Infant Mortality and Low Birth Weight Among Black and White Infants --- United States, 1980--2000

Despite substantial reductions in U.S. infant mortality during the past several decades, black-white disparities in infant mortality rates persist. One of the Healthy People 2010 national objectives for maternal and infant health is to reduce deaths among infants aged <1 year to <4.5 per 1,000 live births among all racial/ethnic groups (objective 16-1c) (1). Important determinants of racial/ethnic differences in infant mortality are low birth weight (LBW), defined as <2500 grams, and very low birth weight (VLBW), defined as <1500 grams. High birth weight--specific mortality rates (BWSMRs) occur at these low birth weights. Healthy People 2010 goals include reducing LBW to 5% and VLBW to 0.9% of live births (objectives 16-10a and 16-10b, respectively). To assess progress toward meeting these national objectives, CDC analyzed birth and death certificate data from the National Center for Health Statistics. This report describes trends in mortality and birth weight among black and white infants, which indicate persistent black-white disparities and underscore the need for prevention strategies that reduce preterm delivery and specific medical conditions that lead to infant death.

CDC analyzed race-specific infant mortality data for 1980--1999 (2) and preliminary mortality data for 2000. Trends in LBW and VLBW were calculated by using birth certificate data for 1980--2000, with 2000 being the most recent year for which data were available (3). BWSMRs were calculated from linked birth and infant death files for 1983--1991 and 1995--1999; LBW infants were divided into VLBW and moderate LBW (MLBW), defined as 1500--2499 grams. Both race-specific LBW/VLBW data and BWSMRs were calculated by using the race of the mother.

In 1980, a total of 3,612,258 live births occurred among all races (2,936,351 to white women and 568,080 to black women). In 2000, a total of 4,064,948 live births occurred among all races (3,202,932 to white women and 619,970 to black women). Although infant mortality declined 45.2% for all races during 1980--2000 (from 12.6 to 6.9 deaths per 1,000 live births) (Table 1), the decline was greater for whites than for blacks. During the same period, infant mortality among whites declined 47.7% (from 10.9 to 5.7), and infant mortality among blacks declined 36.9% (from 22.2 to 14.0). During 1980--2000, the black-white ratio of infant mortality increased 25.0% (from 2.0 to 2.5). However, the ratio remained unchanged during 1990--1998 (2.4 for all years).

During 1980--2000, the percentage of LBW infants increased 11.8% and that of VLBW infants increased 24.3% (Table 1). Although the black-white ratio of LBW births decreased 10.0% (from 2.2 to 2.0), the LBW rate increased 2.4% for blacks (from 12.7% to 13.0%) and increased 14.0% for whites (from 5.7% to 6.5%). Black LBW rates increased 6.3% during the 1980s and decreased 2.3% during 1990--2000. LBW rates remained stable for whites during the 1980s but increased 14.0% during 1990--2000. VLBW rates increased 23.8% for blacks (from 2.48% to 3.07%) and 26.7% for whites (from 0.90% to 1.14%). The VLBW black-white ratio decreased 2.5% (from 2.76 to 2.69) during the entire period but increased 12.7% during the 1980s and decreased 12.4% during 1990--2000. This was due to a reversal in VLBW trends during each decade; during the 1980s VLBW increased 19.0% for blacks and 5.6% for whites, but during 1990--2000, VLBW increased 5.1 % for blacks and 20.0% for whites.

Over time, BWSMRs varied by race of mother (Table 2). During 1983--1999, BWSMRs for LBW declined 36.9% for all races (46.7% for MLBW and 38.3% for VLBW). Both whites and blacks had similar percentage declines in BWSMRs among infants whose birth weights were >2,500 grams, and the black-white gap for this birth weight group increased slightly. However, for both MLBW and VLBW categories, whites had greater declines. White BWSMRs decreased 49.4% for MLBW infants and 41.6% for VLBW infants, and black BWSMRs decreased 38.0% for MLBW infants and 28.4% for VLBW infants. The black-white ratio of BWSMRs increased 39.0% (from 1.03 to 1.43) for all LBW infants, increasing 22.4% (from 0.85 to 1.04) for MLBW infants and 22.3% (from 0.94 to 1.15) for VLBW infants. Accordingly, the historically lower BWSMRs among black MLBW and VLBW infants have disappeared; during the 1980s, BWSMRs were lower among VLBW black infants than among white infants, and during the 1990s, BWSMRs were lower among VLBW white infants than among black infants. Similar reversals in BWSMRs are shown for MLBW infants in 1999.

Reported by: S Iyasu, MBBS, K Tomashek, MD, Div of Reproductive Health, National Center for Chronic Disease Prevention and Health Promotion; W Barfield, MD, EIS Officer, CDC.

Editorial Note:

The findings of this report indicate that although infant mortality has decreased among all races during the past two decades, the overall black-white gap for infant mortality has widened. The lack of progress in closing this gap is a consequence of 1) the persistence of a two- to threefold risk for LBW and VLBW among black infants compared with white infants, and 2) smaller reductions in BWSMRs over time among black VLBW births compared with white VLBW births. Although small reductions occurred in black-white disparities in LBW births and VLBW births during the 1990s, these were attributed partly to greater increases in percentages of LBW and VLBW births among whites, rather than to large reductions in LBW and VLBW among blacks. If these trends persist, the national health objectives to eliminate racial disparities in LBW and VLBW births will not be met. In addition, increases in LBW and VLBW births will slow reductions in infant mortality.

Recent increases in LBW and VLBW births among whites probably are a result of increases in preterm delivery, changes in obstetrical practices, and induction of labor. During 1989--1996, the crude singleton preterm birth rate increased 8% among non-Hispanic whites but decreased 10% among non-Hispanic blacks (4). In addition, increases in white LBW and VLBW births might be attributed partly to increases in multiple births from assisted reproductive therapies. During 1980--1997, the rate of twin births among white mothers increased at a rate nearly twice that of black mothers (5). Although multiple gestations increase the risk for LBW and VLBW births, LBW rates are higher among singleton infants conceived with assisted reproductive technology (6). Nevertheless, blacks continue to have a two- to threefold higher risk than whites for LBW and VLBW. In 1991, >66% of the black-white racial disparity in infant mortality occurred among VLBW infants (7). The specific causes for increased low birth weight and preterm delivery might differ for blacks and whites. The etiology of black-white disparities in low birth weight is complex and is not explained entirely by demographic risk factors such as maternal age, education, or income (8). Factors that might contribute to the disparity include racial differences in maternal medical conditions, stress, lack of social support, bacterial vaginosis, previous preterm delivery, and maternal health experiences that might be unique to black women (9).

Of additional concern are disparate improvements in BWSMRs for blacks and whites over time. During the 1980s, BWSMRs for black VLBW infants were lower than for white VLBW infants. Although these differences are poorly understood, the relative advantage of lower BWSMRs among VLBW blacks has disappeared. Because BWSMRs are influenced by access to quality obstetric and neonatal care, particularly among VLBW births, differential access might exist for blacks compared with whites. Declines in neonatal mortality because of improvements in treatment for specific medical disorders (e.g., respiratory distress syndrome) have been greater for whites than for blacks (10).

The findings of this report are subject to at least three limitations. First, infant mortality was calculated by using the race of the infant as the numerator and the race of the mother as the denominator, and might differ slightly from total BWSMRs. This number might affect calculations of infant mortality in which the race of the mother and that of the infant are reported as different. Second, linked data from 1983 are the earliest linked birth infant death data by race of mother and were not available for 1980--1982 and 1992--1994. Finally, vital records data contain mostly demographic information and do not explain specific reasons for racial disparities in outcomes. Studies that examine quality of health-care delivery, specific maternal and neonatal interventions, and social and environmental determinants might identify the reasons for these differences.

Prevention strategies must focus on reducing LBW and VLBW births to eliminate racial disparities in infant mortality. During the last decade, these disparities have decreased, not because of reductions in LBW births among blacks but because of increases in LBW births among whites. Research should be aimed at preventing preterm delivery and associated factors (e.g., infection, medical complications of pregnancy, or poor prenatal care), and the promotion of effective and culturally sensitive intervention programs (9). Strategies to reduce black-white disparities also should address disparate reductions in specific medical conditions that lead to infant death.

References

1. U.S. Department of Health and Human Services. Healthy People 2010 (conference ed, 2 vols). Washington, DC: U.S. Department of Health and Human Services, 2000.
2. Hoyert DL, Arias E, Smith BL, Murphy SL, Kochanek KD. Deaths: final data for 1999. Natl Vital Stat Rep 2001;49:1--114.
3. Martin JA, Hamilton BE, Ventura SJ, Menacker F, Park MM. Births: final data for 2000. Natl Vital Stat Rep 2002;50;1--102.
4. CDC. Preterm singleton births---United States, 1989--1996. MMWR 1999;48:185--9.
5. Martin JA, Park MM. Trends in twin and triplet births: 1980--97. Natl Vital Stat Rep 1999;47:1--20.
6. Schieve LA, Meikle SF, Ferre C, Peterson HB, Jeng G, Wilcox LS. Low and very low birth weight in infants conceived with use of assisted reproductive technology. N Engl J Med 2002;346:731--7.
7. Carmicheal SL, Iyasu S. Changes in the black-white infant mortality gap from 1983 to 1991 in the United States. Am J Prev Med 1998;15:220--7.
8. Schoendorf K, Hogue CJ, Kleinman J, Rowley DL. Mortality among infants of black as compared to white college graduates. N Engl J Med 1992;326:1522--6.
9. Hogan VK, Richardson JL, Ferre CD, Durant T, Boisseau M. A public health framework for addressing black and white disparities in preterm delivery. J Am Med Womens Assoc 2000;56:177--80.
10. Carmicheal SL, Iyasu S, Hatfield-Timajchy K. Cause-specific trends in neonatal mortality among black and white infants, United States, 1980--1995. Matern Child Health J 1998;2:67--76.

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Table 2


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