The cause of SIDS (sudden infant death syndrome) is unknown. However, numerous theories have been proposed regarding potential causes of SIDS.  

Theories about the potential causes of SIDS focus on three areas – physical, developmental and environmental. One theory is that a physical defect in the brainstem may prevent an infant from waking up when experiencing a lack of oxygen. Developmental theories focus on the fact that many SIDS cases occur among infants between the ages of 2 months and 4 months. This is a period of rapid changes in the body’s functioning as sleep and wake patterns are established and regulatory aspects of the cardiac, respiratory and nervous systems also develop. Any abnormality during a vulnerable period of development may affect the infant. Environmental theories focus on the infant’s exposure to various substances (e.g., cigarette smoke) either before or after birth.

No one theory has explained particular cases of SIDS. The causes may vary or may occur in different combinations for different infants. No single factor has been identified as increasing the risk of SIDS in an otherwise healthy infant. Instead, multiple factors may combine to create an increased risk of SIDS.

Certain factors popularly believed to be related to SIDS have been ruled out as causes. Despite common belief, the following do not cause SIDS:

• Immunizations
• Allergies
• Exposure to electrical or magnetic fields
• Choking or vomiting

Although the cause of SIDS is not known, certain risk factors for the condition have been identified. Risk factors are elements that increase an infant’s risk of SIDS. However, not every infant with risk factors will have SIDS and more importantly, infants with no risk factors may also die of SIDS. Risk factors may be divided into three categories: postnatal environmental, prenatal and unchangeable.

Postnatal environmental risk factors include:

• Sleep position. SIDS occurs more often among babies who are placed on their abdomen or their sides to sleep. This may increase pressure against a child’s jaw and impair breathing. Or, it may trap an infant’s exhaled breath, leading to a lack of oxygen and suffocation. Babies used to sleeping on their backs who are suddenly switched to sleeping on their stomach face a higher risk of SIDS.
  
  
• Soft or loose bedding. Infants who sleep on softer surfaces or with loose bedding are at an increased risk of SIDS. Soft surfaces include couches or waterbeds. Loose bedding may include pillows or comforters or stuffed animals.
  
  
• Exposure to smoke. Second-hand smoke may double a baby’s risk of SIDS. It is believed that exposure to smoke may affect a child’s central nervous system and may make a child harder to rouse from sleep, increasing the risk of SIDS.
  
  
• Overheating. Too much sleepwear on a baby or excessive bedding material may cause an infant to overheat while sleeping.

The mother’s environment or behavior during pregnancy may also affect her infant’s likelihood of developing SIDS. Prenatal risk factors for SIDS include:

• Smoking, alcohol consumption or recreational drug use during pregnancy
  
  
• Poor prenatal care
  
  
• Pregnancy complications (e.g., placenta previa [placenta blocks the opening to the uterus])
  
  
• Being under age 20 during first pregnancy

Non-modifiable risk factors include:

• Age. Most SIDS cases occur in children during the second, third and fourth months of life.
  
  
• Premature or low-birthweight infants. Premature babies or those weighing less than 4.5 pounds (2 kilograms) at birth have a higher risk of SIDS.
  
  
• Race. American Indian and African American babies are more likely to die of SIDS.
  
  
• Gender. More boys die of SIDS than girls.