The 1969 international definition refers to SIDS (sudden infant death syndrome) as the sudden, unexpected death of an infant for which no explanation can be found after a thorough investigation, including autopsy and assessment of circumstances and history (anamnesis) of death. This definition was further subdivided in 2004:
| SIDS category | Description |
| Ia | Cases in which “classic” signs of SIDS are present and documented. |
| Ib | Cases where these factors are present but not fully documented. |
| II | Cases in which the child dies at <3 weeks or >9 months of age, similar deaths occurred in sibling(s), close relatives, or children in the care of the same person, or perinatal events such as prematurity, mechanical asphyxia (i.e., airway obstruction), or marked inflammatory changes that, by themselves, do not adequately explain the occurrence of death |
| “non-classified sudden infant death syndrome”” | Cases that do not meet the previously mentioned criteria or for which no autopsy was performed should be called “unclassified sudden infant death” according to this recommendation. |
