RELATED STORIES • Sudden infant death syndrome: New guidelines for prevention

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Every year, nearly 3,000 seemingly healthy babies in the United States die of sudden infant death syndrome (SIDS).

Sudden infant death syndrome strikes suddenly and silently, usually at night. Typically, a peacefully sleeping baby simply never wakes up. In most cases, no cause is ever found. Most SIDS deaths occur in children who are between 2 months and 4 months old. Sudden infant death syndrome rarely occurs before age 2 weeks or after age 6 months.

Although the cause of sudden infant death syndrome is still unknown, researchers have discovered a number of factors that may put babies at risk. They've also identified simple measures you can take to help protect your child from sudden infant death syndrome. Perhaps the most important of these is placing your baby to sleep on his or her back.

Exactly why SIDS occurs remains elusive. Over the years, researchers have investigated a number of possible causes, including suffocation, vomiting or choking, birth defects, metabolic abnormalities and infection. A 2006 study found that infants who die of SIDS have abnormalities in a part of the brain that helps control breathing and arousal. Other research has focused on the way babies breathe while they're asleep — especially their response to low blood oxygen levels (hypoxia) — and on heart problems.

Researchers are also investigating a possible link between SIDS and long QT syndrome, a subtle electrical disturbance in the heart that causes sudden, extremely rapid heart rates. Researchers suspect that some SIDS babies have a gene for this syndrome. On the other hand, most deaths from long QT syndrome occur during exercise, rather than during sleep as SIDS deaths do. And if long QT syndrome is the cause of SIDS, it doesn't explain why back sleeping reduces a child's risk or why only very young infants are affected.

Still, if you have a history of SIDS in your family, your doctor will want to check for the presence of long QT syndrome in your infant. This can usually be done withanelectrocardiogram (ECG) and confirmed, if necessary, with genetic studies.

Some factors that probably don't cause crib death include:

• A toxin or poison in the baby's environment. The notable exception to this is cigarette smoke. Babies exposed to secondhand smoke have a harder time waking up or being awakened from sleep, which researchers believe increases the risk of SIDS.
• Immunizations or a lack of immunizations. Some people believe that the multiple vaccinations American infants receive may play a role in SIDS. But a statement issued by the American Academy of Pediatrics in October 2005 says there's no association between immunizations and SIDS. At the same time, there's no evidence that children who aren't vaccinated are at increased risk.
• Exposure to electrical or magnetic fields or to household pets. In addition, SIDS doesn't seem to be caused by allergies or allergic reactions.

In the long run, it may be that SIDS doesn't have a single cause but rather occurs because of a complex interaction among many factors, including the way infants develop, their physiological responses to stress and exposure to environmental stressors.

For now, you can take preventive steps that can reduce the risk to your infant. And if a baby dies suddenly or unexpectedly, it can be extremely important to have a detailed autopsy performed to look for signs of infection or metabolic problems.

Although SIDS can strike any infant, researchers have identified several factors that may increase a baby's risk. At higher risk are babies who are:

• Male. Boy babies are more likely to die of SIDS than girl babies are.
• Between 2 weeks and 6 months of age. Infants are most vulnerable in the second and third months of life.
• Premature or of low birth weight. Your baby is more susceptible to SIDS if he or she is premature or weighs 4.4 pounds or less at birth.
• Black or American Indian. For reasons that aren't well understood, race appears to play a role in SIDS. A higher incidence of certain metabolic conditions may put some children at greater risk. Researchers have also identified a collection of genetic mutations, mainly affecting black babies, that may play a role. And cultural differences in child-care practices — such as whether babies are placed to sleep on their backs — are likely a factor.
• Placed to sleep on their stomachs. Babies who sleep on their stomachs are much more likely to die of SIDS than are babies who sleep on their backs. At highest risk are babies used to sleeping on their backs who are suddenly switched to stomach sleeping. At one time, doctors recommended stomach sleeping because babies rest more soundly in that position. But it's now known that stomach sleeping greatly increases a baby's risk.
• Born to mothers who smoke or use drugs. Smoking cigarettes during or after your pregnancy or using drugs such as cocaine, heroin or methadone while you're pregnant puts your baby at considerably higher risk of SIDS. The risk from smoking increases with the number of cigarettes smoked.
• Exposed to tobacco smoke. Infants exposed to secondhand smoke are harder to rouse from sleep, which increases the risk of SIDS.
• Born to mothers with placenta previa. A serious complication of pregnancy, placenta previa occurs when the placenta covers all or part of the cervix, often leading to heavy bleeding in the second or third trimester. Placenta previa poses real risks for both women and their unborn children and usually requires complete bed rest until delivery. When bleeding can't be controlled, doctors are likely to deliver the baby immediately by Caesarean section.

Because a baby's head is still soft and pliable, some newborns placed to sleep on their backs develop a flattening of the back of the head (positional plagiocephaly). In most cases, the flattening is harmless and easily treated. If caught early enough, simply changing the position of your baby's head will cause the flattening to disappear. At other times, your baby may wear an open-topped helmet to help restore a normal head shape.

But you can prevent flattening from occurring in the first place. Hold your baby when he or she is awake. With close supervision, place your baby on his or her tummy to play. When you place your baby on his or her back to sleep, alternate the direction your baby's head faces. It also may help to change the position of the crib or the location of interesting objects, such as mobiles. That way, your baby won't consistently look in one direction. Once your baby is old enough to sit up, head flattening becomes less of a concern.

There's no guaranteed way to prevent SIDS, but you can help your baby sleep safely. Consider the latest guidelines from the American Academy of Pediatrics:

• Put your baby to sleep on his or her back. Place your baby to sleep resting on his or her back, rather than on the stomach or side. This isn't necessary when your baby's awake or able to roll over both ways without your help. By the time your baby has learned to roll over, the risk of SIDS has decreased.
• Be sure your baby is placed to sleep on his or her back when staying with relatives or at child care. If your baby is used to sleeping on his or her back, it's especially important to avoid switching to the stomach position. Don't assume that others will place your baby to sleep in the correct position. Be sure to let them know what you want for your infant.
• Don't smoke. A smoke-free environment is especially important during pregnancy and in your baby's first year of life. Infants whose mothers smoke during and after pregnancy are much more likely to die of SIDS than are infants of nonsmoking mothers.

• Select bedding carefully. Use a firm mattress, rather than a water bed or beanbag. Avoid placing your baby on thick, fluffy padding, such as lambskin or a thick quilt. These may interfere with breathing if your baby's face presses against them. For the same reason, don't leave fluffy toys or stuffed animals in your infant's crib.

  To keep your baby warm, try a sleep sack or other sleep clothing that doesn't require additional covers. If you choose to use a blanket, make sure it's lightweight. Tuck the blanket securely at the foot of the crib, with just enough length to cover your baby's shoulders. Then place your baby in the crib, near the foot, covered loosely with the blanket.

• Place your baby to sleep in a crib or bassinet — not in your bed. Adult beds aren't safe for infants. A baby can become trapped and suffocate between the headboard slats, the space between the mattress and the bed frame, or the space between the mattress and the wall. A baby can also suffocate if a sleeping parent accidentally rolls over and covers the baby's nose and mouth.
• Keep your baby close by. Consider keeping your baby's crib or bassinet in your room at first. Infants who sleep in the same room as their mothers have a lower risk of SIDS.
• Offer a pacifier. Sucking on a pacifier at naptime and bedtime may reduce the risk of SIDS. One caveat — if you're breast-feeding, wait to offer a pacifier until your baby is 1 month old and you've settled into a comfortable nursing routine. If your baby's not interested in the pacifier, try again later. Don't force the issue. If the pacifier falls out of your baby's mouth while he or she is sleeping, don't pop it back in.
• Moderate room temperature. Keep the temperature in your baby's room at a level that's comfortable for you, not warmer than normal. If your baby is sweating around the neck or face, it probably means he or she is too warm or has a fever or illness. When this happens, use fewer covers — not more.

Some parents feel more secure when their newborn's heart rate and breathing are monitored electronically. Unfortunately, monitoring is unlikely to prevent SIDS deaths. Positioning devices aren't effective either — or even proved safe.

Facing the death of a child can be overwhelming. Parents are often torn by guilt as well as grief. But SIDS can occur no matter how much you love and protect your baby. Right now, SIDS remains a mystery.

At this time, the emotional support of others is especially important. You may find it comforting to talk to other SIDS parents. If so, your doctor may be able to recommend a support group in your area, or you can visit an online SIDS chat room. But support groups aren't for everyone. For some people, talking to a trusted friend or counselor may be more helpful.

If you can, keep an open line of communication with friends and family about how you're feeling. People want to help but sometimes may not know how to approach you. Parents, especially, need to be as open as possible with one another. Losing a child can put a terrible strain on a marriage. Counseling may help some couples understand and express their feelings.

Finally, give yourself time to grieve. Don't worry if you find yourself crying unexpectedly, if holidays and other celebratory times are especially difficult, or if you're tired and drained much of the time. This is normal. You're dealing with a devastating loss. Healing takes time.