SIDS is a diagnosis of exclusion and should be applied to only those cases in which an infants death is sudden and unexpected, and remains unexplained after the performance of an adequate postmortem investigation, including:

an autopsy (by an experienced pediatric pathologist, if possible);
investigation of the death scene and circumstances of the death; and
exploration of the medical history of the infant and family.After investigation, some of these infant deaths are found to be caused by suffocation, hyperthermia or hypothermia, neglect or some other defined cause.Australia and New Zealand shifted to sudden unexpected death in infancy (SUDI) for professional, scientific, and coronial clarity. The term SUDI is now often used instead of sudden infant death syndrome (SIDS) because some coroners prefer to use the term undetermined for a death previously considered to be SIDS. This change is causing diagnostic shift in the mortality data. In addition, the US Centers for Disease Control and Prevention have proposed that such deaths be called sudden unexpected infant deaths (SUID) and that SIDS is a subset of SUID. Age
SIDS has a four-parameter lognormal age distribution that spares infants shortly after birth — the time of maximal risk for almost all other causes of non-trauma infant death. By definition, SIDS deaths occur under the age of one year, with the peak incidence occurring when the infant is two to four months old. This is considered a critical period because the infants ability to rouse from sleep is not yet mature. Risk factors
The exact cause of SIDS is unknown.
Although studies have identified risk factors for SIDS, such as putting infants to bed on their bellies, there has been little understanding of the syndromes biological process or its potential causes. Deaths from SIDS are unlikely to be due to a single cause, but rather to multiple risk factors. The frequency of SIDS does appear to be influenced by social, economic, or cultural factors, such as maternal education, race or ethnicity, or poverty. SIDS is believed to occur when an infant with an underlying biological vulnerability, who is at a critical development age, is exposed to an external trigger. The following risk factors generally contribute either to the underlying biological vulnerability or represent an external trigger:

Tobacco smoke
SIDS rates are higher in babies of mothers who smoke during pregnancy. Between no smoking and smoking one cigarette a day, on average, the risk doubles. About 22% of SIDS in the United States is related to maternal smoking. SIDS correlates with levels of nicotine and its derivatives in the baby. Nicotine and derivatives cause alterations in neurodevelopment. Sleeping
Placing an infant to sleep while lying on the belly or side rather than on the back increases the risk for SIDS. This increased risk is greatest at two to three months of age. Elevated or reduced room temperature also increases the risk, as does excessive bedding, clothing, soft sleep surfaces, and stuffed animals in the bed. Bumper pads may increase the risk of SIDS due to the risk of suffocation.