Current Trends National Birthweight-Specific Infant Mortality Surveillance: Preliminary Analysis -- United States, 1980

Low birthweight (LBW), the greatest single problem associated with infant mortality (1), is related to many infant and maternal characteristics (2), including several that can be examined using vital records. Such records include infant's race, sex, gestational age at birth, birth order and plurality, and mother's age, educational attainment, and prenatal-care history. These maternal and infant characteristics are also related to neonatal* and postneonatal** mortality in varying degrees, in part because of their association with LBW. To examine these associations in the population, it is necessary to link individual birth and infant death certificates. While such linkage has been conducted in various states, the last national birth-death linkage was performed for infants born during 1960 (3). CDC, in collaboration with all states, the National Institute of Child Health and Human Development, the National Center for Health Statistics (NCHS), and the Health Resources and Services Administration, compiled birth-death linked data for live births occurring in 1980 based on linkages performed within each state, New York City, the District of Columbia, and Puerto Rico.

Preliminary analysis from this National Infant Mortality Surveillance (NIMS) shows that neonatal mortality (NNM) ranged from 647.6/1,000 live-born infants 500-999 gS to 1.4/1,000 live-born infants 3,500-3,999 g (Figure 1). However, at 3,500 g or greater, NNM increased with increasing birthweight. Compared with black infants, white infants under 3,000 g had higher NNM, but whites 3,000 g or greater had lower NNM than blacks with comparable birthweight.

Postneonatal mortality (PNM) among neonatal survivors ranged from 135.2/1,000 survivors 500-999 g at birth to 1.9/1,000 survivors 4,000-4,499 g at birth (Figure 2). PNM rose with birthweight 4,500 g or greater for both blacks and whites. Black neonatal survivors experienced higher PNM in all birthweight categories.

There was a 93.9-fold relative risk (RR) of infant mortality for singleton infants under 1,500 g at birth (469.4 deaths/1,000 live births), compared with infants 2,500 g or greater (5.0 deaths/1,000 live births) (Table 1). When smaller white singleton infants were compared with larger white singleton infants, the RR was somewhat higher (108.0) than the RR (93.9) for those of all races. When smaller black singleton infants were compared with larger, the RR was lower (61.6), primarily because infant mortality among black infants did not improve as greatly with increasing birthweight. Moreover, black infants under 2,500 g had a lower infant mortality than white infants of comparable birthweight (under 1,500 g and 1,500-2,499 g), while black infants 2,500 g or greater had a greater risk of infant mortality. The overall twofold higher risk of infant mortality for blacks than for whites reflects their excess mortality among the majority of births that occur in the larger-weight categories as well as the higher risk of LBW among black infants. For singleton black infants, 2.1% were under 1,500 g, and 9.2% weighed 1,500-2,499 g; percentages for singleton white infants were 0.7% and 4.2%, respectively.

For multiple-born infants (e.g., twins, triplets), birthweight-specific infant mortality also declined with increasing birthweight (Table 1); black multiple-born infants experienced lower infant mortality than whites, only at birthweight under 1,500 g. Within races, multiple-born infants under 2,500 g had lower infant mortality than singleton infants of comparable birthweight (under 1,500 g and 1,500-2,499 g), but multiple-born infants 2,500 g or greater had higher infant mortality. Among black multiple-born infants, 16.3% were under 1,500 g, and 45.5% were 1,500-2,499 g; among whites, 9.4% were under 1,500 g, and 39.8% weighed 1,500-2,499 g. Overall, multiple-born infants experienced higher infant mortality than singleton infants because of a greater risk of both LBW and death among heavier infants.

Infant mortality was higher among singleton males than among singleton females (Table 2). Among white singletons, infant mortality improved with longer gestations to 42 weeks and was lowest for second-born infants; for blacks, infant mortality was lowest for third-born infants and decreased with increasing gestation to 40 weeks. Infant mortality was lowest for black infants born to mothers 25-34 years of age; for white infants, infant mortality was lowest among those born to mothers 30-34 years of age. For all races combined, infant mortality was lowest among infants born to college graduates and women who received prenatal care beginning in the first trimester. Reported by all state health depts; the health depts of New York City, the District of Columbia, and Puerto Rico; Demographic and Behavioral Sciences Br, Center for Population Research, National Institute of Child Health and Human Development, National Institutes of Health; Div of Maternal and Child Health, Health Resources and Svcs Administration; National Center for Health Statistics; Pregnancy Epidemiology Br, Research and Statistics Br, Div of Reproductive Health, Center for Health Promotion and Education, CDC.

Editorial Note

Editorial Note: The reduction of infant mortality is a major health objective in the United States (1). Success in improving survival of LBW infants since 1960 has not been paralleled by a decline in the incidence of LBW (2,4-8). Lower infant mortality rates in other industrialized nations point to further improvements that can be achieved in the United States (8-10), especially for black infants who continue to suffer neonatal and postneonatal mortality rates approximately twice as high as those for white infants (11,12).

Analysis of infant mortality by birthweight provides a powerful tool at both national and state levels for identifying problems in maternal and infant care and for developing intervention plans. The NIMS project represents an intermediate step towards the routine annual collection of linked birth and death information as proposed by NCHS. This project required major efforts from all states and other vital registration reporting areas to provide the first national data on birthweight-specific infant mortality since 1960.

With the exception of infant's sex and race, information on the relationship between these infant and maternal characteristics and the risk of infant death can be obtained only through linkage of the birth certificate and the death certificate with information on cause and age at death. This preliminary analysis has revealed gradients in infant mortality for all reported characteristics, including at least a twofold gradient by maternal age and education for both blacks and whites. These data confirm the findings of previous studies that indicate a crossover in NNM between blacks and whites, with black infants experiencing lower NNM at lower birthweights and white infants experiencing lower NNM at higher birthweights (12,13). This crossover is also evident by reported gestational age. For birthweight under 3,000 g, black infants have been reported to experience lower NNM for all combinations of birthweight and gestational age in individual states (12,13).

These data will be disseminated through detailed tabulations in forthcoming reports that will include numbers and rates for NNM and PNM divided into 500-g intervals (under 4,500 g). In addition to the characteristics for singleton births listed in Table 2 (i.e., infant's sex, gestation, and live-birth order; and mother's age, education, and month prenatal care began), data will be available for type of delivery, number of other previous terminations, and number of prenatal-care visits. All tables for singleton infants will be reported separately for blacks, whites, and all races. For multiple gestations, data include plurality, gestational age, type of delivery, race, and birthweight. For all singleton deaths, data will also be tabulated by underlying cause, birthweight, race, and age at death. Preliminary analysis by cause of death has previously been reported (14).

References

1. U.S. Public Health Service. Pregnancy and infant health. In: Promoting health/preventing disease: objectives for the nation. Washington, D.C.: U.S. Department of Health and Human Services, 1980:15-20.

2. Institute of Medicine. Preventing low birthweight. Washington, D.C.: National Academy Press, 1985.

3. Armstrong RJ. A study of infant mortality from linked records by birth weight, period of gestation, and other variables, United States. Hyattsville, Maryland; National Center for Health Statistics, 1972; DHEW publication no. (HSM) 72-1055 (Vital and health statistics; series 20; no. 12).

4. Lee KS, Paneth N, Gartner LM, Pearlman MA, Gruss L. Neonatal mortality: an analysis of the recent improvement in the United States. Am J Public Health 1980;70:15-21.

5. Kleinman JC. Trends and variation in birth weight. Health United States 1981. Hyattsville, Maryland: U.S. Public Health Service, 1981; DHHS publication no. (PHS) 82-1232.

6. Williams RL, Chen PM. Identifying the sources of the recent decline in perinatal mortality rates in California. N Engl J Med 1982;306:207-14.

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