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Reports on Selected Racial/Ethnic Groups Special Focus: Maternal and Child Health Contribution of Birth Defects to Infant Mortality Among Racial/Ethnic Minority Groups, United States, 1983

Michele C. Lynberg, Ph.D., M.P.H. Muin J. Khoury, M.D., Ph.D. Birth Defects and Genetic Disease Branch Division of Birth Defects and Developmental Disabilities Center for Environmental Health and Injury Control

Summary

Linked birth/infant death data from the National Center for Health Statistics (NCHS) for the 1983 U.S. birth cohort, the latest year for which linked data were available, were evaluated in order to assess the contribution of birth defects to infant mortality among racial/ethnic groups. Of the 34,566 singleton infant deaths with specified birth weight born to U.S. residents, birth defects were listed as an underlying cause of death for 7,678 (22.2%) infants and as a contributing cause of death for an additional 1,006 (2.9%) infants. Infant mortality rates due to birth defects were highest among American Indians (2.9 deaths/1,000 live births), followed by Asians and Hispanics (2.6), and blacks (2.5). Proportional mortality due to birth defects varied among racial/ethnic groups; it was greatest among Asians (27%), followed by whites (25%), Hispanics (24%), American Indians (18%), and blacks (13%). Also, infant mortality rates due to birth defects were high among minority infants of low birth weight, particularly among those born weighing between 1,500 and 2,499 g. Within this group of infants, proportional mortality due to birth defects ranged from 52% among Asians to 29% among blacks. These data indicated that birth defects were an important contributor to infant mortality among all racial/ethnic groups. Birth-defects surveillance systems should be used to evaluate whether racial/ethnic differences in infant mortality from birth defects are due to differences in incidence and/or survival among minority infants with birth defects.

INTRODUCTION

Birth defects are the leading cause of infant mortality in the United States (1) and the fifth leading cause of years of potential life lost (YPLL) (2). Also, birth defects contribute substantially to childhood morbidity and long-term disability. Approximately 80,000 infants are born with a major birth defect in the United States each year. Of these, approximately 6,000 die during the first 28 days of life, and another 2,000 die before reaching their first birthday, leaving 72,000 living children affected to various degrees (3). Children with birth defects account for approximately 25%-30% of pediatric hospital admissions; total costs for care of children with birth defects exceed $1 billion annually (4,5). Using reported age-specific survival rates and age-specific medical-care costs, CDC estimates that $90 million (based on the 1985 value of the dollar) will be spent on medical care in 1990 for 12,000 surviving infants with spina bifida born since 1980 (6).

All segments of the U.S. population share the burden of birth defects, with frequency and type varying by race, ethnicity, and socioeconomic status (7-11). Between 1981 and 1986, American Indians showed the highest incidence rates of major defects per 1,000 total births (22.0). Other minorities had lower rates: blacks (18.0), Hispanics (14.4), and Asians (15.8) (12). In comparison, the incidence of major defects was 19.0/1,000 live births among whites.
This report summarizes the contribution of birth defects to infant mortality among racial/ethnic minorities.

METHODS

Information on birth and death data was obtained from NCHS' linked birth/infant death data set for the 1983 birth cohort, which consists of two separate data files (13). Information on births and deaths was obtained from NCHS' natality statistical file and NCHS' mortality statistical file. The numerator file contained information from birth and death certificates concerning infants born in 1983 who died in 1983 or 1984 before their first birthday. The denominator file included information from birth certificates concerning all live births for 1983. Each record contained information coded from birth and death certificates supplied by each state. Information was available on 3,341,274 live births and 39,704 infant deaths. The analysis was limited to singleton infants with known birth weight born to U.S. residents.

Birth-certificate information included information on residence; birth weight; gestational age; sex; race of baby, mother, and father; origin (or descent) of mother; plurality; and other information on pregnancy history. Death-certificate information included data on underlying and contributing causes of death. In addition, age of death, autopsy, and other identifying information was available.

Mother's race and ethnicity were used to classify infants into racial/ethnic minority groups. Twenty-three states and the District of Columbia reported racial/ethnic origin on the birth certificate. These reporting areas represented 96% of the Hispanic population. Although the remaining states did not have ethnic information included on the birth certificate, the mother's reported place of birth permitted the further identification of persons of Hispanic descent (those born in Mexico, Puerto Rico, or Cuba). Using mother's descent--in combination with mother's place of birth--permitted the delineation of Hispanic individuals who would otherwise have been classified as white.

Infants whose mothers were either of African descent or of the black race were grouped into the category "black." The category "American Indian" included American Indians, Aleuts, Eskimos, Alaskans, and Canadian Indians. "Asian" included Chinese, Japanese, Hawaiians, Filipinos, and other Asian or Pacific Islanders. Infants whose mothers reported other descent (Northern European, French, etc) or indicated they were white were included in the category "white." Only 162 (0.5%) infants had "unknown" race/ethnicity, and 13 infants were classified as "other" races. These 175 infants were excluded from the minority-group analysis. Causes of infant mortality were coded using the International Classification of Diseases, 9th Revision (ICD-9). Table 1 details the ICD-9 codes included in each of the underlying cause groups. Birth defects were defined as conditions coded from 740.0 to 759.9 of the ICD-9. Conditions included in this group were heterogeneous, including defects in all organ systems with varying degrees of severity. Some "birth defects" are a result of being born prematurely, and are not birth defects per se. Among infants with low birth weight (LBW) (i.e., less than 2,500 g), the authors looked for birth defects that could have been a result of prematurity. These conditions were lung hypoplasia (748.5), patent ductus arteriosus (747.0), and hydrocephalus (742.3) associated with intraventricular hemorrhage (772.1). When these conditions were associated with LBW infants, they were not considered to be birth defects. Of 34,566 infants who died in their first year of life, 1,037 had at least one of these three types of "false" birth defects. Of these, 495 had other birth defects as well, whereas 542 had false birth defects as their only birth defects listed. False birth defects were listed as the underlying cause of death for 353 infants. For these infants, the underlying cause of death was reassigned.
When describing the occurrence of birth defects among premature infants, the authors looked not only at deaths among infants with unspecified LBW (ICD-9 765, disorders relating to short gestation and unspecified LBW), but also specified LBW (ICD-9 764) and respiratory distress syndrome (RDS) (ICD-9 769). Technically, only premature infants can have RDS (or hyaline membrane disease). Although some of the infants with ICD-9 code 769 may have had respiratory distress unassociated with prematurity, we had no means of separating them from those having RDS associated with prematurity. In this analysis, the authors considered infants with the RDS code as premature. For some analyses, infants with an underlying cause of RDS, LBW, or prematurity were grouped into one category.

RESULTS

Table 2 shows the number of live births and infant deaths by race/ethnicity. Blacks had the highest infant mortality rate (18.5 deaths/1,000 live births); American Indians had the next highest rate (15.8/1,000 live births). All minority groups exceeded the 1990 objective of nine infant deaths/1,000 live births (14).

Figure 1 shows that, overall, birth defects were the leading cause of death in the 1983 birth cohort. Of 34,566 singleton infant deaths with known birth weight, 7,678 (22.2%) were attributed to birth defects. Low birth weight, prematurity, and RDS combined were the cause of death for 5,654 (16.4%). Sudden infant death syndrome (SIDS) was the cause of death for 5,049 (14.6%) infants.

Birth defects that contributed most to infant mortality, grouped by organ system, are shown in Table 3. Regardless of the racial/ethnic group, cardiovascular-system defects were the most commonly recorded defects on the death certificate. Central-nervous-system defects and chromosomal anomalies were also common.

In Figures 2-4, infant mortality rates, as well as the proportional mortality due to the three leading underlying causes of infant death, are shown. The underlying cause of death, by racial/ethnic group, is shown in Figure 2. Birth defects were the leading cause of infant mortality among Hispanics, Asians, and whites. Prematurity, LBW, and RDS combined were the leading causes of infant mortality among blacks, whereas SIDS was the leading cause of infant mortality among American Indians. Although infant mortality varied substantially by racial/ethnic group, the rate of infant mortality due to birth defects was less variable. Infant mortality rates due to birth defects were highest among American Indians (2.9/1,000 live births), followed by Asians and Hispanics (2.6) and blacks (2.5). In comparison, white infant mortality due to birth defects was 2.3/1,000 live births.

The proportional mortality due to birth defects varied greatly by racial/ethnic group. The proportional mortality was greatest among Asians (27%) and Hispanics (24%), and less among American Indians (18%) and blacks (13%). In comparison, proportional mortality among whites was 25%.

Infant mortality rates were further analyzed by age at death. Figure 3A shows infant mortality rates among infants who died in the neonatal period ( less than 28 days of life). Again, although mortality rates varied substantially by racial/ethnic group, deaths due to birth defects were relatively constant (between 1.7 and 1.9 deaths/1,000 live births), regardless of racial/ethnic group. Low birth weight, prematurity, and RDS combined contributed the largest number of deaths during the neonatal period, especially among black infants.

Although the risk of death from birth defects was stable across racial/ethnic groups, the proportional mortality varied greatly. During the neonatal period, proportional mortality was greatest among Asians (32%), whites (30%), and Hispanics (27%) and lowest among blacks (15%) and American Indians (25%).

Figure 3B shows mortality, by racial/ethnic group, in the postneonatal period. The risk from birth defects remained relatively constant (between 0.6/1,000 live births and 1.0/1,000 live births). The largest contributor to infant mortality during the postneonatal period was SIDS; American Indians had the highest rate, and blacks had the next highest rate. Proportional mortality from birth defects remained relatively higher among Asians (20%), whites (18%), and Hispanics (17%) than among American Indians (12%) and blacks (11%).

Assessing infant mortality by age at death indicated that LBW, prematurity, and RDS together contributed the largest number of deaths in the neonatal period, whereas SIDS contributed the largest number in the postneonatal period. However, birth defects remained the overall leading cause of infant mortality, because neonates infrequently died from SIDS and postneonates infrequently died from LBW.

Figure 4 shows infant mortality rates by weight group ( less than 1,500 g, 1,500 g-2,499 g, and greater than or equal to 2,500 g). Infants born weighing less than 1,500 g had a high rate of infant mortality, with the risk of death in the first year being approximately 400 deaths/1,000 live births for each of the racial/ethnic groups. The importance of birth defects in this group decreased compared with problems associated with prematurity and LBW. However, a relatively constant risk of death from birth defects remained. In all but the black group, the risk of death attributed to birth defects was 30-40/1,000 live births, accounting for approximately 7% of all mortality among these infants.

Among infants born weighing between 1,500 and 2,499 g, birth defects played a more important role. Asians had the highest rate of infant deaths in this group at 16.1/1,000 live births, a rate equivalent to that observed for white infants. American Indians had a rate of 14.3/1,000 live births, followed by Hispanics at 13.7/1,000 live births. Blacks had the lowest rate at 8.5/1,000 live births.

Overall, proportional mortality was high in this weight group, although it varied by racial/ethnic group. Proportional mortality was highest among Asians (52%), followed by whites (44%), Hispanics (39%), American Indians (31%), and blacks (29%). Among infants born weighing greater than or equal to 2,500 g, the risk of death from birth defects was highest among American Indians at 2.0/1,000 live births, followed by Hispanics at 1.7/1,000 live births. The remaining racial/ethnic groups had rates between 1.4 and 1.5/1,000 live births.

The proportional mortality was less in this group than in the group weighing from 1,500 to 2,499 g: whites and Asians (32%), Hispanics (31%), blacks (21%), and American Indians (20%). Data on the underlying cause provided an incomplete picture of the factors contributing to infant mortality. Because many factors may have contributed to an infant's death, data supplying multiple causes provided a better understanding of infant mortality. The authors looked at birth defects that were listed with other causes of death. Table 4 shows the number of records with each of the 10 leading causes of death listed on the record. The percentage of records with birth defects listed are also shown. In the total population, 8,684 (25.1%) of all infant deaths had at least one birth defect listed as a contributing cause of death. Of the 13,068 infants with LBW or prematurity listed as one of the causes of death, 986 (7.5%) had birth defects also listed as a contributing cause. Infants with hypoxia/asphyxia, unintentional injuries, pneumonia, and perinatal infections had a relatively high proportion of birth defects listed as well. A large proportion of infants with both birth defects and unintentional injuries listed among the multiple causes had injuries which were a result of medical or surgical procedures.

Table 5 shows data on multiple cause by racial/ethnic group. Asian infants had the largest proportion of infants with at least one birth defect listed as a multiple cause of death, whereas black infants had the smallest proportion. Substantial variation in the contribution of birth defects existed between racial/ethnic groups when looking at data on multiple causes. For example, among records with unintentional injuries listed as a multiple cause, the percentage of records with birth defects also listed ranged from 12.9% among American Indians to 28.4% among whites. On the other hand, among infants with perinatal infections listed as a cause of death, American Indians had the highest proportion of infants with birth defects also listed, whereas black infants had the lowest proportion.

DISCUSSION

Overall, birth defects were the leading cause of infant mortality. Among racial/ethnic minorities, birth defects were the leading cause of infant mortality among Hispanics and Asians. The risk of infant death from birth defects varied among racial/ethnic groups, but for all ethnic groups, the risk was higher than for white infants. American Indians had the highest rates of lethal birth defects, followed by Asians, Hispanics, and blacks. The variation in rate of lethal birth defects among racial/ethnic groups may have been related to both incidence and survival. In turn, variation in survival may have been related to delivery and access to the health care system. These variations among racial/ethnic minorities warrant further investigation. The proportional mortality due to birth defects varied greatly by racial/ethnic group predominately because of the increased contribution of LBW, prematurity, and RDS among blacks and because of SIDS among American Indians. The proportional mortality due to birth defects was greatest among Asians, followed by whites, Hispanics, American Indians, and blacks. In addition, data on multiple causes indicated that birth defects most often contributed to infant mortality among Asians, followed by whites, Hispanics, American Indians, and blacks.

Because the proportional mortality due to birth defects varied greatly among racial/ethnic groups, the substantial gap of infant mortality that existed between minority and non-minority infants will not be ameliorated by addressing birth defects alone. Black and American Indian infants continued to have the highest rates of overall infant mortality, which probably reflected differences in access to medical care, socioeconomic status, nutrition, and maternal lifestyles. These issues should be addressed in order to close the gap represented by infant mortality rates.

Although black infants showed the highest overall rate of infant mortality, American Indian infants actually had higher rates of infant mortality when stratified by birth weight and postneonatal period. This apparent discrepancy resulted from the higher proportion of LBW infants among the black group than among the American Indian group. Although 11.7% of black infants were born weighing less than 2,500 g, only 5.5% of American Indians were born weighing less than 2,500 g. The high rates of mortality among LBW infants heavily influenced the overall mortality among blacks as compared with American Indians.

Some analysts have attributed the rapid decline of infant mortality rates in the 1970s to rising medical technology in the care of premature and other critically ill newborns. In the 1980s, this decline in infant mortality rates slowed considerably--partly because of a lack of progress in primary prevention of conditions that lead to infant death. As a consequence, the 1990 health objective of nine infant deaths/1,000 live births is unlikely to be met (14). Additionally, to meet the year 2000 objectives, health agencies will have to make substantial efforts to prevent the leading causes of infant mortality.

To decrease infant mortality due to birth defects, population-based surveillance systems on birth defects must continue to proliferate and expand in the United States and throughout the world (15-17). Collaborative epidemiologic studies are being done as part of the International Clearinghouse for Birth Defects Monitoring Systems (18-20) and the European Registry of Congenital Abnormalities and Twins (EUROCAT) project (21,22). These surveillance systems have been invaluable resources as population-based registries for evaluation of health services and in the conduct of both descriptive epidemiologic studies and follow-up studies. One example is the large-scale case-control study conducted by the CDC between 1982 and 1984 to test whether offspring of male Vietnam veterans were at greater risk of having serious birth defects than offspring of men who did not serve in Vietnam (23). Cases from the existing Metropolitan Atlanta Congenital Defects Program (MACDP), which began in 1968, were studied (24). Similarly, collaborative efforts among state-based surveillance systems would improve the ability to perform etiologic studies of birth defects. CDC has increasingly focused efforts to standardize data-collection instruments (on the basis of the MACDP system) in order to facilitate this collaboration.

In summary, birth defects were an important contributor to infant mortality among all racial/ethnic minorities. Epidemiologic data obtained from surveillance systems and birth-defects registries would provide the basis for the development of prevention programs for infant mortality associated with birth defects among racial/ethnic minorities. Specifically, surveillance systems on birth defects should be used to evaluate whether racial/ethnic differences in infant mortality from birth defects are due to differences in incidence and/or survival among minority infants with birth defects.

References

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