The Lowest Birth-Weight Infants and the US Infant Mortality Rate: NCHS 1983 Linked Birth/Infant Death Data Mary D. Overpeck MPH, Howard J.
Intrdudion Low- and very low-birth-weight perinatal mortality has decreased dramatically over the past few decades."2 California linked birth and death certificates show that the decline in perinatal mortality rates in the 1970s occurred primarily from improvements in the survival of infants born at weights above 750 g.3 Virtually no change was observed for infants weighing under 751 g, and only a small decrease was observed for infants weighing between 751 and 1000 g. Prospective studies of reproductive outcomes also show higher losses among -fetuses from 16 to 24 gestational weeks than among fetuses from 25 to 29 gestational weeks, with clinical assessment of very poor viability for fetuses under 500 g or 22 weeks of fertilization age.4-8 The Linked Birth and Infant Death Data Set, 1983 birth cohort, from the National Center for Health Statistics (NCHS) is the first national linkage of individual birth and death certificates since 1960.9,10 Linkage overcomes limitations of infant mortality rates (IMRs), in which the numerator is based only on annual counts of death certificates; it also permits calculation of mortality rates by birth weight for the whole United States. A 1980 analysis based on linkages by individual states shows the impact of veiy low-birth-weight (VLBW) infants on the US IMR. This tabular data set, however, did not include births under 500 g or allow comparisons of infants born at weight intervals smaller than 500 g."1 The present analysis focuses on individually linked data for the least viable infants (those weighing less than 500 g and those weighing 500 to 749 g), their contribution to the total IMR, and the rate and age at which they die. Black and White births are analyzd separately because of differential rates of extremely low-weight births and attributable proportions of deaths.
Mehods The methodology to create the 1983 linked file is described in both the NCHS
Hoffinan, AM, and Kate Prager,
ScD
Public Use Data Tape Documentation and a working paper.1213 The file contains linked records for all infants born in 1983 in the United States who died in 1983, or in 1984 before their first birthday. Records were linked for 39 683 resident infant deaths, 98.4% of all infant deaths occurring in the United States during the time specified. Analysis of the 651 unlinked death certificates shows them to be similar to the linked certificates in terms of race and age at death. If no death certificate was filed, an infant with a birth certificate was assumed to have survived. Specific rates may be slightly understated owing to unlinked records or to missing information on the linked records. Birth weight is missing for 2.9% of linked infant deaths and for 0.1% of all births. Birth weight is used here instead of gestational age because it is more completely and accurately reported.'4.'5 Differences larger than 2% in Figures 1 and 2 are statistically highly significant based on Fleiss' confidence interval procedures.'6 Otherwise, statistical significance has been assessed using chi-square tests on relative risk (RR) estimates, as discussed for Table 1.
Results Forty-three percent of all infant deaths came from 1.2% of all live births, the VLBW infants weighing less than 1500 g (Table 1). The majority of deaths among VLBW infants came from those weighing 500 to 749 g and those weighing less than 500 g, with 25% of total deaths coming Mary Overpeck and Howard Hoffinan are with the National Institute of Child Health and Human Development, National Institutes of Health, in Bethesda. Kate Prager is with the National Center for Health Statistics, Centers for Disease Control, in Hyattsville. Requests for reprints should be sent to Mary D. Overpeck, MPH, Division of Epidemiology, Statistics and Prevention Research, National Institute of Child Health and Human Development, Executive Plaza North, Room 640, Bethesda, MD 20892. This paper was submitted to the Journal February 19, 1991, and accepted with revisions July 18, 1991.
American Journal of Public Health 441
Pubfic Health Briefs
> 2500 9
1500-2499 g 14%
1500-2499 g
UnstUnstate
16%
Unstated
d
Usae
3%
50074 g 750-1499
5001~~9 9
g
Whites
00-914% 19%
Blacks
FIGURE 1-AtbIbutable proportions of infant deaths within each race for the US 1983 birth cohort
Soure.-National Center for Health Statistics, National Vital Statistics System (unpublished data). from the 0.3% of births at these lowest weights (14.7% from White births, 9.2% from Black births). Infants weighing 500 to 749 g had a mortality rate more than three times the rate of those weighing 750 to 1499 g; those weighing under 500 g had nearly four times the rate. Mortality rates with weight not stated indicate a bias toward the smallest infants. Higher birth-weight-specific mortality rates for Blacks occurred only at normal weights (RR = 1.66) and unknown weights (RR = 2.16). Relative risks of death for Black infants compared with White infants were 0.85 at 1500 to 2499 g, 0.77 at 750 to 1499 g, 0.93 at 500 to 749 g, and 0.98 at under 500 g. Relative risks indicate better survival only for those Black infants weighing between 750 and 1499 g and 1500 and 2499 g. The attributable proportion of Black deaths from births weighing less than 750 g was 33%, compared with 21% for White deaths (Figure 1). Infants born weighing
442 American Journal of Public Health
750 to 1499 g contributed similar proportions of deaths within each race. Because there were 5.3 times more White births than Black births weighing 2500 g or more, compared with only 1.5 times more White births weighing less than 500 g (Table 1), a much larger fraction of Black deaths resulted from extremely low weights. The risk of death was highest in the early neonatal period (from 0 to 6 days of life), especially for VLBW infants (Table 2). The early neonatal mortality rate increased exponentially as birth weight decreased by 250-g intervals. The late neonatal and postneonatal mortality rates also increased as birth weight decreased. The small numbers of late neonatal and postneonatal deaths reported for infants weighing less than 500 g may have produced unreliable rates. Survival decreased rapidly as birth weight decreased. Births weighing less than 750 g resulted in 41% of all early neonatal deaths (Table 3). Of those, 18% were attributable
to births weighing less than 500 g. Deaths of the smallest infants contributed to more early neonatal mortality among Blacks than among Whites (Figure 2). Twentyfive percent of Black and 15% of White early neonatal deaths were due to infants weighing
Intrdudion Low- and very low-birth-weight perinatal mortality has decreased dramatically over the past few decades."2 California linked birth and death certificates show that the decline in perinatal mortality rates in the 1970s occurred primarily from improvements in the survival of infants born at weights above 750 g.3 Virtually no change was observed for infants weighing under 751 g, and only a small decrease was observed for infants weighing between 751 and 1000 g. Prospective studies of reproductive outcomes also show higher losses among -fetuses from 16 to 24 gestational weeks than among fetuses from 25 to 29 gestational weeks, with clinical assessment of very poor viability for fetuses under 500 g or 22 weeks of fertilization age.4-8 The Linked Birth and Infant Death Data Set, 1983 birth cohort, from the National Center for Health Statistics (NCHS) is the first national linkage of individual birth and death certificates since 1960.9,10 Linkage overcomes limitations of infant mortality rates (IMRs), in which the numerator is based only on annual counts of death certificates; it also permits calculation of mortality rates by birth weight for the whole United States. A 1980 analysis based on linkages by individual states shows the impact of veiy low-birth-weight (VLBW) infants on the US IMR. This tabular data set, however, did not include births under 500 g or allow comparisons of infants born at weight intervals smaller than 500 g."1 The present analysis focuses on individually linked data for the least viable infants (those weighing less than 500 g and those weighing 500 to 749 g), their contribution to the total IMR, and the rate and age at which they die. Black and White births are analyzd separately because of differential rates of extremely low-weight births and attributable proportions of deaths.
Mehods The methodology to create the 1983 linked file is described in both the NCHS
Hoffinan, AM, and Kate Prager,
ScD
Public Use Data Tape Documentation and a working paper.1213 The file contains linked records for all infants born in 1983 in the United States who died in 1983, or in 1984 before their first birthday. Records were linked for 39 683 resident infant deaths, 98.4% of all infant deaths occurring in the United States during the time specified. Analysis of the 651 unlinked death certificates shows them to be similar to the linked certificates in terms of race and age at death. If no death certificate was filed, an infant with a birth certificate was assumed to have survived. Specific rates may be slightly understated owing to unlinked records or to missing information on the linked records. Birth weight is missing for 2.9% of linked infant deaths and for 0.1% of all births. Birth weight is used here instead of gestational age because it is more completely and accurately reported.'4.'5 Differences larger than 2% in Figures 1 and 2 are statistically highly significant based on Fleiss' confidence interval procedures.'6 Otherwise, statistical significance has been assessed using chi-square tests on relative risk (RR) estimates, as discussed for Table 1.
Results Forty-three percent of all infant deaths came from 1.2% of all live births, the VLBW infants weighing less than 1500 g (Table 1). The majority of deaths among VLBW infants came from those weighing 500 to 749 g and those weighing less than 500 g, with 25% of total deaths coming Mary Overpeck and Howard Hoffinan are with the National Institute of Child Health and Human Development, National Institutes of Health, in Bethesda. Kate Prager is with the National Center for Health Statistics, Centers for Disease Control, in Hyattsville. Requests for reprints should be sent to Mary D. Overpeck, MPH, Division of Epidemiology, Statistics and Prevention Research, National Institute of Child Health and Human Development, Executive Plaza North, Room 640, Bethesda, MD 20892. This paper was submitted to the Journal February 19, 1991, and accepted with revisions July 18, 1991.
American Journal of Public Health 441
Pubfic Health Briefs
> 2500 9
1500-2499 g 14%
1500-2499 g
UnstUnstate
16%
Unstated
d
Usae
3%
50074 g 750-1499
5001~~9 9
g
Whites
00-914% 19%
Blacks
FIGURE 1-AtbIbutable proportions of infant deaths within each race for the US 1983 birth cohort
Soure.-National Center for Health Statistics, National Vital Statistics System (unpublished data). from the 0.3% of births at these lowest weights (14.7% from White births, 9.2% from Black births). Infants weighing 500 to 749 g had a mortality rate more than three times the rate of those weighing 750 to 1499 g; those weighing under 500 g had nearly four times the rate. Mortality rates with weight not stated indicate a bias toward the smallest infants. Higher birth-weight-specific mortality rates for Blacks occurred only at normal weights (RR = 1.66) and unknown weights (RR = 2.16). Relative risks of death for Black infants compared with White infants were 0.85 at 1500 to 2499 g, 0.77 at 750 to 1499 g, 0.93 at 500 to 749 g, and 0.98 at under 500 g. Relative risks indicate better survival only for those Black infants weighing between 750 and 1499 g and 1500 and 2499 g. The attributable proportion of Black deaths from births weighing less than 750 g was 33%, compared with 21% for White deaths (Figure 1). Infants born weighing
442 American Journal of Public Health
750 to 1499 g contributed similar proportions of deaths within each race. Because there were 5.3 times more White births than Black births weighing 2500 g or more, compared with only 1.5 times more White births weighing less than 500 g (Table 1), a much larger fraction of Black deaths resulted from extremely low weights. The risk of death was highest in the early neonatal period (from 0 to 6 days of life), especially for VLBW infants (Table 2). The early neonatal mortality rate increased exponentially as birth weight decreased by 250-g intervals. The late neonatal and postneonatal mortality rates also increased as birth weight decreased. The small numbers of late neonatal and postneonatal deaths reported for infants weighing less than 500 g may have produced unreliable rates. Survival decreased rapidly as birth weight decreased. Births weighing less than 750 g resulted in 41% of all early neonatal deaths (Table 3). Of those, 18% were attributable
to births weighing less than 500 g. Deaths of the smallest infants contributed to more early neonatal mortality among Blacks than among Whites (Figure 2). Twentyfive percent of Black and 15% of White early neonatal deaths were due to infants weighing
