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This is a clinician-written, evidence-based summary aligned to the 2026 MLA Content Map. It is intended for medical students and junior doctors preparing for the UKMLA. Always cross-reference with NICE guidance, local protocols, and clinical judgement.
Key points
- SIDS = sudden unexpected death of an infant <1 year that remains unexplained after thorough investigation
- Peak age 2–4 months. Most occur during sleep. Incidence: ~200 deaths per year in the UK (~0.3 per 1,000 live births)
- Risk factors: prone sleeping (most important modifiable risk factor), co-sleeping (especially on sofa, or with alcohol/drugs), parental smoking, overheating, prematurity, low birth weight
- Protective factors: supine sleeping (Back to Sleep), breastfeeding, room-sharing (same room, not same bed) for first 6 months, dummy use, immunisations
- Safe sleeping advice: place baby on their BACK, feet to foot of cot, firm flat mattress, room temperature 16–20C, no pillows/duvets/bumpers, do not smoke near the baby
- All sudden unexpected deaths in infancy (SUDI) must be investigated: joint agency response, autopsy, death scene investigation
Overview
SIDS is a diagnosis of exclusion made when the sudden death of an infant under 1 year remains unexplained after a thorough investigation including autopsy, examination of the death scene, and review of the clinical history. The broader term Sudden Unexpected Death in Infancy (SUDI) encompasses all sudden infant deaths, including those eventually explained by infection, metabolic disease, or non-accidental injury. The pathophysiology of SIDS is believed to involve a vulnerable infant (brainstem abnormality in arousal/cardiorespiratory control), a critical developmental period (2–4 months), and an exogenous stressor (prone sleeping, overheating, infection) — the triple risk model.
Epidemiology
The incidence of SIDS in the UK has fallen dramatically since the Back to Sleep campaign in 1991, from approximately 2 per 1,000 to 0.3 per 1,000 live births. Approximately 200 infants die from SIDS each year in the UK. Peak age is 2–4 months, with 90% occurring before 6 months. Male:female ratio approximately 1.5:1. Risk is higher in preterm, low birth weight, and multiple gestation infants.
Clinical Features
Symptoms
Infant found unresponsive, not breathing, or dead during sleep
No preceding illness or only minor upper respiratory symptoms
Signs
Cardiorespiratory arrest on arrival of emergency services
Assessment of the death scene may reveal unsafe sleeping practices
Investigations
First-line
Joint agency responsePolice, paediatrician, and health visitor joint investigation — initiated for all SUDI cases
Post-mortem examinationDetailed autopsy by paediatric pathologist — essential to exclude other causes of death (infection, metabolic, NAI)
Death scene investigationDetailed assessment of where and how the infant was sleeping — sleeping position, bedding, room temperature, co-sleeping
Second-line
Metabolic screenBlood and urine samples taken at time of death — to exclude fatty acid oxidation disorders (MCADD) and other metabolic conditions
MicrobiologyBlood, CSF, and tissue cultures — to exclude overwhelming infection
Skeletal surveyTo exclude non-accidental injury
Specialist
Genetic testingCardiac channelopathies (long QT, Brugada), MCADD, and other inherited conditions may be identified post-mortem
1
Prevention — safe sleeping advice
- Place baby on their BACK to sleep — every sleep, every time
- Feet to foot of cot (feet touching the foot of the cot to prevent slipping under bedding)
- Firm, flat, waterproof mattress in a clear cot — no pillows, duvets, bumpers, toys, or loose bedding
- Room temperature 16–20C — do not overheat
- Room-share (same room as parents) for first 6 months — NOT bed-share
- NEVER sleep with baby on a sofa or armchair
- Do not smoke during pregnancy or allow smoking in the house
- Breastfeed if possible — protective effect
- Immunisations are protective — ensure up to date
2
After a SUDI
- Compassionate support for the family — SUDI is devastating
- Joint agency investigation (Kennedy protocol/SUDIC investigation)
- Refer to the Lullaby Trust for bereavement support
- Subsequent pregnancies: CONI (Care of Next Infant) programme — apnoea monitor, increased health visitor contact
Complications
- Bereavement and psychological trauma: Parents experience profound grief, guilt, and sometimes wrongful suspicion of abuse
- Subsequent pregnancy anxiety: CONI programme provides structured support
- Relationship breakdown: Common after loss of a child
UKMLA Exam Tips
- 1Most important modifiable risk factor = prone sleeping. Back to Sleep campaign halved SIDS rates
- 2Co-sleeping on a sofa is the most dangerous sleeping arrangement
- 3SIDS is a diagnosis of EXCLUSION — requires autopsy, death scene investigation, and full clinical review
- 4Protective factors: supine sleeping, breastfeeding, room-sharing (not bed-sharing), dummy use, immunisations
- 5MCADD (medium-chain acyl-CoA dehydrogenase deficiency) can present as SUDI — screened for on newborn blood spot
- 6Peak age 2–4 months. 90% occur before 6 months
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