National underascertainment of sudden unexpected infant deaths associated with deaths of unknown cause.

Objective. To investigate underascertainment of unexpected infant deaths at the national level as a result of probable classification as attributable to unknown cause. Methods. Using linked birth and death certificates for all US birth cohorts from 1983-1991 and 1995-1996, we identified 53 470 sudden infant death syndrome (SIDS) fatalities, 9071 unintentional injury deaths, 3473 injury deaths classified with intentional or suspicious intent, and 8097 deaths with unknown underlying cause. For these deaths, we compared relative risks (RRs) for maternal and infant variables available on birth certificates known to be predictive of SIDS, unintentional injury, and homicides. Variables available on death certificates were compared for unlinked and linked records. Factors related to state and national management of cases pending final cause determination are reviewed. Results. For deaths from unknown cause, rates were consistently high among the same risk groups that have been shown to be at increased risk for SIDS, unintentional injury, and homicides. For most risk factors, RRs for deaths attributable to unknown causes were somewhat lower than for RRs for intentional/suspicious injury deaths but higher than for SIDS or unintentional injury, indicating combined contributions from all causes. For example, age at death from unknown cause includes RRs that more strongly resemble patterns of intentional/suspicious injuries than SIDS or unintentional injury. Deaths from unknown cause were more likely to occur during the first week of life for unattended births occurring outside clinical settings or when birth certificates were not found, similar to intentional/suspicious injury deaths. Conclusions. Risk profiles indicate that deaths of unknown cause are likely to represent a mixture of unexpected deaths. The process for determination of cause of unexpected death affects national underascertainment of SIDS and injury deaths. Better coordination among child fatality review teams and local, state, and national officials should reduce underascertainment and improve documentation of circumstances surrounding deaths for prevention efforts.