Sudden infant death syndrome (SIDS), commonly called cot or crib death, denotes the sudden and unexpected death of an infant younger than one year that remains unexplained after a thorough scene investigation, clinical history review and complete autopsy. SIDS is a subcategory of the broader term sudden and unexpected infant death (SUID), which covers all sudden infant deaths regardless of cause. Most SIDS deaths occur in the first year of life, with a peak incidence between two and four months when an infant's sleep arousal and cardiorespiratory control systems are still developing. For more background, see overview and clinical definitions at diagnostic guidance.

Characteristics and diagnosis

By definition, SIDS is diagnosed only after investigators exclude known causes such as congenital conditions, infection, trauma or accidental suffocation. The scene and circumstances of sleep are important: SIDS typically occurs during sleep and often without a witness. A complete autopsy and review of the infant's medical history and death scene determine whether a case is classified as SIDS or another form of SUID. Public health summaries and guidance on classification are available from resources like forensic review and epidemiology reports.

Proposed mechanisms and scientific theories

The exact biological cause of SIDS is unknown, but researchers propose a convergence of factors: a vulnerable infant, a critical developmental period, and an external stressor such as sleep environment. Hypotheses include brainstem dysfunction affecting arousal and breathing control, abnormalities in serotonin regulation in the brain, metabolic or cardiac conduction problems, and the possible contribution of respiratory infection or environmental toxins. Specific lines of research can be found via neurological studies, sleep physiology, and work on neurotransmitters such as serotonin (serotonin research). Some investigators have explored infectious agents and toxins as contributors; historical inquiries have examined bacteria and their toxins (microbiology, toxin effects).

Risk factors and prevention strategies

Several demographic and environmental factors are associated with increased SIDS risk: male sex, younger age (peak 2–4 months), and seasonal variation with more deaths in colder months. Modifiable risks include prone sleeping (on the stomach), soft bedding, overheating, parental smoking and bed-sharing under unsafe conditions. Protective measures that have reduced SIDS rates in many countries include placing infants on their backs to sleep, using a firm sleep surface without loose bedding, avoiding exposure to tobacco smoke, room-sharing without bed-sharing, breastfeeding, and considering a pacifier at naps and bedtime. These recommendations are summarized in public health campaigns and educational materials (risk guidance, prevention programs).

Investigation, misclassification and family support

When an infant dies suddenly, thorough medicolegal investigation is required to identify cause and rule out homicide or accidental suffocation. Misclassification can occur: some deaths initially labeled SIDS are later reassigned to other causes after new evidence; conversely, intentional harm may be missed without careful review. Families affected by SIDS need sensitive communication, accurate information and bereavement support because caregivers may face suspicion or unresolved grief. Resources on medicolegal practice and family support include forensic protocols, child protection guidance and community bereavement services (support networks).

Public health impact and notable facts

SIDS was once a leading cause of postneonatal infant death in many high-income countries, but rates fell sharply after evidence-based prevention campaigns promoting supine sleep positions. Despite progress, SIDS and other forms of SUID remain an important focus for research, prevention and family support. Accurate classification helps target prevention; distinguishing SIDS from accidental suffocation, infection-associated deaths or inflicted injury is essential. For policy, research, and additional reading see official recommendations and summaries at policy resources and research overviews.

  • Key prevention actions: back sleeping, smoke-free environment, firm sleep surface, no loose bedding.
  • Investigative priorities: autopsy, scene examination, medical history review.
  • Support: empathetic communication, bereavement counseling and community resources.