Sudden infant death syndrome (SIDS) is the abrupt and unexplained death of an infant less than 1 year old. Although the etiology of SIDS remains unclear, evidence suggests that it is caused by a combination of environmental triggers and cardiorespiratory impairment, which then leads to prolonged hypoxia. Most SIDS cases occur in the first 6 months of life. Parents should receive information on how to prevent SIDS during prenatal care and in pediatric check-ups after birth: Recommendations include placing the infant on his/her back to sleep, ensuring a safe sleep environment, and avoiding overheating and second-hand smoke. SIDS is a diagnosis of exclusion; an autopsy is therefore important to rule out differential diagnoses (e.g., congenital cardiac anomalies or ).
The etiology of SIDS remains unclear. Evidence suggests that it is caused by a combination of both extrinsic and intrinsic factors, which ultimately lead to acute or chronic hypoxia. Over 90% of cases of SIDS occur during sleep.
Extrinsic factors (triggers)
- Sleeping in the prone position
- Exposure to nicotine during pregnancy and after birth (including 2nd-hand smoking)
- Unsafe sleeping environment or CO2 rebreathing: e.g., a shared blanket, stuffed animals in the crib (because of the grasping reflex, newborns tend to drag items to their faces)
- Many more correlations: SIDS in siblings, babies born prematurely, young mothers (< 20 years), low socioeconomic status, etc.
- Brainstem disorder that includes morphologic/biochemical abnormalities of serotonin (known as 5-hydroxytryptamine or 5-HT), which impacts the respiratory drive, the ability to wake up, blood pressure, upper respiratory reflexes , and body temperature.
- Congenital anomalies that could lead to infant death (e.g., cardiac anomalies)
- Intentional suffocation; evidence of
The differential diagnoses listed here are not exhaustive.
- Definition: a sudden and unexpected event occurring in an infant that is considered life-threatening by the observer and is characterized by some combination of the following:
- Epidemiology: reported incidence is 0.05–6%. 
- See “Definition.”
- May occur while the infant is awake or asleep
- Not associated with SIDS 
- Prognosis: recurrence is high, but overall mortality is low (< 1%) 
- During pregnancy
Protective factors after birth
- The infant should be placed to sleep in the supine position
- Safe sleep environment: firm mattress; no pillows, blankets, stuffed animals, or bumper pads in the crib.
- In the first 6 months, co-sleeping in the same room without bed-sharing
- Second-hand smoke and overheating should be avoided
- Use of pacifier during sleep 
- Breastfeeding until the 4th–6th month
- "Tummy time"
- Immunization in line with the official schedule