Sudden infant death syndrome (SIDS) is the sudden, unexpected death of an apparently healthy baby under 1 year of age, most often during sleep, that remains unexplained even after medical investigation and autopsy. It is a leading cause of death in infants between 1 and 12 months old, especially around 2–4 months.

What SIDS Is (Quick Scoop)

  • SIDS is a diagnosis used when a baby dies suddenly and no clear cause is found after full evaluation (autopsy, scene investigation, medical history review).
  • It usually happens during sleep, which is why people sometimes call it “crib death.”
  • It is classified under the broader group “sudden unexpected infant death” (SUID), which also includes accidental suffocation and unclear circumstances.

A simple way to picture it

Think of SIDS as a tragic event where a baby’s body suddenly “shuts down” during sleep without an obvious reason, even when doctors look very closely afterward. The mystery is that nothing obvious like infection, injury, or known disease clearly explains it.

What Causes SIDS? (Current Understanding)

Experts agree there is no single known cause, but rather a combination of underlying vulnerability plus triggers in the baby’s environment.

The “triple risk” idea

Many researchers use a “triple risk” model to explain SIDS:

  1. A vulnerable baby
  2. A critical period of development (roughly the first 6 months)
  3. An external stressor during sleep (like unsafe sleep positions or overheating)

When all three overlap, the risk of SIDS is highest.

What Might Be Going Wrong in the Body?

These are not guaranteed causes but patterns seen in some SIDS cases.

  • Brain abnormalities: Some babies have differences in the brain areas that control breathing, heart rate, and waking from sleep, making it harder to respond when oxygen drops or carbon dioxide rises.
  • Problems sensing low oxygen / high carbon dioxide: Their bodies may not “wake them up” properly if they rebreathe their own air or their airway is partly blocked.
  • Immune system changes: Some SIDS babies have unusual immune markers, and a minor infection might combine with other vulnerabilities to tip things over.
  • Metabolic disorders: Rare inherited metabolic problems (for example, fatty‑acid oxidation disorders) can cause sudden death in infancy and sometimes get labeled as SIDS if not detected.
  • Recent mild infection: Many babies who die of SIDS recently had a cold or respiratory infection, which may worsen breathing or oxygen levels during sleep.

Known Risk Factors (Things That Raise Risk)

These do not “cause” SIDS by themselves but make it more likely.

Baby-related factors

  • Age 2–4 months (peak risk), under 1 year overall.
  • Being born premature or with low birth weight.
  • Having a sibling who died of SIDS or sudden unexplained infant death.
  • Recent respiratory infection (like a cold).
  • Possible genetic or brain-development vulnerabilities.

Sleep environment factors

  • Sleeping on the stomach (prone position) rather than on the back.
  • Sleeping on soft surfaces (pillows, couches, soft mattresses) or with loose bedding and soft toys around the face.
  • Bed‑sharing with adults or other children, especially on a soft mattress, couch, or when adults are very tired or have used alcohol or drugs.
  • Overheating: too many layers, heavy blankets, or a very warm room.

Parent and pregnancy factors

  • Smoking during pregnancy or around the baby (both active and secondhand smoke).
  • Maternal age under 20.
  • No or late prenatal care.
  • Drug or heavy alcohol use in pregnancy.
  • Short gaps between pregnancies and living in high-stress or poverty conditions.

What Does NOT Cause SIDS?

  • Routine vaccines: Large studies show vaccines do not increase SIDS risk; in fact, vaccinated babies may have slightly lower risk.
  • Minor parenting mistakes: SIDS is not caused by “bad parenting”; it is usually a tragic overlap of factors, many outside anyone’s control.

Ways to Reduce the Risk (Current Recommendations)

No method can guarantee prevention, but certain habits clearly lower risk.

  • Always place baby on their back to sleep, for nights and naps.
  • Use a firm, flat sleep surface with a fitted sheet only; no pillows, bumpers, or stuffed animals.
  • Keep the crib or bassinet in the parents’ room for at least the first 6 months, but baby should sleep in their own separate sleep space.
  • Avoid bed‑sharing, especially on soft surfaces or when tired, using substances, or with multiple people in the bed.
  • Do not smoke during pregnancy or around the baby; keep the home and car smoke‑free.
  • Avoid overheating; dress the baby in light sleepwear and keep the room at a comfortable temperature for an adult.
  • Breastfeed if possible, at least for several months, as it is linked with lower SIDS risk.
  • Keep up with regular checkups and recommended vaccinations.

A gentle note if you feel anxious

Many parents today see a lot of SIDS stories online, which can make the risk feel overwhelming and constant. The reality is that, especially in recent years, safe-sleep campaigns have significantly reduced SIDS rates, and following basic safe-sleep steps makes the individual risk for any one baby relatively low.

If you tell me whether you want this framed more like a medical explainer or a parent-focused reassurance guide, I can reshape it and add practical checklists tailored to that.