What is sudden infant death syndrome?

Sudden infant death syndrome (SIDS) is also known as 'cot death' and 'crib death'. SIDS is defined as any death of an infant under one year of age that cannot, after thorough investigation, be attributed to a known cause.

SIDS is a leading cause of death of healthy babies in developed countries. The incidence of SIDS has dropped 50-80% in recent decades, so there is now about one case per 2000-3000 births.

The syndrome was first defined in 1969. In 1994, a 'Back to Sleep' public health campaign was run in the United States, encouraging caregivers to place sleeping infants on their backs, with similar campaigns run in other countries. Since then, SIDS rates have dropped worldwide, but levelled off in the early 2000s. Some of the decline in SIDS numbers may be explained by 'diagnosis shift' - the attribution of death to a cause other than SIDS.

SIDS cannot be totally prevented. There are some factors, particularly an unsafe sleep position and parental smoking, that greatly increase the chance of SIDS.

Kinney H.C. and Thach B.T. (2009) The Sudden Infant Death Syndrome. New England Journal of Medicine 361:795–805.

Kinney H.C. and Thach B.T. (2009) The Sudden Infant Death Syndrome. New England Journal of Medicine 361:795–805.

Moon R.Y. (2011) SIDS and other sleep-related infant deaths: expansion of recommendations for a safe infant sleeping environment. Pediatrics 128:e1341–e1367.


The causes of SIDS are, by definition, unknown. As the name suggests, SIDS occurs when a seemingly healthy infant is found dead, usually while sleeping.

SIDS is part of a larger category of deaths known as sudden unexpected infant death (SUID). An infant may be found to have died from another cause, such as accidental suffocation, infection, or a previously unrecognized birth defect. If post-mortem investigation has ruled out other causes, then the death is defined as SIDS.

Overall, about 80% of sudden unexpected infant deaths are eventually categorized as SIDS.

So far, researchers believe that SIDS occurs when the brain stem (which is in charge of regulating our breathing, heart rate and sleep) fails to respond well to a temporary lack of oxygen.

The triple risk model

The 'triple risk' model suggests that no single factor is solely responsible for SIDS. Instead, SIDS occurs when factors from at least two of the following three categories converge:

  • Critical development period - this typically occurs between 2-4 months of age, when infants are rapidly growing and their bodily systems can be unstable;
  • Individual factors - include genetic predispositions, premature birth, low birth weight and developmental abnormalities that can make the infants more vulnerable, and;
  • External 'stressors' - a stomach-sleeping position, exposure to smoking, soft bedding or overheating are some predisposing factors.

Sudden infant death syndrome is believed to be influenced by three categories of factors: a critical development period (2-4 months of age), individual factors, and environmental factors.The triple risk model for SIDS for a 2-4 month old baby. 

Brain stem

The area at the base of the brain that controls many of the body's most fundamental motor, sensory and reflex functions, as well as the facial muscles.

Kinney H.C. and Thach B.T. (2009) The Sudden Infant Death Syndrome. New England Journal of Medicine 361:795–805.

Wong F. Yiallourou S.R. Odoi A. et al. (2013) Cerebrovascular control is altered in healthy Term infants when they sleep prone. Sleep 36: 1911–1918.

Risk factors and prevention

Unfortunately, SIDS cannot be totally prevented. However, there are factors that increase the risk of SIDS and some that help prevent it.

Factors that have been shown to increase the chance of SIDS are an unsafe sleeping environment, which include the infant lying on their stomach or side, sleeping on soft surfaces or having objects in bed (blankets, pillows, toys) and an overheated room.

Adopting a safe sleeping position can reduce the chances of SIDS.

Safe sleep

The safest sleep position is:

  • On the back (for short naps as well as longer sleep periods);
  • In a smoke-free environment;
  • Alone in the bed/cot;
  • On a firm surface;
  • With baby's feet at or near the foot of the bed, and;
  • With no other objects in the bed.

A safe sleeping position for a baby. 

Sleeping in the same bed

It is not yet clear whether a baby sleeping in one bed with a parent does indeed increase a baby's chance of SIDS. It is, however, clear that a baby sleeping in the same bed with a parent who is under the influence of alcohol or medication, or is very tired, greatly increases the chances of SIDS.

Factors with a protective effect

Evidence suggests that the following behaviors may help protect against SIDS and reduce the chances of it happening:

  • Room sharing - the infant sleeping in a cot next to parents for the first six to 12 months of life;
  • Breastfeeding, and;
  • Offering a dummy (pacifier) before sleep. Note that forcing the dummy on a baby who does not want it is not recommended.

Additional findings

Evidence also shows that:

  • Vaccination does not increase the risk of SIDS;
  • Baby monitors and other commercial products are not effective in preventing SIDS in healthy babies;
  • Infants who are used to sleeping on their backs are at greatly increased risk of SIDS when placed on their stomachs;
  • Infants that are placed on their backs who then proceed to roll onto their sides or stomachs are not at increased risk of SIDS and do not need to be placed on their backs again;
  • Infants are not at increased risk of choking when placed on their backs;
  • About 20% of SIDS cases occur in professional child care settings. The first week of child care is a particularly vulnerable period;
  • Placing an infant for long periods of time in one position may cause a flattening of its head. These conditions (positional plagiocephaly, brachycephaly), are caused by pressure on the baby's head from lying on the back. They are not harmful and go away within months. They can be prevented by providing 'tummy time' (lying on the stomach) to the infant when they are awake and someone is watching them, and by holding or carrying them upright.

Moon R.Y. Patel K.M. and Shaefer S.J. (2000) Sudden infant death syndrome in child care settings. Pediatrics 106:295–300.


An unexplained infant death is an emotionally difficult time. A grieving family often finds itself struggling with strong emotions that can include sorrow, guilt, anger and a sense of futility. In addition to coping with the death itself, the family will also have to face the uncertainty and distress that accompany the investigations into the circumstances of death.

At this time, it is important for the family to be able to receive emotional and professional support and counselling.

Moon R.Y. Patel K.M. and Shaefer S.J. (2000) Sudden infant death syndrome in child care settings. Pediatrics 106:295–300.

FAQ Frequently asked questions