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Sudden infant death syndrome (or SIDS) occurs when a baby under 12 months old dies suddenly for no apparent reason. When doctors investigate the death (do an autopsy), they can't find anything that might have been the cause.

Sudden infant death syndrome happens without any warning when the baby is thought to be sleeping. It can occur when the baby is in a crib. But babies can also die when they're sleeping somewhere else, such as in a stroller or their parent's arms.¹

Babies who die of SIDS usually do so peacefully in their sleep and show no signs of pain or distress.¹ There aren't any symptoms or warning signs.

Doctors don't know what causes SIDS. But research has found several things that seem to be linked to babies dying unexpectedly. These are known as risk factors. Babies seem to be more at risk if:^{2 3 4}

• They are placed to sleep on their front (this is the most important factor)
• Their mother or another family member smokes.

Risk factors make SIDS more likely. But it's important to realize that having a risk factor doesn't mean SIDS will happen. We know only that babies who die of SIDS are more likely to have been exposed to these risks.

Avoiding these risk factors can help make SIDS less likely. To read more about things you can do, see What helps to reduce the risk of sudden infant death syndrome?

Some companies sell devices that monitor a baby's breathing. Doctors sometimes suggest using these for babies who have a medical condition that affects their breathing. And they're sometimes given to parents who've lost a baby to SIDS and go on to have another child. But there's no evidence that home breathing monitors can reduce the risk of SIDS.⁵ And doctors don't usually recommend them.

Parents who have lost a child to SIDS need a lot of emotional support. It's completely natural to feel grief, and many parents feel guilty or that they should have done something differently.

Counseling, advice and practical help are available. Your doctor may be able to put you in touch with a support group in your area. Many parents find it helps to talk with other people who've been in the same situation.

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.