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Reports on Selected Racial/Ethnic Groups Special Focus: Maternal and Child Health Infant Mortality Among Racial/Ethnic Minority Groups, 1983-1984

Joel C. Kleinman, Ph.D.
Division of Analysis
National Center for Health Statistics

Summary

Infant mortality varies considerably among racial/ethnic groups in the United States. For groups other than whites and blacks, previously published rates based on the vital statistics system have been underestimated because of inconsistencies in the classification of race and Hispanic status on birth and death certificates. For this report, infant mortality rates (IMRs) are based on the 1983 and 1984 linked birth and infant-death files, and mother's race and Hispanic origin are reported in accordance with information shown on the birth certificates.

Overall, Asians have somewhat lower infant mortality rates than whites, but the rates vary from 6.0/1,000 among Japanese mothers to 9.0/1,000 among "other Asian" mothers. Hispanic mothers show even wider variation: from 7.8/1,000 among Cubans to 12.9/1,000 among Puerto Ricans. Blacks have an IMR twice as high as that for whites, and the rate for American Indians is nearly 60% above the rate for whites.

Mexicans are the third largest minority group in the United States, accounting for one-quarter million births per year. Despite a high rate of poverty and low use of prenatal care, Mexicans have approximately the same IMR (9.0/1,000) as non-Hispanic whites. Further study of this group could assist in the development of prevention strategies.

INTRODUCTION

Although infant mortality has been decreasing since the mid-1960s, progress in reducing infant mortality slowed in the 1980s. Approximately 20 countries have infant mortality rates (IMRs) lower than the IMR in the United States. A major concern related to infant mortality in the United States is the high rate among black infants. In 1987, the IMR among blacks was twice that among whites. Furthermore, the black-white ratio of IMRs has not improved since 1950 (1).

Although the difference between black and white infant mortality has received much attention, limited information is available on infant mortality among other minority groups in the United States. For such information to be provided on a regular basis, a national surveillance system for monitoring trends would be required. However, because of inconsistencies in the recording of race on birth and death certificates, the use of routine vital statistics for monitoring trends can be misleading for minority groups other than blacks. A better method of assessing infant mortality is to link infant death certificates with the corresponding birth certificates. These linked files provide IMRs for minorities other than blacks that differ considerably from IMRs obtained from routine vital statistics. Furthermore, because birth certificates in the United States contain more information about the mother (e.g., education, marital status, prenatal care) and the infant (e.g., birth weight, period of gestation) than death certificates, more detailed analysis is possible to identify high-risk populations and to develop and evaluate interventions for reducing mortality among high-risk groups. The National Center for Health Statistics, CDC, has implemented a program to produce such linked files on an annual basis, beginning with the 1983 birth cohort. Data from the 1983 and 1984 cohorts are now available and are used in this report to examine IMRs among minority groups.

Infant mortality rates are usually calculated by dividing the number of infant deaths in a given year (obtained from death certificates) by the number of live births in the same year (obtained from birth certificates). Race-specific IMRs calculated in this way are valid only when the coding of race on both birth and death certificates is comparable. However, results from the 1983 and 1984 National Linked Birth-Death Files show that the coding for races other than white or black is not comparable. In studies based on these files, the race of the child on the birth certificate (used as the denominator of the usual IMR) is compared with race of the child on the death certificate (used as the numerator of the usual IMR) for all infant deaths. For whites and blacks, the race coding on the birth certificate differed from that on the corresponding death certificate in less than 2% of the linked files; however, 25%-40% of infant deaths among births coded as American Indian/Alaskan Native or Asian on the birth certificate were coded to a different race on the death certificate. For this reason, the IMRs for minorities other than blacks need to be tracked by using the National Linked Birth-Death Files.

METHODS

Infant mortality rates from the linked birth-death files were calculated by using the race and Hispanic origin* of the mother recorded on the birth certificate. The race of the mother was used instead of the race of the child primarily for two reasons. First, the race of the child is assigned in an arbitrary manner, depending upon the race of the mother and father. For example, if one parent is white and the other is not, the race of the child is coded as the race of the parent who is not white. Second, the race of the father is unknown for nearly 20% of the birth records compared with less than 1% for which the race of the mother is unknown (the race of the child is coded to the known race if one parent's race is unknown). Similarly, in approximately 4% of birth records, the mother's Hispanic origin is not recorded compared with 14% for the father's.

Infant mortality rates are presented by mother's race for the entire United States and by mother's Hispanic origin for the 23 states in which data on Hispanic origin are collected and recorded on the birth certificate. Data are aggregated for 1983 and 1984 in order to provide more stable estimates of rates based on the small numbers of deaths that occur in some minority groups.

Postneonatal mortality among normal birth-weight infants (greater than or equal to 2,500 g) has been used as an indicator of preventable mortality (2). Excluding deaths from congenital anomalies further refines this indicator. This measure is not a perfect indicator of preventable mortality, because some congenital anomalies are not lethal when optimal medical care is given, and other deaths among these infants could not be prevented even with the best of medical care. Nevertheless, as a practical indicator, this measure has the advantage of being easily implemented. This method is also least subject to reporting differences among population subgroups. Thus, for this report, preventable mortality among minority groups was estimated by using postneonatal mortality among normal birth-weight infants from all causes except congenital anomalies.

RESULTS

Table 1 shows infant, neonatal, and postneonatal mortality rates by racial/ethnic category. Japanese mothers had substantially lower IMRs than any other group. This advantage was evident in both the neonatal and postneonatal period. Cuban mothers, however, had even lower postneonatal rates than Japanese mothers. Black mothers had the highest rates, followed by American Indians and Puerto Ricans. American Indians had relatively low neonatal mortality but the highest postneonatal mortality rates of all the groups. They are unique in that more than half of their infant deaths occurred in the postneonatal period compared with approximately one-third for the other groups.

The five leading causes of infant death are shown in Table 2. Congenital anomalies were the leading cause for all groups except blacks and American Indians; in these groups, sudden infant death syndrome (SIDS) ranked first. Of all the causes of death, congenital anomalies showed the least variation among ethnic groups: from 198/100,000 live births among Cubans to 255/100,000 among Puerto Ricans. SIDS mortality, on the other hand, showed the greatest variation. SIDS rates among blacks were twice as high as those among whites, and SIDS rates among American Indians were nearly three times higher than those among whites. Asians, Cubans, Mexicans, and (especially) Central and South Americans had unusually low SIDS rates. Mortality rates from respiratory distress syndrome (RDS) and other disorders related to short gestation and low birth weight were much higher among blacks and Puerto Ricans, with the differential especially pronounced for the latter cause among blacks. Complications of pregnancy were twice as high among blacks and 35% lower among Asians than among whites.

Birth weight is strongly associated with infant mortality; therefore, data were analyzed to determine whether differences in IMRs resulted from differences in birth-weight distribution or from differences in birth-weight-specific mortality. Following methods used in other analyses (3), Table 3 disaggregates low birth weight (LBW) into two components: very low birth weight (VLBW) ( less than 1,500 g) and moderately low birth weight (MLBW) (1,500-2,499 g). Blacks had nearly three times and Puerto Ricans 1.6 times the incidence of VLBW compared with non-Hispanic whites. Other minority groups showed little excess VLBW incidence. However, Chinese and Japanese mothers had 20% lower VLBW rates than whites. Blacks and Puerto Ricans had high MLBW rates, but some of the other groups--Filipinos, other Asians, and other and "unknown Hispanics" (i.e., Hispanic mothers whose Hispanic origin is not known)--also had MLBW rates 30%-40% above the rate for whites. Although Japanese mothers had low VLBW rates, they had somewhat higher MLBW rates than whites. The high rates of both LBW components among black and Puerto Rican mothers is reflected in their high IMRs, especially from RDS and disorders related to short gestation and LBW.

Table 4 shows birth-weight-specific IMRs. Little variation among minority groups was found in mortality for VLBW infants, although Japanese and Filipino mothers had particularly low rates. American Indian mothers had nearly 50% greater IMRs for MLBW infants than did white mothers, whereas Japanese, Filipino, and Cuban mothers had rates approximately 40% lower. The IMRs among normal birth-weight infants were particularly high for black, American Indian, and Puerto Rican mothers.

Tables 3 and 4 imply that the high IMRs among blacks and Puerto Ricans were due to adverse outcomes for both birth weight and birth-weight-specific survival. The high IMR among American Indians, on the other hand, was due almost entirely to poorer birth-weight-specific survival.

Postneonatal mortality rates among normal birth-weight infants (excluding congenital anomalies) provide an indicator of preventable mortality. American Indians had the highest rate, almost three times the rate for whites (Table 5). Blacks had twice the rate of whites. Puerto Ricans were the only other group with an elevated risk (29% above the rates for whites). If these groups could achieve the same rate of postneonatal mortality (excluding congenital anomalies) among normal birth-weight infants as the white non-Hispanic group, the overall IMR would decline by almost 25% among American Indians, 10% among blacks, and 5% among Puerto Ricans. Asians, Mexicans, and Central and South Americans had similar rates; other and unknown Hispanics had a somewhat higher risk (16% above whites).

DISCUSSION

Infant mortality among racial/ethnic minority groups in the United States varies widely. For groups other than whites and blacks, previously published rates based on the usual vital statistics system have been underreported because of inconsistencies in the classification of race and Hispanic status on birth and death certificates. Now that a national system of linked birth and death records is available, surveillance of these groups will be possible.

Infant mortality rates based on linked files differ in other respects from the those based on routine vital statistics. In particular, they are measures of risk for the birth cohort, i.e., they measure the probability of death among the cohort of live births, whereas the usual IMR is a ratio of deaths to births occurring in the same calendar year. However, some deaths are not accounted for because of the lack of a match on birth files. In 1983, 1.6%--and in 1984, 2.2%--of the infant deaths could not be matched to a birth certificate.

Overall, Asians had somewhat lower IMRs than whites, but the rates varied from 6.0/1,000 among Japanese mothers to 9.0/1,000 among "other Asian" mothers. Hispanic mothers showed even wider variation: from 7.8/1,000 among Cubans to 12.9/1,000 among Puerto Ricans, with Mexicans (the largest group) having about the same rate (9.0/1,000) as non-Hispanic whites. Blacks had an IMR twice as high as that of whites, and the rate for American Indians was nearly 60% above that of whites. Other studies have shown considerable variation in infant mortality among different American Indian communities (4).

Congenital anomalies were the leading cause for all groups except blacks and American Indians (for whom SIDS ranked first). SIDS mortality varied the most, with rates being twice as high among blacks and nearly three times as high among American Indians as they were among whites. Cubans, Mexicans, and (especially) Central and South Americans had unusually low SIDS rates. Mortality rates from RDS and other disorders related to short gestation and LBW were much higher among blacks and Puerto Ricans.

These results are consistent with data on birth-weight distribution among live births. Blacks and Puerto Ricans had a higher incidence of VLBW and MLBW than non-Hispanic whites. The other minority groups had little excess VLBW.

Infant mortality varied little among the minority groups for VLBW infants, and only American Indian mothers had substantially higher IMRs than white mothers for MLBW infants. However, this lack of variation should be interpreted with caution for two reasons. First, finer birth-weight intervals need to be used in these ranges because of the very steep gradient in IMR with increasing birth weight and the differences in birth-weight distribution among minority groups. Second, some investigators argue that comparison of birth-weight-specific mortality should be made with explicit reference to the birth-weight distribution (5). That is, birth-weight-specific mortality rates in two populations with different mean birth weights should be compared on the basis of how far a particular birth weight is from the mean, rather than using the absolute value of the birth weight. When additional years of data become available, such detailed analysis will be possible.

The high IMRs among normal-birth-weight infants for black, American Indian, and Puerto Rican mothers identifies a problem that would be evident even with more detailed birth-weight-specific analysis. A further refining of this comparison by using postneonatal mortality among normal birth-weight infants (excluding congenital anomalies) as an indicator of preventable deaths shows that American Indians had three times and blacks had twice the risk of preventable deaths that whites had. If these groups could achieve the rate among whites, their overall IMRs would decline by nearly 25% among American Indians and 10% among blacks.

Risk profiles for these minority groups varied widely. Blacks, American Indians, Puerto Ricans, and Mexicans had the lowest proportion with prenatal care beginning in the first trimester (approximately 60% in 1984) compared with nearly 80% for non-Hispanic whites, Cubans, and Asians (1). The proportion of births to unmarried mothers was highest among blacks (59% in 1984), Puerto Ricans (51%), and American Indians (40%). Mexicans had a lower proportion (24%), but it was still above the proportions for Cubans (16%), non-Hispanic whites (11%), and Asians (10%). The proportion of births to teenagers was higher among blacks (24%), Puerto Ricans (21%), American Indians (20%), and Mexicans (18%) than among non-Hispanic whites (11%), Cubans (9%), and Asians (6%).

The low IMR among Mexicans is of particular interest because Mexican mothers have relatively high maternal risk profiles compared with non-Hispanic whites. Another finding is that American Indians have risk profiles that are nearly as high as those of blacks, yet their incidence of LBW and their neonatal mortality rates are only 10%-15% above those of whites. Some of these anomalous results could be due to underreporting of infant deaths and live births of VLBW. For example, illegal immigrants have incentives to obtain U.S. birth certificates for infants who survive but to avoid the reporting of infants who die shortly after birth (6). Specific studies based on field work outside the hospital and vital statistics system could help clarify this issue.

Continued surveillance of infant mortality among minority groups in the United States is important for several reasons. Black mothers are at particularly high risk of virtually all adverse pregnancy outcomes. Puerto Rican mothers have an intermediate risk, between the risks for blacks and whites. American Indians are at especially high risk for postneonatal mortality. Each of these minority groups will have population targets set up in the year 2000 objectives. Mexicans are the third largest minority group in the United States, accounting for one-quarter million births per year. They appear to have relatively good pregnancy outcomes, despite a high rate of poverty and low use of prenatal care. Further study of this group might provide helpful information for prevention strategies.

Further specificity of minority-group categories would also be helpful. The group "other Asian or Pacific Islanders" is now the largest Asian group and the one with the highest IMR. Specific groups within this category may warrant special attention. Similarly, the group "other and unknown Hispanic" is the second largest category of Hispanic births. Coding of birth certificates does not allow this group to be further subdivided; therefore, whether the unknown Hispanic is the dominant category within this group cannot be determined. The further consideration of coding guidelines and the development of studies in localities with relatively large concentrations of minority groups should be encouraged.

References

  1. National Center for Health Statistics. Health, United States, 1989. Hyattsville, Maryland: Public Health Service, 1990.
  2. WHO Collaborating Center in Perinatal Care. Unintended pregnancy and infant mortality/morbidity. In RW Amler, HB Dull (eds). Closing the gap: the burden of unnecessary illness. New York: Oxford University Press, 1987.
  3. Kleinman JC, Kessel SS. Racial differences in low birth weight: trends and risk factors. N Engl J Med 1987;317:749-53.
  4. Honigfeld L, Kaplan D. Native American postneonatal mortality. Pediatrics 1987;80:575-8.
  5. Wilcox A, Russell I. Why small black infants have a lower mortality rate than small white infants: the case for population-specific standards for birth weight. J Pediatr 1990;116:7-10.
  6. Kleinman JC. Underreporting of infant deaths: then and now. Am J Public Health 1986;76:365-6. *For reporting purposes, "Hispanic" is defined as "a person of Mexican, Puerto Rican, Cuban, Central or South American or other Spanish culture or origin, regardless of race."


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