Sudden infant death syndrome

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Sudden Infant Death Syndrome
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Safe to Sleep Public Education Campaign, NICHD
Classification and external resources
eMedicineemerg/407 ped/2171
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Sudden Infant Death Syndrome
Safe Sleep logo
Safe to Sleep Public Education Campaign, NICHD
Classification and external resources
eMedicineemerg/407 ped/2171

Sudden infant death syndrome (SIDS) is marked by the sudden death of an infant that is not predicted by medical history and remains unexplained after a thorough forensic autopsy and detailed death scene investigation.[1] As infants are at the highest risk for SIDS during sleep, it is sometimes referred to as cot death or crib death.


SIDS was the third leading cause of death of infant mortality in the U.S. in 2011. According to the Centers for Disease Control and Prevention, SIDS deaths have been declining since 1988.[3] The cause of SIDS is unknown, but some characteristics associated with the syndrome have been identified and appear to interact with other characteristics: A triple-risk model states that SIDS occurs when an infant with an underlying, biological vulnerability who is at a critical developmental age is exposed to an external trigger.[4]

SIDS prevention strategies include: putting the infant to sleep on his/her back, a firm mattress separate from but close to caregivers, no loose bedding, a relatively cool sleeping environment, using a pacifier, and avoidance of exposure to tobacco smoke.[5] The "Safe to Sleep" campaign is considered a significant public health success, credited with leading to a measurable reduction in SIDS rates.[6]

Infanticide and child abuse cases may be misdiagnosed as SIDS due to lack of evidence, and caretakers of infant with SIDS are sometimes falsely accused.[7][8] Accidental suffocations are also sometimes misdiagnosed as SIDS and vice versa.[9] Grief support for families impacted by SIDS is particularly important because the death of the infant is typically sudden, without witnesses, and requires an investigation.[4]


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

  1. an autopsy (by an experienced pediatric pathologist, if possible);
  2. investigation of the death scene and circumstances of the death;

After investigation, some of these infants deaths are found to be caused by accidental suffocation, hyperthermia or hypothermia, neglect or some other defined cause.[10]

Australia and New Zealand are shifting to the term "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.[11]

In addition, the U.S. Centers for Disease Control and Prevention (CDC) has recently proposed that such deaths be called "sudden unexpected infant deaths" (SUID) and that SIDS is a subset of SUID.[12]

Risk factors[edit]

The cause of SIDS is unknown. Although studies have identified risk factors for SIDS, such as putting infants to bed on their stomachs, there has been little understanding of the syndrome's biological process or its potential causes. The frequecy of SIDS does appear to be influenced by social, economic, and cultural factors, such as maternal education, race or ethnicity, and poverty.[13] SIDS occurs when an infant with an underlying, biological vulnerability who is at a critical development age is exposed to an external trigger.[4] The following risk factors generally contribute either to the underlying biological vulnerability or represent an external trigger.


Exposure to nicotine from maternal smoking — SIDS rates are higher for infants of mothers who smoke during pregnancy.[14][15][16]


Placing an infant to sleep while lying on the stomach or the side increases the risk.[5] This increased risk is greatest at two to three months of age.[5] Elevated or reduced room temperature also increases the risk.[17] As does excessive bedding, clothing, soft sleep surfaces, and stuffed animals.[18] Bumper pads may increase the risk and as there is little evidence of benefit from their use; they are not recommended.[5]

Sharing a bed with parents or siblings increases the risk for SIDS.[19] This risk is greatest in the first three months of life, when the bed is soft, when more than one other person is present, or when the bed partners are using drugs or alcohol or are smoking.[5] The risk remains however even in parents who do not smoke or use drugs.[20] The American Academy of Pediatrics thus recommends "room-sharing without bed-sharing", stating that such an arrangement can decrease the risk of SIDS by up to 50%. Furthermore, the Academy recommended against devices marketed to make bed-sharing "safe", such as in-bed co-sleepers.[21]

Pregnancy and infant factors[edit]

Maternal age — SIDS rates decrease with increasing maternal age, with teenage mothers at greatest risk.[14] Delayed or inadequate prenatal care [14] Low birth weight — in the United States from 1995–98, the SIDS death rate for infants weighing 1000–1499 g was 2.89/1000; for a birth weight of 3500–3999 g, it was only 0.51/1000.[22][23] Premature birth— increases risk of SIDS death roughly fourfold.[14][22] From 1995–1998 the U.S. SIDS rate for births at 37–39 weeks of gestation was 0.73/1000; the SIDS rate for births at 28–31 weeks of gestation was 2.39/1000[22] A lack of breastfeeding is linked to SIDS.[24]

Anemia has also been linked to SIDS[25] (note, however, that per item 6 in the list of epidemiologic characteristics below, extent of anemia cannot be evaluated at autopsy because "total hemoglobin can only be measured in living infants."[26]). Age of infant — SIDS incidence rises from zero at birth, is highest from two to four months of age, and declines toward zero after the infant's first year.[27] Male sex — male children have a ~50% higher risk of SIDS than female children.[28]


Genetics plays a role, as SIDS is more prevalent in males.[29][30] There is a consistent 50% male excess in SIDS per 1000 live births of each sex. Given a 5% male excess birth rate, there appears to be 3.15 male SIDS cases per 2 female, for a male fraction of 0.61.[29][30] This value of 61% in the US is an average of 57% black male SIDS, 62.2% white male SIDS and 59.4% for all other races combined. Note that when multiracial parentage is involved, infant "race" is arbitrarily assigned to one category or the other; most often it is chosen by the mother. The X-linkagehypothesis for SIDS and the male excess in infant mortality have shown that the 50% male excess could be related to a dominant X-linked allele, occurring with a frequency of 13 that is protective of transient cerebral anoxia. An unprotected XY male would occur with a frequency of 23 and an unprotected XX female would occur with a frequency of 49. The ratio of 23 to 49 is 1.5 to 1, which matches the observed male 50% excess rate of SIDS.

Critical developmental age[edit]

The unique signature characteristic of SIDS is its log-normal 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 1, with the peak incidence occurring when the infant is at 2 to 4 months of age. This is considered a critical period because the infant's ability to arouse from sleep is not yet mature.[4]


There is a tentative link with Staphylococcus aureus and Escherichia coli.[31] Vaccination does not increase the risk of SIDS, and may reduce the risk slightly.[32][33]

A 1998 report found "no evidence to substantiate the toxic gas hypothesis that antimony- and phosphorus-containing compounds used as fire retardants in PVC and other cot mattress materials are a cause of SIDS. Neither was there any evidence to believe that these chemicals could pose any other health risk to infants."[34] The report also states that "in normal cot-like conditions it is not possible to generate toxic gas from antimony in mattresses" and that "babies have also been found to die on wrapped mattresses."

A set of risk factors SIDS have been identified: seasonality: winter maximum, summer minimum; increasing SIDS rate with live birth order; low increased risk of SIDS in subsequent siblings of SIDS; apparent life threatening events (ALTE) are not a risk factor for subsequent SIDS; SIDS risk is greatest during sleep.[35]

Differential diagnosis[edit]

Some conditions that are often undiagnosed and could be confused with or comorbid with SIDS include:

For example, an infant with MCAD deficiency could have died by "classical SIDS" if found swaddled and prone with head covered in an overheated room where parents were smoking. Genes indicating susceptibility to MCAD and Long QT syndrome do not protect an infant from dying of classical SIDS. Therefore, presence of a susceptibility gene, such as for MCAD, means the infant may have died either from SIDS or from MCAD deficiency. It is currently impossible for the pathologist to distinguish between them.

A 2010 study looked at 554 autopsies of infants in North Carolina that listed SIDS as the cause of death, and suggested that many of these deaths may have been due to accidental suffocation. The study found that 69% of autopsies listed other possible risk factors that could have led to death, such us unsafe bedding or sleeping with adults.[43]

Several instances of infanticide have been uncovered where the diagnosis was originally SIDS.[7][8] Estimate of the percentage of SIDS deaths that are actually infanticide vary from less than 1% to up to 5% of cases.[44]

Some have underestimated the risk of two SIDS deaths occurring in the same family and the Royal Statistical Society issued a media release refuting this expert testimony in one UK case in which the conviction was subsequently overturned.[45]


A number of measures have been found to be effective in preventing SIDS including: sleeping position, breastfeeding, limiting soft bedding, immunizing the infant, and the use of pacifiers.[5] The use of electronic monitors have not been found to be useful and are thus not recommended.[5] Evidence regarding fans and swaddling is unclear.[5]

Sleep positioning[edit]

A plot of SIDS rate from 1988 to 2006

Sleeping on the back has found to reduce the risk of SIDS.[46] It is thus recommended by the American Academy of Pediatrics and promoted as a best practice by the US National Institute of Child Health and Human Development (NICHD) "Safe to Sleep" campaign. The incidence of SIDS has fallen in a number of countries in which this recommendation has been widely adopted.[47] Sleeping on the back does not appear to increase the risk of choking even in those with gastroesophageal reflux disease.[5] While infants in this position may sleep more lightly this is not harmful.[5] Sharing the same room as ones parents but in a different bed may decrease the risk by half.[5]


The use of pacifiers appears to decrease the risk of SIDS although the way they do this is unclear.[5] The American Academy of Pediatrics considers pacifier use to prevent SIDS to be reasonable.[5] Pacifiers do not appear to affect breastfeeding in the first four months, even though this is a common misconception.[48]


Product safety experts advise against using pillows, overly soft mattresses, sleep positioners, bumper pads, stuffed animals, or fluffy bedding in the crib and recommend instead dressing the child warmly and keeping the crib "naked."[49]


Sleep sacks[edit]



Families who are impacted by SIDS should be offered emotional support and grief counseling.[52] The experience and manifestation of grief at the loss of an infant is impacted by cultural and individual differences.[53]


Globally SIDS resulted in about 22,000 deaths as of 2010, down from 30,000 deaths in 1990.[54] Rates vary significantly by population from 0.05 per 1000 in Hong Kong to 6.7 per 1000 in American Indians.[55]

SIDS was responsible for 0.54 deaths per 1,000 live births in the US in 2005.[22] It is responsible for far fewer deaths than congenital disorders and disorders related to short gestation, though it is the leading cause of death in healthy infants after one month of age.

SIDS deaths in the US decreased from 4,895 in 1992 to 2,247 in 2004.[56] But, during a similar time period, 1989 to 2004, SIDS being listed as the cause of death for sudden infant death (SID) decreased from 80% to 55%.[56] According to Dr. John Kattwinkel, chairman of the Centers for Disease Control and Prevention (CDC) Special Task Force on SIDS "A lot of us are concerned that the rate (of SIDS) isn't decreasing significantly, but that a lot of it is just code shifting".[56]

Rates of SIDS by race/ethnicity in the U.S., 2009, CDC, 2013


In 2013, there are persistent disparities in SIDS deaths among racial and ethnic groups in the U.S. In 2009, the rates of death range from 20.3 for Asian/Pacific Islander to 119.2 for American Indians/Alaska Native. Rates are per 100,000 live births and enable more accurate comparison across groups of different total population size.

Research suggests that factors which contribute more directly to SIDS risk—maternal age, exposure to smoking, safe sleep practices, etc. -- vary by racial and ethnic group and therefore risk exposure also varies by these groups.[4] Cultural factors can be protective as well as problematic.[57]

Society and culture[edit]

Much of the media portrayal of infants shows them in non-recommended sleeping positions.[5]


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  2. ^ Randall B (1996). "Witnessed sudden infant death syndrome". Journal of Sudden Infant Death Syndrome and Infant Mortality 1: 55–57. 
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  11. ^ NZ Ministry of Health
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  22. ^ a b c d CDC WONDER online database
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  33. ^ Sudden Infant Death Syndrome (SIDS) and Vaccines
  34. ^ See FSID Press release.
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  57. ^ Brathwaite-Fisher, T, Bronheim, S. Cultural Competence and Sudden Infant Death Syndrome and Other Infant Death: A Review of the Literature from 1990-2000. National Center for Cultural Competence, Georgetown University Center for Child and Human Development 2001. DOI:

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