Changes in the classification of sudden unexpected infant deaths: United States, 1992-2001

Sudden infant death syndrome (SIDS) makes up the largest component of sudden unexpected infant death in the United States. Since the first recommendations for supine placement of infants to prevent SIDS in 1992, SIDS postneonatal mortality rates declined 55% between 1992 and 2001.

The objective of this analysis was to examine changes in postneonatal mortality rates from 1992 to 2001 to determine if the decline in SIDS was due in part to a shift in certification of deaths from SIDS to other causes of sudden unexpected infant death. In addition, the analysis reviews the change in mortality rates attributed to the broad category of sudden unexpected infant death in the United States since 1950.

US mortality data were used. The International Classification of Diseases (ICD) chapters "Symptoms, Signs, and Ill-Defined Conditions" and "External Causes of Injury" were considered to contain all causes of sudden unexpected infant death. The following specific ICD (ninth and tenth revisions) underlying-cause-of-death categories were examined: "SIDS," "other unknown and unspecified causes," "suffocation in bed," "suffocation-other," "aspiration," "homicide," and "injury by undetermined intent." The average annual percentage change in rates was determined by Poisson regression. An analysis was performed that adjusted mortality rates for changes in classification between ICD revisions.

The all-cause postneonatal mortality rate declined 27% and the postneonatal SIDS rate declined 55% between 1992 and 2001. However, for the period from 1999 to 2001 there was no significant change in the overall postneonatal mortality rate, whereas the postneonatal SIDS rate declined by 17.4%. Concurrent increases in postneonatal mortality rates for unknown and unspecified causes and suffocation account for 90% of the decrease in the SIDS rate between 1999 and 2001.

The failure of the overall postneonatal mortality rate to decline in the face of a declining SIDS rate in 1999-2001 raises the question of whether the falling SIDS rate is a result of changes in certifier practices such that deaths that in previous years might have been certified as SIDS are now certified to other non-SIDS causes. The observation that the increase in the rates of non-SIDS causes of sudden unexpected infant death could account for >90% of the drop in the SIDS rates suggests that a change in classification may be occurring.