Sudden infant death syndrome (SIDS) is a sudden and unexpected death of a seemingly healthy infant, usually less than one year of age. In order to be considered a case of SIDS, the death must be unexplained, even after a full investigation. SIDS usually occurs while the baby is asleep with no outward signs of distress or struggle – the infant may simply stop breathing.

Most deaths occur among infants under 6 months of age, with the majority occurring under 4 months of age. The syndrome is more common in male babies than in female babies.

Some deaths that are originally thought to be SIDS actually turn out to be the result of other conditions such as meningitis, encephalitis, and overlooked congenital or metabolic problems.

Although the actual cause of SIDS deaths remains unexplained, the search for a cause continues. It is not clear whether a single cause for all SIDS deaths will emerge. Below are various clues and possible triggers that researchers have identified.

Sleeping position

Some cases of SIDS may occur due to the baby sleeping on their side, or face-down on their stomach. The Canadian Paediatric Society, the Canadian Foundation for the Study of Infant Deaths and Baby’s Breath Canada all recommend that normal, healthy babies should be placed on their back for sleep. Parents should give their baby some carefully supervised “tummy time” several times each day while the baby is awake. This can help prevent “flat head” and counteract any effects of back sleeping on muscle development.

Poor temperature control

Some infants may have poor temperature control. It used to be that the majority of SIDS deaths occurred in winter, possibly due to babies being over-wrapped in clothes and at risk of becoming overheated. Now, there is evidence showing no difference in the number of SIDS deaths associated with different seasons. However, overheating is still considered a risk for SIDS.

Central nervous system abnormalities

Although SIDS babies may appear to be in perfectly good health, they may have an abnormality of the central nervous system (brain, nerves, or spinal cord) that has yet to be identified. For example, some infants may have underdeveloped parts of their central nervous systems that are not yet mature.

Flawed breathing control

Affected babies may have some flaw in their breathing control or ability. Research has shown that some babies may have borderline breathing abnormalities that may be the result of delayed maturation of the part of the central nervous system that controls breathing.

Exposure to tobacco smoke

Exposure to smoke is a health risk for unborn babies and for growing babies. Exposure to smoke has been identified as one of the great risk factors for SIDS.

Risk factors

Possible risk factors for SIDS include:

• prone sleeping position (e.g., on stomach or side)
• pre-term (premature) birth or low birth weight
• slower-than-normal growth rate
• sleeping on a soft surface
• sleeping in an area that is too warm
• smoking or drug use during pregnancy
• sleeping in a car seat or other device (swing, stroller, etc.)
• socio-economically disadvantaged background
• Indigenous or African descent
• cluttered sleeping area (e.g., toys, pillows, blankets in the crib)
• sharing a bed with another child
• parents who are smokers
• a sibling who has died of SIDS
• young birth parent (less than 20 years of age)
• late or no prenatal care
• being male
• sharing a bed with parent/caregiver
• a history of needing resuscitation because breathing has stopped
• recent illness
• low Apgar score at birth (an Apgar score measures a baby's overall condition and is taken soon after a baby is born. It is based on the baby's skin colour, breathing, heart rate, responsiveness, and muscle tone.)

Profiles of birth parents of babies who die of SIDS show that problems during pregnancy may predispose their babies to the syndrome. For example, birth parents who smoke, are anemic, or are undernourished appear to have babies who are at higher risk for SIDS.

There are no symptoms that are known to be specific to SIDS. It's considered very dangerous for breathing to stop for more than 20 seconds. (Please see below for some suggestions about what to do if your baby stops breathing.)

By definition, there can be no prediagnosis of SIDS; it can only be diagnosed after the baby has died. SIDS is usually only diagnosed in cases where exhaustive medical investigation has failed to determine the exact cause of death.

Parents should first realize that they aren't to blame in cases when a child dies from SIDS.

Although there's no proven way to prevent SIDS with 100% certainty, the suggestions below may help.

• If you notice any breathing stoppages or "blue spells," get immediate medical attention.  
• If you notice a pause longer than 10 seconds in your baby's breathing, wake them up with a small jolt or stimulus such as a flick of your finger on the feet. If this doesn't set the baby breathing again, the next step is more vigorous stimulation such as a pinch. If that doesn't work, begin mouth-to-mouth resuscitation and have someone call 9-1-1. Never shake the baby, as it could harm the baby. Even minimal shaking may cause a serious head injury.  
• During the pregnancy, make sure that the expectant parent receives good medical care and adequate nutrition.  
• Do not let your baby sleep on their stomach or side. Put the baby to sleep on their back. Some babies may need to sleep on their stomach because of a medical condition. If your doctor has told you that your baby should sleep on their stomach (for example, because of problems with their airway or breathing), then it's best to follow the doctor's recommendations. In such cases, the doctor may recommend ways to elevate the head and trunk of the baby in order to reduce the risk when sleeping in this prone position.  
• Use a pacifier, starting once breast-feeding is well established and stopping before one year of age (before dental complications may arise).  
• Keep your baby in a smoke-free environment. A birth parent who smokes during pregnancy, as well as second-hand smoke, are factors linked to SIDS.  
• Put your baby to sleep on a firm mattress, without thick blankets, pillows, or bumpers in the crib. Avoid puffy under-padding such as lambskin or quilts. Do not put your baby on a waterbed or air mattress. Do not let the baby sleep in your bed; this poses dangers of accidental smothering and suffocation, and it is a risk factor for SIDS.  
• The baby should sleep in a crib, cradle, or bassinet in the same room as their parents or caregivers until at least 6 months of age. Research indicates that sharing a room (but not a bed) with the baby is linked with a reduced risk of SIDS. Place the crib within arm's reach of where you sleep.  
• Never let your baby's face be covered by clothing or bedclothes. Avoid using blankets or comforters. Dress your baby in fitted, one-piece sleepwear that makes them comfortable at room temperature.  
• Try not to let your baby get too hot (don't use blankets). The temperature in your child's room should be comfortable for you. Babies don't need a warmer environment than adults.  
• Breast-feed if possible.  
• A small number of high-risk babies may require electronic monitoring of the heart rate and breathing. This practice has not been shown to protect babies, and you should be aware that monitoring doesn't help a baby breathe. 
• Avoid commercial devices marketed to reduce the risk of SIDS as they have not been adequately studied. These devices often claim to help maintain the baby's sleep position or protect against re-breathing (breathing in the air that has just been breathed out).  
• If your baby falls asleep in a baby swing or baby bouncer, move them to their crib, cradle, or bassinet. If your baby falls asleep in a car seat or stroller, move them to their crib, cradle, or bassinet (and place them on their back) once you reach your destination.

Parents who have lost a baby due to SIDS are often overcome with grief and guilt and feel responsible for the death of their child. It's important to seek support and counselling to help them get through this difficult time. Health care professionals, support groups (in-person or online), grief or SIDS organizations, and friends and family can provide a good support network.