction

Infant mortality rates are associated with maternal health, quality of and access to medical care, socioeconomic conditions, and public health practices, which makes infant mortality an increasingly important public health concern (1,2). After large declines throughout the twentieth century, the U.S. infant mortality rate did not decline significantly during 2000–2005 (3). Analysis of 2000–2004 infant mortality in the United States indicated considerable disparities by race and Hispanic origin (4). Race and ethnic disparities in U.S. infant mortality have been apparent since vital statistics data began to be collected more than 100 years ago. These disparities have persisted over time, and research indicates that not all groups have benefited equally from social and medical advances (5–7).

The infant mortality analysis and discussion that follows is part of the second CDC Health Disparities and Inequalities Report (CHDIR) (4). The 2011 CHDIR (8) was the first CDC report to assess disparities across a wide range of diseases, behavioral risk factors, environmental exposures, social determinants, and health-care access. The criteria for inclusion of topics that are presented in the 2013 CHDIR are based on criteria that are described in the 2013 CHDIR Introduction (9). This report provides more current information on infant mortality rates on the basis of race/ethnicity, mother's place of birth, and by state and region. The purposes of this infant mortality analysis are to raise awareness of differences in infant mortality by selected maternal and infant characteristics, and to prompt actions to reduce these disparities.

Methods

To estimate disparities in infant mortality rate by selected characteristics and specified group, CDC analyzed data from the United States linked birth/infant death data sets (linked files) for 2005 through 2008 (the latest year for which accurate race/ethnicity data are available) (5). In these data sets, information from the birth certificate is linked to information from the death certificate for each infant (aged <1 year) who dies in the United States. Characteristics analyzed included sex, maternal race/ethnicity, maternal place of birth, and the state of residence of the mother at the time of birth. Household income and educational attainment were not analyzed because they were either not collected or not collected consistently on birth certificates. Maternal race was defined as white, black, Asian/Pacific Islander, and American Indian/Alaska native. Ethnicity is defined as Hispanic or non-Hispanic. Hispanic data were further subdivided into Mexican, Puerto Rican, Cuban, and Central and South American. Place of birth was defined as born in the 50 states and DC, or born outside of the 50 states and DC.

Infant mortality rates were calculated as the number of infant deaths per 1,000 live births in the specified group (i.e., by maternal race/Hispanic origin, maternal birthplace, state of residence, and infant gender). Ratios of non-Hispanic black to non-Hispanic white infant mortality rates were computed to assess the magnitude of the disparity in non-Hispanic black and non-Hispanic white infant mortality rates by state. Data from 2006–2008 were aggregated to obtain statistically reliable state-specific rates by race and Hispanic origin; rates are not shown for cells with <20 infant deaths. Rates based on <20 infant deaths are not shown separately as they do not meet standards of reliability or precision. Differences between infant mortality rates were assessed for statistical significance by using the z test (p<0.05).

Disparities were measured as the deviations from a "referent" category rate. Absolute difference was measured as the simple difference between a population subgroup mortality rate and the rate for its respective reference group. The relative difference, a percentage, was calculated by dividing the difference by the value in the referent category and multiplying by 100.

Results

The U.S. infant mortality rate declined 10% from 2005 to 2010, from 6.86 infant deaths per 1,000 live births in 2005 to a preliminary estimate of 6.14 in 2010 (5,13). In 2008, the overall U.S. infant mortality rate was 6.61 infant deaths per 1,000 live births, with differences by race and Hispanic origin (Table 1). The highest infant mortality rate was for non-Hispanic black women (12.67), with a rate 2.3 times that for non-Hispanic white women (5.52) (Table 1). Compared with non-Hispanic white women, infant mortality rates were 53% higher for American Indian/Alaska Native* women (8.42) and 32% higher for Puerto Rican women (7.29). Infant mortality rates for Asian/Pacific Islanders* (4.51) and Central or South American women (4.76) were lower than those for non-Hispanic white women. From 2005 to 2008, infant mortality rates declined approximately 4% for the total population and for non-Hispanic white women, approximately 7% for non-Hispanic black women, and 12% for Puerto Rican women; changes for other racial/ethnic groups were not statistically significant. When examined by place of birth of the mother, the 2008 infant mortality rate was 38% higher for women born in the 50 states and DC than for women born elsewhere (Table 1). The infant mortality rate was 21% higher for male than for female infants.

Differences also exist in infant mortality rates between various states, with a twofold or greater difference in rates between the states with the highest and lowest rates for the total population and for each race/ethnic group studied. Across the United States, infant mortality rates are generally higher in the South and Midwest and lower in other parts of the country. During 2006–2008, total infant mortality rates ranged from a high of 11.97 per 1,000 live births for DC and Mississippi 10.16 to a low of 4.94 for Massachusetts and Utah. However, because DC has high concentrations of high-risk women, its rate is more appropriately compared with rates for other large U.S. cities. For non-Hispanic white women, Alabama had the highest rate (7.67) and New Jersey the lowest rate (3.78). For non-Hispanic black women, the rate was highest in Hawaii (18.54) and lowest in Washington (7.66). For Hispanic women, the rate was highest in Pennsylvania (7.94) and lowest in Louisiana (3.92).

Ratios of non-Hispanic black to non-Hispanic white infant mortality rates were computed to assess the magnitude of the disparity in non-Hispanic black and non-Hispanic white infant mortality rates by state (Figure). Although the average rate ratio in the United States was 2.35, seven areas (Connecticut, DC, Hawaii, Massachusetts, New Jersey, New York, and Wisconsin) had rate ratios of 2.60 or greater. In contrast, seven other states (Arkansas, Alabama, Kentucky, Mississippi, Oklahoma, Oregon, and Washington) had ratios <2.10. Rate ratios are not shown for states with <20 non-Hispanic black infant deaths.

Discussion

The U.S. infant mortality rate has declined 10% from 2005 (6.86) to 2010 (preliminary estimate: 6.14) (5,13); however, disparities have persisted. Higher infant mortality rates for male than for female infants have persisted for many years and occur among most world populations, and have been explained in part by differences in genetic susceptibility to disease (14). Differences in infant mortality rates by race/ethnicity, maternal birthplace, and geographic area might reflect in part different population profiles, with regard to sociodemographic and behavioral risk factors. For example, infant mortality rates are higher than the U.S. average for adolescents, women aged ≥35 years, unmarried mothers, smokers, those with lower educational levels, or inadequate prenatal care (5). Substantial differences between groups in income and access to health care also might contribute to differences in infant mortality (15). Population groups with the lowest infant mortality rates tended to have a smaller percentage of births to women with some or all of these characteristics, whereas groups with the highest infant mortality rates tended to have a higher percentage of births in women with some or all of these characteristics. Other factors that might contribute to racial/ethnic differences in infant mortality include differences in maternal preconception health, infection, stress, racism, and social and cultural differences (7,16–21). However, the influence of an individual risk factor can vary considerably between population groups, indicating different medical profiles and life experiences for women of different backgrounds (7,16,21–22).

The risk factors for infant mortality discussed earlier (e.g., maternal age, tobacco use, lower income or educational levels, and inadequate prenatal care) are very similar to the risk factors for preterm or low birthweight delivery, and these risk factors can affect infant mortality either directly or through the mechanism of preterm or low birthweight delivery. In 2008, the percentage of infants born preterm (<37 completed weeks' gestation) was higher for non-Hispanic black (17.5%), Puerto Rican (14.1%), and American Indian/Alaska Native (13.6%) mothers, than for non-Hispanic white mothers (11.1%) (5). Infant mortality rates are substantially higher for preterm and low birthweight infants, and even limited changes in the percentages of preterm or low birthweight births can have a major impact on infant mortality (5,6). In fact, the recent decline in U.S. infant mortality is linked to a recent decline in the percentage of preterm births, from a high of 12.8% in 2006 to 12.0% in 2010 (5,22). Still the U.S. infant mortality rate was higher than for the majority of other developed countries, in part because of a substantially higher percentage of preterm births, a critical risk factor for infant mortality (23–24).

Limitation

The findings in this report are subject to at least one limitation. Differences in infant mortality rates for smaller states and certain race/ethnic groups (e.g., American Indians/Alaska Natives, Asians/Pacific Islanders, and Cubans) should be interpreted with caution, as small numbers of infant deaths (i.e., <20) in specific subcategories might lead to a lack of statistical precision.

Conclusion

Infant mortality remains a complex and multifactorial problem that will continue to challenge researchers and policymakers in the years ahead. Despite recent declines in the overall infant mortality rate, the longstanding disparities in infant mortality by racial/ethnic group, mother's birthplace, and geographic area persist. One of the Healthy People 2020 objectives is to achieve an infant mortality rate of 6.0 for the total population and for each race/ethnic group. Although the U.S. infant mortality rate of 6.14 in 2010 approximates the Healthy People 2020 objective, rates for several racial/ethnic groups are substantially higher than the goal (25). Prevention of preterm birth is critical to both lowering the overall infant mortality rate and to reducing racial/ethnic disparities (5,6).

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