Perspectives in Disease Prevention and Health Promotion Years of Potential Life Lost Attributable to Low Birthweight -- United States, 1980 Birth Cohort

To determine the years of potential life lost before age 65 (YPLL) attributable to low birthweight for all causes of infant mortality, cause- and birthweight-specific mortality data were examined from CDC's National Infant Mortality Surveillance (NIMS) project for singleton births in 1980. This is the most recent file of birthweight-specific infant mortality for the United States.

For the 1980 birth cohort, all states, the District of Columbia, New York City, and Puerto Rico linked birth and infant death certificates and provided CDC with tabulations of deaths by birthweight and other characteristics, including The International Classification of Diseases, 9th Revision (ICD-9), codes for underlying cause of death. For each reporting area, the number of live births for calculation of mortality rates was obtained from computer tapes of national natality data prepared by the National Center for Health Statistics (NCHS).*

This analysis included only those singleton infants with known birthweight, in three groups: 500-1,499 grams (very low birthweight (VLBW)); 1,500-2,499 grams (intermediate low birthweight ); and 2,500 grams or more. Underlying causes of death were aggregated into 10 categories using a modification of previously described classification schemes (Table 3) (1,2). To calculate YPLL, an average age at death of 3.3 days and 127 days was used for neonatal (under 28 days) and postneonatal (28-365 days) deaths, respectively, reflecting the actual age distribution of deaths to infants born in 1980. For each cause of death, the percentage of infant deaths attributable to the increased risk of death associated with VLBW and ILBW (population-attributable risk) (3) was calculated, using the rate of death among infants 2,500 grams or more as the referent.** This percentage was then applied to the total YPLL from each cause of infant death to determine YPLL attributable to VLBW and ILBW.S

The neonatal mortality rates were 362.1, 23.1, and 2.0 deaths/1,000 live births, and the postneonatal mortality rates were 64.6, 12.3, and 2.8 deaths/1,000 neonatal survivors for VLBW, ILBW, and 2,500-gram or more infants, respectively. These calculations are based on 30,919 VLBW infants, 176,812 ILBW infants, and 3,324,881 infants weighing 2,500 grams or more.

A total of 2,257,988 YPLL was due to infant deaths. Neonatal deaths accounted for 63.3%, and postneonatal deaths, for 36.7%, of total YPLL; 802,326 (35.5%) YPLL were attributable to VLBW; and 347,773 (15.4%), to ILBW. The increased risk of death associated with low birthweight (VLBW plus ILBW) accounted for 67.5% of YPLL due to neonatal deaths and 22.3% of YPLL due to postneonatal deaths.

The leading causes of YPLL were perinatal conditions, Pcongenital anomalies, sudden infant death syndrome (SIDS), and infections (Table 3). The pattern was similar for YPLL attributable to low birthweight, except that infections were more prominent than SIDS. There were 414,116 YPLL due to causes coded by ICD-9 as prematurity, low birthweight, or respiratory distress syndrome, including 344,093 (83.1%) YPLL attributable to VLBW and 47,737 (11.5%) YPLL attributable to ILBW. Reported by state health departments of all 50 states, New York City, District of Columbia, and Puerto Rico; Pregnancy Epidemiology Br, Research and Statistics Br, Div of Reproductive Health, Center for Health Promotion and Education, CDC.

Editorial Note

Editorial Note:Many infant deaths could probably be prevented by better application of existing technologies and skills (1). The importance of this public health problem is highlighted through the use of YPLL to assess infant mortality, because deaths early in life are weighted heavily in the calculation of YPLL (4).

Data in this report are derived by linking birth and infant death records by states and reported to the NIMS project. This linkage is estimated to include 95% of reported infant deaths. The number of YPLL attributable to low birthweight depends on the proportion of infants with VLBW and ILBW, their mortality rate, and their relative risk of death compared to infants with birthweight of 2,500 grams or more. YPLL calculated in this manner is less than the YPLL for VLBW and ILBW infants, because not all deaths among them are attributable to the risk associated with low birthweight alone. Use of the population-attributable risk to estimate the proportion of YPLL attributed to low birthweight allows a description of the increase in risk of death associated with VLBW and ILBW. This approach permits assessment of both the importance of individual causes of death and the contribution of low birthweight to each cause of infant mortality.

YPLL due to congenital anomalies and SIDS in this report differs somewhat from two recent CDC reports, because a different age at death was used in calculations (5,6). Congenital anomalies contribute substantially to YPLL overall (24.0%), and YPLL attributable to low birthweight (38.5%). Because certain congenital anomalies may increase the risk of low birthweight, some of the YPLL attributable to low birthweight among infants with congenital anomalies may be more properly attributed to the congenital anomaly per se.

Infants with low birthweight may continue to be at an increased risk of death through the fourth year of life (7). Thus, the actual YPLL due to low birthweight is higher than the estimates presented here, which are limited to infant deaths (under 1 year).

Although infants with VLBW and ILBW represent less than 6% of newborns with known birthweight of 500 grams or more, they account for approximately half of YPLL due to infant deaths. Efforts to reduce infant deaths will depend to a high degree on the prevention of low birthweight (8).

References

1. Brann AW, McCarthy BJ, Adams MJ, et al. Unintended pregnancy and infant mortality and morbidity: strategies for closing the gap. In: Closing the gap: National Health Policy Consultation. Carter Center of Emory University, Atlanta, Georgia, November 26-28, 1984.

2. Buehler JW, McCarthy BJ, Holloway JT, Sikes RK. Infant mortality in a south Georgia health district, 1974-1981. South Med J (in press).

3. Kleinbaum DG, Kupper LL, Morgenstern H. Epidemiologic research, principles and quantitative methods. Belmont, California: Lifetime Learning, 1982:159-80.

4. CDC. Changes in premature mortality--United States, 1983-1984. MMWR 1986;35:29-31.

5. CDC. Premature mortality due to congenital anomalies. MMWR 1986;35:97-9.

6. CDC. Premature mortality due to sudden infant death syndrome. MMWR 1986;35:169-70.

7. Meirik O. Birthweight specific survivorship of children through age four in Sweden. Presented at the American Public Health Association, 113th Annual Meeting, Washington D.C., November 17-21, 1985.

8. Institute of Medicine. Preventing low birthweight. Washington, D.C.: National Academy Press, 1985. *Data from Puerto Rico are not included in this analysis. For New Mexico, number of births was obtained from state computer tapes rather than NCHS tapes. **Population-attributable risk = D - (BR)/T, where D = number of deaths due to a specific cause in a low birthweight category (VLBW or ILBW); B = number of births in a specific birthweight category; R = cause-specific death rate for infants with birthweight 2,500 grams or more; and T = total number of cause-specific infant deaths. SYPLL = T(65 - (A/365.25)), where A = average age at death, in days. P"Perinatal conditions" refers to conditions classified by ICD-9 (codes 760-779, excluding congenital infections) as originating in the perinatal period and does not refer to the time of death.

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