Cot mattress safety

A crucial part of safe sleeping is choosing the best cot mattress. 

The Lullaby Trust, the UK's foremost organisation on baby safe sleeping, recommends that the safest place for your baby to sleep is on their own sleep surface, in the same room as you, for at least the first six months. 

Below are some of our top tips to choose the safest mattress for your child:

• Make sure you choose the right size of cot mattress. The mattress should fit the cot with no gaps down the side of more than 3cm.
• Make sure the mattress is firm and flat, and protected by a waterproof cover (sometimes called a mattress protector).
• Avoid second-hand mattresses where possible. Check any mattress you use conforms to current safety requirements.
• Any mattress should carry the BSI number BS 1877-10:2011+A1:2012. It’s preferable if it also carries BS 7177:1996 and BS EN 16890:2017. See below for more information about these standards and what they mean.

Find out which mattress came out top in our tests in our round-up of the Best Buy cot mattresses.

Safety standards for cot mattresses

Cot mattresses that comply with the current British Standards give you confidence that what you’re buying for your baby is safe, and we would always encourage you to check whether your products are marked with a BS number.

With cot mattresses it’s slightly more complicated, as there are a number of BS numbers to look out for. BS 1877-10:2011+A1:2012 is an older mandatory standard that specifies the kinds of materials, construction and dimensions required when manufacturing mattresses, while BS 7177:1996 guarantees it has passed flammability standards.

There’s also a much newer cot mattress standard called BS EN 16890:2017, which was approved in September 2017. Although it’s currently a voluntary standard, it includes a number of requirements designed to test the performance and safety of cot mattresses. This includes:

• Checking that no part of the mattress, for example the zip, can detach and become a choking hazard.
• Checking the firmness of the top of the mattress, so if your baby accidentally rolls onto their front, their face won’t sink into the mattress and potentially suffocate them.
• Checking whether the removable cover shrinks after washing, in case it compresses the mattress and causes a gap between the mattress and the bed frame where your child could trap a limb.

The BS EN 16890:2017 standard encompasses a wider set of tests and risks than the current mandatory standard.

We test to this standard because we believe it goes further to ensure a safe sleeping environment for babies and young children.

Which cot mattress is best? 

We test the firmness, body support and durability of each cot mattress we review, so we can recommend the best to you.

If a mattress isn't firm enough to aid safe sleep and isn't sufficiently supportive, it becomes a Don't Buy cot mattress.

Our Best Buy cot mattresses have passed all our robust safety tests. 

Not a member? Join Which? to read our full cot mattress reviews.

Should you buy a second-hand cot mattress?

We do not recommend buying or using a second-hand cot mattress. Where possible, always buy new. This is also the advice of The Lullaby Trust.

Which? tests have found that cot mattresses can lose more than 25% of their firmness in just a few years of use. This means using a second-hand or hand-me-down cot mattress may not provide the firm support all babies need, and which reduces the risk of suffocation or smothering.

The Lullaby Trust also says there is some research that found an increased chance of sudden infant death syndrome (SIDS) when using a second-hand mattress, although the link is not yet proven.

When we surveyed 1,878 parents in February 2021 about cot mattresses, almost two in five told us they use a second-hand cot mattress.

Of these parents, 60% said the second-hand mattress was given to them, whilst the rest bought it themselves.

Cot mattress ownership:

• New (61%)
• Second-hand - given to them (23%)
• Second-hand - purchased (16%)

Out of all the second-hand cot mattress bought, nine in 10 came from Facebook Marketplace, Amazon Marketplace or eBay.

Buying from an online marketplace carries some risk as you can't be certain of the history and quality of a second-hand cot mattress, meaning there is a greater chance of your second-hand cot mattress having problems.

This is backed up by our survey: around half of parents with a new cot mattress said they had issues with it, but this increases to around nine in 10 parents facing problems when the cot mattress was second-hand.

What's wrong with using a second-hand cot mattress?

As you can see in the breakdown below, all types of problems are significantly more common in second-hand cot mattresses.

Most concerning is that a quarter of second-hand cot mattress owners reported indentations forming in the mattress or it sagging too much. A mattress that isn't firm and flat won't provide enough support for your baby, and could possibly be a suffocation risk. 

As such it's vital you buy cot mattresses new, as you will have fewer issues and greater piece of mind.

Buying new doesn't have to break the bank. We compared the prices of cot mattresses that we've tested and found the average price of a Best Buy is £96, while the average price of a Don't Buy is £135!*  So you don't have to spend the most money to buy the best.

If you have to buy second-hand, it's worth seeing if you can check the cot mattress yourself before parting with your money. If a reseller won't let you find out more about the history of a second-hand cot mattress, we'd advise looking elsewhere.

Use our checklist below to make sure a second-hand cot mattress (whether bought or a hand-me-down) is going to be fit for use:

• Make sure the cot mattress was previously protected by a waterproof cover - this will reduce the risk of unhealthy bacteria lurking in the mattress.
• Test to see if it's still firm and lies flat, with no holes or tears - when you press down there should be resistance and the mattress should spring back immediately after you remove your hand.
• Check the mattress isn't sagging in places - otherwise that part of the cot mattress won't provide enough support for your child, and if a baby ended up face down in the sagging area it could be a suffocation risk.
• Make sure it fits the cot bed with no gaps around the edges - take a tape measure and check the length and width of the mattress at multiple places on the mattress. The standard size mattress for a UK cot bed is 140cm x 70cm, but double check the cot mattress size your cot bed needs beforehand.
• Clean the waterproof layer and air-dry thoroughly before reusing the mattress - to make sure it's dirt and bacteria-free before your little one uses it.

*Price of cot mattresses tested by Which? as of May 2021

What is Sudden infant death syndrome (SIDS)? 

Sudden infant death syndrome (SIDS) is a term used to classify any sudden and unexpected death of a baby 12 months of age or younger. Sometimes referred to as 'cot death', it still isn't known for sure what causes SIDS, but there are steps parents can take to reduce the risk of it happening. Parents are urged to follow safer sleep advice when putting their baby into his or her cot. This includes:

• Keep your baby's cot in your room with you for the first six months. 
• Always place your baby on his or her back to sleep, not front or side (unless your doctor advises otherwise). If your baby rolls onto his or her tummy you should turn them onto their back again. Your baby should be put down to sleep in this position until he or she is able to roll from back to front and back again.  
• Place your baby with feet to the foot of the cot, so he or she can’t wriggle down under the covers. 
• Don’t worry if he or she wriggles up and gets uncovered. 
• It can be dangerous if your baby’s head gets covered when he or she sleeps. To avoid this, tuck in the bedclothes firmly around your child and no higher than his or her shoulders.
• Never use a pillow, quilt or duvet if your baby is under a year old. Instead, use cotton sheets or lightweight blankets.
• Alternatively, you can use a baby sleeping bag instead of bedding. 
• The recommended room temperature for a baby to sleep in is 16-20ºC (61-68ºF) – use a room thermometer to check the temperature before putting your baby to bed.  
• To check whether your baby is too hot, look for sweating or feel the back of your baby’s neck or his or her tummy, not hands or feet.

Overheating while sleeping and SIDS

It is very important your baby does not overheat while sleeping as the chance of SIDS is higher in babies who get too hot while sleeping. 

Many cot mattress manufacturers make claims about their mattresses having extra features that provide a cooler sleeping surface for your baby, such as special construction or fabric.

It's important to know that your cot mattress won't single-handedly be able to keep your baby from overheating, so make sure you follow the temperature advice above.

Also be careful of letting marketing claims on mattresses sway your buying decision. One of the things we test cot mattresses for is insulation and we classify each mattress in terms of the level of insulation it provides.

Compare our cot mattress reviews to see the differences.