SIDS is the sudden death of an infant younger than 1 year of age, which remains unexplained after a thorough case investigation, including performance of a complete autopsy, examination of the death scene, and review of the clinical history. SIDS is considered a diagnosis of exclusion. The etiology of SIDS is unknown. A popular hypothesis is the triple risk model, which suggests that SIDS occurs in infants with predisposing factors (i.e., genetic pattern) who experience a trigger (i.e., maternal smoking), at a vulnerable developmental stage of the central nervous or immune system.

Deaths from SIDS have been declining since 1992, the year a major intervention was sponsored by the American Academy of Pediatrics (AAP) to encourage parents to place their babies in nonprone positions when putting them in a crib to sleep. In 1991, according to the National Center for Health Statistics (NCHS), the SIDS death rate was 1.3 deaths per 1,000 live births. By 2004, the rate had fallen to 0.55 deaths per 1,000 live births. According to the NCHS, the total number of deaths from SIDS in 2004 was 2,246. The occurrence of SIDS is rare during the first month of life, increases to a peak between 2 and 3 months of age, and then becomes rare after 6 months. Despite reductions in SIDS deaths, it remains the number one cause of death for infants between ages of 1 month and 1 year.

Risk factors for SIDS include the following: