Analysis of unlinked infant death certificates from the NIMS project.

The National Infant Mortality Surveillance (NIMS) project used linked birth and infant death certificates to calculate birth weight-specific infant mortality risks for the 1980 U.S. birth cohort. Record linkage depends on complete registration of vital events, interstate exchange of vital records, accurate information on certificates, and a comprehensive linkage system. States reported 2,604 unlinked infant death certificates for 1980, ranging from 0 to 397 per State. Age at death for these infants ranged from 1 minute to 11 months. More than 41 percent of the unlinked death certificates were for postneonates, compared with 32.5 percent found in the cohort's total infant death experience. Only 38.2 percent of the unlinked infant death certificates showed strictly intrastate events (birth and death occurrence, and residence at death all in one State), compared with 92.9 percent in the cohort's total infant death experience. Estimates of the percentage successfully linked by State ranged from 86.0 to 100.0. After adjusting for the certainly unlinked infant death certificates, nine States' infant mortality risks increased by more than 0.2 per 1,000 live births. Improvements are needed both within and between States to ensure more complete birth and infant death certificate linkage.