Sudden Infant Death Syndrome

Sudden infant death syndrome (SIDS), also known as cot death or crib death, is the sudden unexplained death of a child of less than one year of age. Diagnosis requires that the death remain unexplained even after a thorough autopsy and detailed death scene investigation. SIDS usually occurs during sleep. Typically death occurs between the hours of 00:00 and 09:00. There is usually no noise or evidence of struggle. SIDS remains the leading cause of infant mortality in Western countries, contributing to half of all post-neonatal deaths.

The exact cause of SIDS is unknown. The requirement of a combination of factors including a specific underlying susceptibility, a specific time in development, and an environmental stressor has been proposed. These environmental stressors may include sleeping on the stomach or side, overheating, and exposure to tobacco smoke. Accidental suffocation from bed sharing (also known as co-sleeping) or soft objects may also play a role. Another risk factor is being born before 39 weeks of gestation. SIDS makes up about 80% of sudden and unexpected infant deaths (SUIDs). The other 20% of cases are often caused by infections, genetic disorders, and heart problems. While child abuse in the form of intentional suffocation may be misdiagnosed as SIDS, this is believed to make up less than 5% of cases.

The most effective method of reducing the risk of SIDS is putting a child less than one year old on their back to sleep. Other measures include a firm mattress separate from but close to caregivers, no loose bedding, a relatively cool sleeping environment, using a pacifier, and avoiding exposure to tobacco smoke. Breastfeeding and immunization may also be preventive. Measures not shown to be useful include positioning devices and baby monitors. Evidence is not sufficient for the use of fans. Grief support for families affected by SIDS is important, as the death of the infant is sudden, without witnesses, and often associated with an investigation.

Rates of SIDS vary nearly tenfold in developed countries from one in a thousand to one in ten thousand. Globally, it resulted in about 19,200 deaths in 2015, down from 22,000 deaths in 1990. SIDS was the third leading cause of death in children less than one year old in the United States in 2011. It is the most common cause of death between one month and one year of age. About 90% of cases happen before six months of age, with it being most frequent between two months and four months of age. It is more common in boys than girls. Rates have decreased in areas with "safe sleep campaigns" by up to 80%.

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Video explanation


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; and
  3. 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 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.

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.


SIDS has a 4-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 at 2 to 4 months of age. This is considered a critical period because the infant's ability to rouse from sleep is not yet mature.

Risk factors

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. Deaths from SIDS are unlikely to be due to a single cause, but rather multiple risk factors. The frequency of SIDS does appear to be influenced by social, economic, and cultural factors, such as maternal education, race or ethnicity, and 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 1 cigarette a day, on average, the risk doubles. About 22% of SIDS in the United States is related to mother's smoking. SIDS correlates with levels of nicotine and derivatives in the baby. Nicotine and derivatives cause alterations in neurodevelopment.


Placing an infant to sleep while lying on the stomach or side rather than on their back increases the risk. 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. Bumper pads may increase the risk of SIDS due to the risk of suffocation. They are not recommended for children under one year of age as this risk of suffocation greatly outweighs the risk of head bumping or limbs getting stuck in the bars of the crib.

Sharing a bed with parents or siblings increases the risk for SIDS. This risk is greatest in the first three months of life, when the mattress is soft, when one or more persons share the infant's bed, especially when the bed partners are using drugs or alcohol or are smoking. The risk remains, however, even in parents who do not smoke or use drugs. 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.

Room sharing as opposed to solitary sleeping is known to decrease the risk of SIDS.


Breastfeeding is associated with a lower risk of SIDS. It is not clear if co-sleeping among mothers who breastfeed without any other risk factors increases SIDS risk.

Pregnancy and infant factors

SIDS rates decrease with increasing maternal age, with teenage mothers at greatest risk. Delayed or inadequate prenatal care also increases risk. Low birth weight is a significant risk factor. In the United States from 1995 to 1998, the SIDS death rate for infants weighing 1000–1499 g was 2.89/1000, while for a birth weight of 3500–3999 g, it was only 0.51/1000. Premature birth increases the risk of SIDS death roughly fourfold. From 1995 to 1998, the U.S. SIDS rate for births at 37–39 weeks of gestation was 0.73/1000, while the SIDS rate for births at 28–31 weeks of gestation was 2.39/1000.

Anemia has also been linked to SIDS (note, however, that per item 6 in the list of epidemiologic characteristics below, extent of anemia cannot be evaluated at autopsy because an infant's total hemoglobin can only be measured during life.). 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.


Genetics plays a role, as SIDS is more prevalent in males. 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. 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-linkage hypothesis 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 male would occur with a frequency of ​23 and an unprotected female would occur with a frequency of ​49.

About 10 to 20% of SIDS cases are believed to be due to channelopathies, which are inherited defects in the ion channels which play an important role in the contraction of the heart.


Drinking of alcohol by parents is linked to SIDS. One study found a positive correlation between the two during New Years celebrations and weekends. Another found that alcohol use disorder was linked to a more than twofold risk.


There is a tentative link with Staphylococcus aureus and Escherichia coli. Vaccinations do not increase the risk of SIDS. Vaccines are linked to a 50% lower risk of SIDS.

SIDS has been linked to cold weather with this association believed to be due to over bundling and thus overheating. Premature babies are at four times the risk of SIDS, possibly related to an underdeveloped ability to control the cardiovascular system

A 1998 report found that antimony- and phosphorus-containing compounds used as fire retardants in PVC and other cot mattress materials are not a cause of SIDS. The report also states that toxic gas cannot be generated from antimony in mattresses and that babies suffered SIDS on mattresses that did not contain the compound.


Differential diagnosis

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

  • medium-chain acyl-coenzyme A dehydrogenase deficiency (MCAD deficiency);
  • infant botulism;
  • long QT syndrome (accounting for less than 2% of cases);
  • Helicobacter pylori bacterial infections;
  • shaken baby syndrome and other forms of child abuse;
  • overlaying, child smothering during carer's sleep

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 as unsafe bedding or sleeping with adults.

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

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.


A number of measures have been found to be effective in preventing SIDS including changing the sleeping position, breastfeeding, limiting soft bedding, immunizing the infant and using pacifiers. The use of electronic monitors has not been found to be useful as a preventative strategy. The effect that fans might have on the risk of SIDS has not been studied well enough to make any recommendation about them. Evidence regarding swaddling is unclear regarding SIDS. A 2016 review found tentative evidence that swaddling increases risk of SIDS, especially among babies placed on their stomachs or side while sleeping.

Measures not shown to be useful include positioning devices and baby monitors. Companies that sell the monitors do not have FDA approval for them as a medical device.

Sleep positioning

SIDS rate from 1988 to 2006

Sleeping on the back has been found to reduce the risk of SIDS. 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. Sleeping on the back does not appear to increase the risk of choking even in those with gastroesophageal reflux disease. While infants in this position may sleep more lightly this is not harmful. Sharing the same room as one's parents but in a different bed may decrease the risk by half.


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


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

Blankets or other clothing should not be placed over a baby's head.

The use of a "baby sleep bag" or "sleep sack", a soft bag with holes for the baby's arms and head can be used as a type of bedding that warms the baby without covering its head.


Higher rates of DTP immunization is associated with lower rates of SIDS supporting recommendations on timely DTP immunization.

Many other studies have also reached conclusions that vaccinations reduce the risk of SIDS. Studies generally show that SIDS risk is approximately halved by vaccinations.


Families who are impacted by SIDS may be offered emotional support and grief counseling. The experience and manifestation of grief at the loss of an infant are impacted by cultural and individual differences.


Globally SIDS resulted in about 22,000 deaths as of 2010, down from 30,000 deaths in 1990. Rates vary significantly by population from 0.05 per 1000 in Hong Kong to 6.7 per 1000 in Native Americans.

SIDS was responsible for 0.54 deaths per 1,000 live births in the US in 2005. 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. 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%. According to 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".


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 per 100,000 live births for Asian/Pacific Islander to 119.2 per 100,000 live births for Native Americans/Alaska Native. African American infants have a 24% greater risk of having a SIDS related death and experience a 2.5 greater incidence of SIDS than in Caucasian infants. 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. Risk factors associated with prone sleeping patterns of African American families include mother's age, household poverty index, rural/urban status of residence, and infant's age. More than 50% of African American infants were placed in non-recommended sleeping positions according to a study completed in South Carolina. Cultural factors can be protective as well as problematic.

The rate per 1000 births varies in different ethnic groups in the United States:

  • Central Americans and South Americans: 0.20
  • Asian/Pacific Islanders: 0.28
  • Mexicans: 0.24
  • Puerto Ricans: 0.53
  • Whites: 0.51
  • African Americans: 1.08
  • Native American: 1.24

Society and culture

The rate of SIDS varies vastly in different cultures and countries around the globe with SIDS rates lowest among Asian and Pacific Islander infants. Some evidence supports the hypothesis that SIDS is not an ancient phenomenom and that it appears more commonly in western societies.

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

See also

  • Newborn care and safety
  • Sudden unexpected death syndrome
  • Sudden unexplained death in childhood