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Definition: sudden infant death syndrome from The Macquarie Dictionary

a syndrome leading to death in babies up to about two years old, the mechanisms of which are not yet understood but have been variously linked with excessive heat or cold, respiratory ailments giving rise to difficulty in breathing, the failure of the involuntary breathing process during sleep, and other such possible causes; cot death.

from A-Z of Death and Dying, The: Social, Medical, and Cultural Aspects

Sudden infant death syndrome (SIDS) is the sudden and unexpected death of a seemingly healthy infant less than one year of age. For a case to be considered that of SIDS, the death must be unexplained, even following a full investigation. SIDS usually occurs while the baby is asleep with no outward signs of distress or struggle; the infant may simply stop breathing. Most SIDS deaths occur among infants under six months of age, with the majority occurring under four months of age. The syndrome is more common in male babies than in female babies.


Despite a decline in rates of SIDS of more than 50 percent in Canada, the United States, and many other developed countries since the early 1990s, SIDS continues to be a leading cause of death among infants, accounting for approximately 25 percent of all deaths between one month and one year of age. SIDS rates in Canada and the United States in 1990 were 0.8 and 1.3 per 1,000 live births respectively, dropping to 0.3 and 0.8 per 1,000 live births in 2002. The declines in rates have been attributed mostly to public education advocating that infants be placed on their backs to sleep and that specific modifications to the baby's environment be made.


Factors that are thought to be relevant to SIDS include those that can be associated with the mother, the infant, and the environment:

  • Maternal factors

    There has been a higher incidence of SIDS associated with mothers who are very young, who smoke or use illicit drugs, who have other health issues, or where there were complications during the pregnancy.

  • Infant factors

    SIDS has been more commonly associated with infants who are preterm or who have a sibling that previously died from SIDS. In addition, male babies seem to be more prone to SIDS. Some infants may have poor temperature control. Despite the fact that SIDS babies may appear to be in perfectly good health, they may have an abnormality of the central nervous system (brain, nerves, or spinal cord) that has yet to be identified. Some infants, for example, may have underdeveloped parts of their central nervous systems that are not yet mature. Babies affected by SIDS may have some flaw in their breathing control or ability.

  • Environmental or care factors

    These factors may relate to infants who are not placed on their backs to sleep, whose sleeping area is overly soft or has loose bedding, being exposed to smoke, living in environments that are too warm, or living in poverty. It used to be that the majority of SIDS deaths occurred in winter, possibly due to babies being overwrapped in clothes and therefore at risk of becoming overheated. Now, there is evidence showing no difference in the number of SIDS deaths associated with different seasons. Overheating, however, is still considered a risk for SIDS. These factors are all seen as contributory; however, there is no evidence for a specific cause-and-effect relationship between any of these factors individually and death from SIDS.

Social Aspects

Parents who have lost a baby as a result of SIDS are often overcome with grief and guilt, feeling responsible for the death of their child. Not only must these parents struggle with the loss of their baby, but they frequently must also endure a police investigation of the death, and there is often a prolonged wait for the cause of death to be determined. The loss of the baby comes as a profound shock to the parents, and the traumatic nature of the death and the inability to find a concrete cause of death can further complicate their grief (see Grief). Although there is greater public awareness regarding SIDS, a SIDS death can carry stigma for the parents due to erroneous assumptions about causation and perceived parental deficiency or inadequacy of some type. It is important for these parents to have access to information, support, and counseling to help them get through this difficult time. Organizations dedicated to helping families after a SIDS loss, such as First Candle, may provide much-needed information as well as support.

See also: Children; Infant Mortality; Perinatal Death.

Further Reading
  • American Academy of Pediatrics Task Force on Sudden Infant Death Syndrome. “The Changing Concept of Sudden Infant Death Syndrome: Diagnostic Coding Shifts, Controversies Regarding the Sleeping Environment, and New Variables to Consider in Reducing Risk.” Pediatrics, 116(2005): 1245-55.
  • Hunt, Carl E.; Fern R. Hauck. “Sudden Infant Death Syndrome.” Canadian Medical Association Journal, 174(13) (2006): 1861-69.
  • Moon, Rachel Y.; Linda Y. Fu. “Sudden Infant Death Syndrome.” Pediatrics in Review, 28(6) (2007): 209-14.
  • Darcy Harris
    Copyright 2014 by Michael Brennan

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