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Hippokratia. 2011 Apr-Jun; 15(2): 127–131.
PMCID: PMC3209674

The factors contributing to the risk of sudden infant death syndrome


Sudden infant death syndrome (SIDS) is defined as the sudden death of an infant under one year of age which remains unexplained after a thorough case investigation, including performance of a complete autopsy, examination of the death scene and review of the clinical history. SIDS is one of the leading causes of infant mortality and occurs from the first month, until the first year of life for newborns and infants.

The aim of this review was to identify and examine risk factors responsible for causing the sudden infant death and to propose certain measures in order to protect newborns and infants from sudden death. The potential factors that contribute to the occurrence of SIDS include inadequate prenatal care, low birth weight (<2499gr), premature infants, intrauterine growth delay, short interval between pregnancies and maternal substance use (tobacco, alcohol, opiates). Moreover, factors related to infant’s sleep environment such as the prone or side sleeping position and thick coverlet increase the risk of sudden death in infants. Also, the combination of risk factors such as that of prone sleeping position and soft bed mattress are linked to a 20-fold increased risk of death. Finally, polymorphisms in the serotonin transporter gene (5-HTT), viral respiratory infections, long Q-T (responsible for the presence of fatal arrhythmia) are related to the SIDS.

Literature review indicates that each individual risk factor contributes to the appearance of SIDS and the establishment of certain protective measures for parents and health professionals has reduced its prevalence. But the precise identification of the SIDS causes and how these contribute to the occurrence of sudden death in neonates and infants, remains a challenge for health professionals.

Keywords: sudden death, newborn, infant, risk factors, prevention, review

Willinger et al., (1991) first described the sudden infant death syndrome (SIDS), as "the sudden death of an infant under one year of age which remains unexplained after a thorough case investigation, including performance of a complete autopsy, examination of the death scene and review of the clinical history"1. SIDS appears in newborns and infants, without any indication that their life is in danger2.

The annual incidence rate of SIDS in '80s was 1.53% per 1000 live births in the USA, while in 2004 it decreased to 0.51%3. Deaths due to SIDS, in the general population amounted to 2200 per year4. Moreover, SIDS is classified by the Center of Disease Control (CDC) as the third leading cause of overall infant mortality in the USA5.

SIDS occurs between the first month and before the first year of an infant's life. Infants aged 2-4 months endanger greater risk of SIDS, while most deaths occur in infants during the sixth month of their life6.

The differential diagnosis of SIDS in neonates and infants includes asphyxiation or drowning, heart disease (arrhythmia), electrolyte disturbances or dehydration, poisoning, trauma, infections (pneumonia, sepsis, meningitis) and congenital metabolic disorders7.

Few researchers proposed the specification of diagnostic criteria for sudden death in neonates. Beckwith B. (2003), proposed three categories related to SIDS, which classified the neonate or infant in the relevant category. The first category includes infants aged from three weeks to eight months, without the previous appearance of SIDS in the family environment, infants without evidence of abuse, trauma or accident and neonates with a significant degree of anxiety in the thymus, adrenal glands or other organs. The second category consists of infants who exhibit the criteria of the above category, except those that are >1 year old, who have history of sudden death in the family environment and inflammatory disorders. Finally, the last category concerns cases of deaths of infants, which made an autopsy after their death1.

According to the literature review, the possible risk factors that contribute to the incidence of SIDS are demographic, relevant to the pregnancy and the birth, the sleep environment, genetic, infective and cardiovascular.

Demographic factors

Low socioeconomic status, maternal young age, low maternal education and single marital status, are associated with the occurrence of SIDS1. Infants who did not live with their father were 50% higher sensitive in sudden death, unlike those who grew up with their father8. Also, several studies have reported that male infants present 30-50% more chances to die from sudden death9,10.

Factors related with pregnancy and birth

Pregnancy related factors (mothers with inadequate prenatal care11, intrauterine growth retardation, short intervals during pregnancies, second pregnancy or after) are connected with increased incidence of SIDS. The sudden death of a second child in a family comes 80-90% natural. Also, birth related factors such as premature 10 and low birth weight infants (infants that were borned at 1000-1499 gr are threatened 4 times higher to die from SIDS and those borned at 1500-2499 gr are threatened 3 times more respectively)12,13.

Substances abuse like tobacco, drugs and alcohol, in pregnancy period, are correlated with SIDS. Tobacco use in pregnancy affects the intrauterine fetal development. During an antismoking campaign, infants borned by mothers who smoke during pregnancy, have 3 times more risk to appear SIDS before the campaign14. The avoidance of smoking during pregnancy estimated that could have reduce SIDS cases by almost 21%.15 The effect of passive smoking was higher in infants <10 weeks and seems to doubles the possibility of SIDS16. Whereas, a home environment free of tobacco use, reduces the chances of SIDS6.

The chances of sudden death increased by 2-5 times in neonates whose mother have used drugs (opiates)17,18, 6-8 times in cases of maternal alcohol use (particularly the first 3 months of pregnancy), 2-8 times when mothers had drunk alcohol in the last 24 hours before the neonate death19. Infant's siblings with fetal alcohol syndrome reported 10 times more chances of sudden death20.

Sleep environment

The environment of sleep concerns the infant's sleep position, the bed's quality, the sharing of the bed and the occurrence of smoking during infant's sleep.

Newborns vulnerable to sudden death are those who change position during sleep and remain in prone position21. The SIDS risk was 37 times higher at infants who changed position during sleep compare to those who remained in this position throughout the whole sleep duration22,23. Infants who sleep in lateral position provoke twice the risk of sudden death, in contrast to those sleeping in supine position24. Instantly, many parents and health professionals may notice that supine sleeping position impairs neonatal respiratory system and increases the risk of gastro-esophageal reflux25.

Soft, old pillows and sleeping mats from polystyrene have been shown to increase by 2-3 times the risk of sudden death9,17,25. Also, heavy comforters covering the baby's head, implicate to be associated with sudden death26,27. When the newborn's body temperature increases, due to increased room temperature, fever, sweating or excessive clothing the chances of SIDS is increased16. While combinations of risk factors such as the prone position and soft layer or increased body temperature and sleep in prone position, are associated with the risk of sudden death, 2 and 6 times, respectively28,29.

Also, infant's sharing bed with parents or siblings (twins) has been implicated with a high risk of SIDS30. Babies under <13 weeks who sleep with their parents are in greater risk13.

Genetic factors

Polymorphism serotonin transporter gene (5-HTT) was observed at infants who developed sudden death. This gene is affected by the functions of the autonomic nervous system31.


Viral respiratory infections are mainly responsible for the occurrence of sudden death. Mild degree of respiratory viral infection was observed by investigators in cases of sudden death infants up to 80%32.

Toxins produced from bacteria like s.aureus and e.coli, pose a particular risk to the life of infant. The toxins of such bacteria are absorbed by mucosal surfaces, or indicate an upcoming bacteremia, affecting principally the cardiovascular and respiratory systems, creating "channels" in cell membranes, which disrupts the smooth ions33.

Cardiovascular factors

Long Q-T syndrome is associated with changes in the Na+/K+ pump. Abnormalities shown in ion heart channels, are related to the sudden death and hence the baby is at risk of potentially fatal arrythmia34. According to American Academy of Pediatrics, rare genetic disorders are result to mutations of the cardiac tissue, like fatal loss of cardiac function35.

Protective measures against SIDS

Although the causes of SIDS are not precisely identified, it seems that the above factors have positive effects in sudden death. American Academy of Pediatrics has already established protective measures to decrease the incidence of SIDS. These measures are:

  • The position of the infant at bedtime is the most modifiable factor. It is suggested that the appropriate sleep position for infants is the supine one. The side sleeping position is a risk factor for sudden death for this reason it is not recommended35.
  • It is preferred to avoid the high temperature of the infant, keeping tolerable room temperature, avoiding excessive clothing35.
  • It is recommended that the sleep surface should be firm and hard30.
  • Research studies confirm that the pacifiers help to reduce the risk of sudden death when they are administered at the beginning of sleep. The American Academy of Pediatrics recommends that parents should provide pacifiers to their infants from the first month of their life35. However, pacifier use is implicated in the creation of medium otitidas37 and airway obstruction37.
  • The baby crib is recommended to be located in the same room with the parent's bed and is advised not to make changes in the environment of sleeping infants, at least for the first year of the infant’s life38,39.
  • Immunization in time seems also to be protective from SIDS40.
  • The education of parents and health professionals are equally important, contributing decisively in reducing the risk of SIDS41.


Concluding, the SIDS seems to be a multifunctional syndrome, without a clear etiologic mechanism. For this reason, exploring the way each risk factor contributes to the occurrence of sudden death in newborns and the establishment of certain protective measures for parents and health professionals, still remain a challenge.

Table 1
Characteristics of the research studies (during the period 1996-2010), that introduced in this review


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