The cause of sudden infant death syndrome (SIDS) is unknown.

However, some studies have found an association between SIDS and certain risk factors. For example, SIDS is more common in babies who:

• are male
• were born with a low birth weight
• were born prematurely
• had a sibling who died of SIDS (although it is still rare for SIDS to occur more than once in the same family)

The following factors can also increase the risk of SIDS.

Tobacco smoke

Exposing your baby to tobacco smoke (both smoking during pregnancy and after your baby is born) can significantly increase the risk of SIDS.

Smoking during pregnancy means that your baby is four times more likely to die from SIDS than if you did not smoke. Continually exposing your baby to a smoky environment after birth means your child will be eight times more likely to die from SIDS than if they lived in a smoke-free environment.

Sleeping

Where and how your baby sleeps can also affect the risk of SIDS. Babies are more at risk of SIDS if they sleep:

• with an adult rather than in their own crib or cot (especially if the adult has consumed alcohol or drugs)
• on their side or stomach
• with a duvet, quilt or pillow

Your baby's temperature is also important. If your baby is too hot, this can increase the risk of SIDS.

For information on how to reduce the risks of SIDS, see SIDS - prevention.

Sudden infant death syndrome (SIDS) is rare, especially after five months when babies start to become more mobile.

However, although the risk is small, there are some steps you can take to reduce the risk further.

Sleeping position

Babies should always be laid to sleep on their back, with their feet touching the foot of the cot.

Sheets or thin blankets should come no higher than your baby's shoulders, to prevent them wriggling under the covers. Make sure the covers are securely tucked in so they cannot slip over your baby's head.

When your baby is awake, they should spend time in a variety of positions and not just on their back. For example, you can let your baby play on their tummy (but make sure they do not fall asleep on their front). This will help them learn how to control their head, and it encourages healthy development.

Because babies should always sleep on their back, you may notice your baby's head becomes slightly flatter on one side. This is known as plagiocephaly.

Plagiocephaly is not a serious condition and will not cause your baby any health problems. Your baby's head should become rounder again naturally as they continue to grow.

When your baby is old enough to roll over, do not prevent them from doing so.

Stop smoking

Exposing your baby to tobacco smoke can dramatically increase the risk of SIDS. This includes smoking during pregnancy.

If you cannot give up smoking, then make sure your home is a smoke-free environment. Make sure you always smoke outdoors. Simply smoking with a window open or in another room will not be enough.

You cannot see or smell most of your cigarette smoke and it lingers in the air for up to two-and-a-half hours. Make sure that any family, friends or guests in the house do not smoke indoors.

If you want advice on how to stop smoking during your pregnancy, call the Smokers  helpline Wales on 0800 169 0 169. 

Separate cot

The safest place for your baby to sleep is in a crib or cot in a room with you for the first six months. You should never bring your baby into bed with you.

This is because accidents can happen. For example, you may roll over in your sleep, suffocating your baby. Your baby could also fall out of bed and be injured, or they could get caught between the wall and the bed.

It is especially dangerous for your baby to share a bed with you if you have been smoking, drinking alcohol or taking medication that makes you drowsy, or if you feel very tired.

Falling asleep on the sofa with your baby has also been shown to increase the risk of SIDS. You should also not let your baby sleep alone in an adult bed.

Using a dummy

It is possible that using a dummy at the start of any sleep period reduces the risk of cot death. However, the evidence is not robust and not all experts agree they should be promoted.

Do not worry if your baby's dummy falls out while they are sleeping or if your baby does not want to use one. Not all babies take to dummies and you should never force your child to use one if they do not want to.

Room temperature

It is important that you keep your baby's room at a safe and comfortable temperature. Babies who get too hot are at an increased risk of SIDS. A baby can get too hot if the room temperature is too warm or if they are covered with too many blankets or too much clothing.

However, even though babies who get too hot are more at risk of SIDS, you should also make sure that your baby does not get too cold. Aim to keep your baby's room temperature between 16 and 20°C (61-68°F).

Ideally, the room should be 18°C (64°F). The best way to monitor the temperature of the room is to use a room thermometer.

Babies should never sleep with a hot water bottle or electric blanket. You should also make sure their bed is not put next to a radiator, heater, fire or placed in direct sunlight. Use lightweight blankets and never use a duvet, quilt or pillow for babies under 12 months old.