What is SIDS?

Sudden infant death syndrome (SIDS) is the leading cause of death in the United States for babies between 1 month and 1 year old.

SIDS isn't any one illness or disease. Rather, it's the diagnosis given when a child under a year old dies suddenly and an exact cause can't be found after a death scene investigation, an autopsy, and a review of the child's medical history. That it can happen without warning makes SIDS particularly devastating for families.

Approximately 2,300 infants die from SIDS in the United States each year. SIDS most commonly affects babies between the ages of 1 and 4 months; 90 percent of cases involve infants younger than 6 months.

While SIDS can occur outside of cribs, it's also known as crib death because it happens most often during sleep, usually between the hours of 10 at night and 10 in the morning.

Nighttime isn't the only time that SIDS strikes, however. If your infant is in daycare, it's important to know that about 20 percent of SIDS deaths happen in childcare settings, according to a study published in the journal Pediatrics in 2000.

This is a surprisingly high number when you consider that babies spend much less time sleeping at daycare than they do at home. This statistic points out how crucial it is to make sure that everyone who cares for your baby follows safe sleeping guidelines (see "How can I lower my baby's risk of SIDS?").

What causes SIDS?

Researchers have learned a great deal about SIDS in the past three decades, but they still have no definitive answer to that question.

Most experts believe that SIDS occurs when a baby has an underlying vulnerability (such as immature or abnormal functioning of the heart, breathing, or arousal) and is exposed to certain stressors (such as sleeping tummy-down or on soft bedding) during a critical period of development.

A study published in February 2010 in the Journal of the American Medical Association suggested another possible vulnerability. The researchers found that infants who died from SIDS had lower than normal levels of serotonin in the brainstem. Serotonin helps regulate breathing, heart rate, and blood pressure during sleep.

Experts are continuing to study the brain, the autonomic nervous system, infant care and sleep environments, infection and immunity, and genetics in search of answers.

Are certain babies at a higher risk for SIDS?

Although the exact cause of SIDS is unknown, some risk factors have been identified. They include:

Being born prematurely or at a very low birth weight

The earlier a baby is born, the higher his risk of SIDS. Likewise, the lower his birth weight, the higher the risk.

Being born to a mother under the age of 20

Teen moms are more likely than moms over 20 to have a child die of SIDS.

Having a lot of siblings, especially if they are close in age

The risk of SIDS increases with each baby you have. Also, the shorter the time between a woman's pregnancies, the higher her baby's risk of SIDS.

Suffering an apparent life-threatening event (ALTE)

Babies who have suffered what's called an apparent life-threatening event (stopped breathing and turned pale, blue, and limp and required resuscitation) have a higher risk of SIDS.

Certain ethnic backgrounds

SIDS rates are highest for African Americans and American Indians and lowest for Asians and Hispanics. The rates of SIDS among African Americans, American Indians, and Alaska natives are more than double that of Caucasian infants.

Some researchers hypothesize that a cultural preference for putting babies to sleep on their stomach puts certain groups at higher risk.

Being a boy

Boys of all ethnicities are at slightly higher risk than girls, by a ratio of 1.5 to 1.