Updated September 03, 2013.

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

What Is Sudden Infant Death Syndrome (SIDS)?

Sudden infant death syndrome, or SIDS, is a tragedy that strikes more than 2,000 U.S. families each year. By definition, SIDS is mysterious - the National Institutes of Health define SIDS as the sudden death of an infant under 1 year old that cannot be explained even after an autopsy and a close investigation of the scene and the infant's history.

Although the exact cause of SIDS is unknown, research has helped doctors to understand what happens when a baby dies from SIDS.

We know that SIDS is different from suffocation and accidental death. A baby who suffocated on a pillow during sleep or whose parent accidentally rolled onto him did not die from SIDS.

When a baby dies of SIDS, low oxygen saturation and high carbon dioxide levels, acid build up in the blood, and heart dysfunction seem to be what causes the baby to die. The big question in SIDS research is exactly why and how these issues occur and conspire in some babies to cause SIDS. Sleeping on fluffy bedding, belly sleeping, or exposure to tobacco smoke are all risk factors, but doctors aren't sure exactly what makes these risk factors cause SIDS.

Video:Preventing Sudden Infant Death Syndrome (SIDS)

Triple Risk Theory on SIDS

Right now, the best explanation for what causes SIDS is the triple risk theory. According to the triple risk theory on SIDS, a baby is most likely to die from SIDS when all of the following three risk factors are present:

• A vulnerable infant
• An outside stressor
• A critical period in development

Let's look at each of these 3 factors in more detail.

A Vulnerable Infant

A vulnerable infant is one who is at high risk for SIDS. Doctors now believe that subtle defects in the parts of the brain that control breathing and arousal are what make a baby vulnerable to SIDS. The parts of the brain that cause a person to wake up or stir when they start to inhale too much carbon dioxide seem to be slightly abnormal in babies who have died from SIDS.

There are many factors that might make an infant vulnerable to sudden infant death syndrome:

• Smoking:Smoking during pregnancy can increase a baby's risk of dying from SIDS. Smoke in a baby's environment also increases a baby's risk.
• Premature birth or low birth weight: Babies who are born before 37 weeks gestation or who weigh less than 5 lbs, 8 oz at birth are at risk for SIDS.
• Toxins: Prenatal exposure to toxins, including alcohol, cocaine, heroin, and methadone can make a baby vulnerable.
• Problems in pregnancy:Placental abruption, placenta previa, premature rupture of the membranes, and lack of prenatal care are all risk factors for SIDS.
• Other factors:Young maternal age, black race, and male sex all seem to increase a baby's risk for SIDS.

An Outside Stressor

In a vast majority of SIDS cases, just being vulnerable by having one or more of the risk factors above didn't cause SIDS on its own. The infants were also stressed in some way, such as:

• Stomach or side sleeping: The safest position for a baby to sleep in is on his or her back. Babies who sleep on their stomachs may rebreathe their own air, giving them less fresh oxygen. Babies who sleep on their sides may easily roll onto their stomachs.
• Unsafe sleep environment: Extra blankets, crib bumpers, and stuffed animals all reduce the amount of fresh air a baby has to breathe. Adult bedding is too soft for a baby and can also trap carbon dioxide.
• Illness: Many parents report that their babies were sick when they died from SIDS or had recently recovered from a cold or other illness.
• Overheating: A room that's too warm or heavy pajamas, expecially hats, can cause overheating. Babies who become overheated during sleep may have increased oxygen needs that stress their systems.

A Critical Period

There seems to be a critical period in development, from about 1 month to about 6 months old, during which babies are at an especially high risk for dying from SIDS. Babies can die from SIDS at any time during their first year, but about 80% of cases happen during this critical period.

Preventing SIDS

You can help to protect your baby from SIDS by reducing his or her risk factors and by providing a safe sleep environment. Get good prenatal care and take care of yourself during your pregnancy, and place your baby on his back to sleep, in a safe and uncluttered crib.

Learn More:Preventing SIDS in Premature Babies

<sub>Sources:

American Academy of Peciatrics Task Force on Sudden Infant Death Syndrome. "Technical Report SIDS and Other Sleep-Related Infant Deaths: Expansion of Recommendations for a Safe Infant Sleeping Environment." Pediatrics November 2011; 128; e1341 - e1367.

Paterson, D., Trachtenberg, F., Thompson, E., Belliveau, R., Beggs, A., Darnall, R., Chadwick, A., Krous, H., and Kinney, H. "Multiple Serotonergic Brainstem Abnormalities in Sudden Infant Death Syndrome." Journal of the American Medical Association Nov. 1, 2006. 296; 2124 - 2133.

Getahun, D., Amre, D., Rhoads, G., and Demissie, K. "Maternal and Obstetric Risk Factors for Sudden Infant Death Syndrome in the United States." Obstetrics and Gynecology April 2004; 103, 646 - 652.</sub>