Firm, flat and waterproof are the three things to check
Your baby's mattress needs to meet all three checks, whichever filling you choose. The Lullaby Trust's mattress guidance says to use a firm, flat, waterproof mattress in good condition.1 Cot mattresses are sold with foam, pocket-spring and natural-fibre fillings, but the checks stay the same for each.
Firm means your baby's head sinks only a little
Your baby's head should sink no more than a few millimetres into the mattress. That's the check given by both The Lullaby Trust and NHS Best Start in Life.3 The NHS explains that a mattress which isn't firm enough can make it difficult for your baby to lose heat, and they can become too hot. Use that check when assessing firmness instead of relying on the description on the packaging.
Flat means no raised or cushioned edges
Choose a completely flat surface, with no raised or cushioned edges or sides, as The Lullaby Trust advises. Check the whole mattress for sagging as well as looking at its shape. A mattress that dips in places needs replacing, even if the cover still looks clean.
Don't add a sheepskin, pod or nest. The Lullaby Trust advises against these because soft material around a baby's head can cause overheating or cover their mouth and nose. Its full wording is included in the safer sleep advice below.
A waterproof cover lets you keep the mattress clean and dry
A waterproof cover matters because you can wipe it clean and help keep the mattress dry. The NHS says the mattress should be protected by a waterproof cover, and The Lullaby Trust also recommends a waterproof mattress protector for easy wiping. Check that the cover fits properly and follow its cleaning instructions.
The Lullaby Trust's product guide puts waterproofing ahead of a "breathable" description. Look for a waterproof mattress or a fitted waterproof cover or layer when you compare options. A claim about breathability doesn't replace that practical requirement.
Keep checking condition while you use the mattress. Replace it if it sags, develops rips or tears, is damaged, or no longer fits snugly. Those are the useful checks to make each time you remove the cover for cleaning.
The right size means no gaps round the edges
The right mattress fits your particular cot snugly, without spaces round the edges. The NHS guidance on what you'll need for your baby explains that this prevents your baby's head getting trapped.2 Buy the size stated by the cot or cot bed's maker, or measure inside the cot before ordering.
The usual UK sizes below are trade conventions, not a sizing standard. Use them to narrow your search, then check the dimensions your own cot needs.
| What you're buying for | Usual mattress size | What to check |
|---|
| Cot | 120 x 60 cm | Match the maker's stated size or the cot's inside measurements. |
| Cot bed | 140 x 70 cm | Check the cot bed's requirements. A smaller cot mattress leaves a gap. |
Check the fit again when the mattress arrives, with its cover fitted. You need a snug fit all round. A mattress that has shrunk or lost its shape can leave spaces even if its original dimensions were right, so keep checking the edges during use.
Don't stack a second mattress or put anything underneath to fill space. If the mattress doesn't fit, get the correct size. Extra padding cannot make a badly fitting mattress suitable for the cot.
A travel cot mattress is often thinner and feels harder than a mattress in a permanent cot. The Lullaby Trust specifically says not to place folded blankets or a quilt under your baby. Keep that advice in mind if the thinner mattress feels different from what you expected.
Buy new if you can, and check any used mattress carefully
Choose a new mattress for each baby if you possibly can. That's The Lullaby Trust's recommendation, and the NHS says you should ideally buy a new mattress if you're borrowing a cot or using one previously used by another of your children. If buying new isn't possible, use The Lullaby Trust's second-hand checklist in the safer sleep advice.
The Lullaby Trust says some research has found a higher risk of sudden infant death syndrome (SIDS) with a second-hand mattress from outside the family home, although the link hasn't been proven. Its advice recognises that a new mattress isn't always possible.
A Scottish case-control study published by Tappin and colleagues in 2002 included 131 infants who died of SIDS between 1996 and 2000 and 278 matched controls.6 It found a significant association between a mattress previously used by another child and an increased risk of SIDS, particularly when the mattress came from another home. There was insufficient evidence to judge whether the relation was cause and effect.
For a used mattress, its storage history matters alongside its condition now. The Lullaby Trust's product guide says it should have been stored somewhere clean, dry and smoke-free. NHS Best Start in Life gives that same storage condition for reusing a mattress from your own home. Ask about storage if you don't already know its history.
Work through the complete checklist before using it. That includes a well-fitting, fully waterproof cover and a firm, flat mattress without sagging or water damage. Check for rips and tears, check the fit in your cot, and include the fire-resistant label check. A clean-looking cover on its own isn't enough to assess the mattress underneath.
Standards and sales claims tell you different things
Look for the relevant cot mattress standard, then make the firmness, flatness, waterproofing and fit checks yourself. The Lullaby Trust lists BS EN 16890:2017+A1:2021 for cot, travel cot and crib mattresses. It lists BS EN 716-1:2017 for cots and travel cots, while the NHS says to look for BS EN 716 when buying a new cot. Check which item a standard applies to when you read the product information.
The Lullaby Trust says there is no safety standard related to SIDS. A British Standard mark alone therefore doesn't mean a product is safe when it comes to reducing the risk of SIDS.
The fire regulations changed in October 2025
Since 30 October 2025, mattresses intended for babies and children with a length less than 170 cm and a width less than 75 cm have been outside the scope of the 1988 furniture fire regulations. The Office for Product Safety and Standards guidance says these products must meet the General Product Safety Regulations in Great Britain and the EU General Product Safety Regulation in Northern Ireland.5 In both cases, this includes considering the product's fire safety.
A new cot mattress may therefore not carry the old fire label. The Lullaby Trust's second-hand mattress checklist still includes a fire-resistant label, so retain that check when assessing a used mattress.
A familiar name or higher price isn't proof of safety
Choose by the mattress checks, without treating a familiar name or a higher price as proof of safety. The Lullaby Trust's product guide, reviewed in February 2025, says a known name or well-known retailer doesn't mean a product is safe for your baby to sleep in or on. It also says you don't need to buy lots of products or spend more on expensive items for safer sleep.4
Be especially careful with claims about SIDS. The guide says: "Claims that a product will decrease the risk of SIDS are almost always not true." Check the mattress itself against the practical requirements before buying.
Reviews are useful for delivery experiences, fit in another buyer's cot and how a cover washes. The three-star reviews are the most useful to read. They can't establish whether a mattress is safe, so you still need to check your own mattress and its fit.
When your child is ready to move to a bed, use the toddler mattress guide for that next purchase. For a cot or cot bed, keep choosing to the cot's stated mattress requirements.
Avoid damaged mattresses, added softness and gaps
- A second-hand mattress from another home, or a damaged used mattress. Buy new where possible. Avoid any used mattress that sags, is torn, has water damage or fits badly.
- Anything soft added to the surface. Don't add a sheepskin, pod or nest, or put folded blankets or a quilt under your baby, including in a travel cot. These additions conflict with The Lullaby Trust's mattress advice.
- A mattress that leaves gaps. A cot mattress in a larger cot bed is too small. Don't stack a second mattress to fill space, because your baby needs the correctly fitting mattress.
- Buying on claims alone. Don't choose "breathable" over waterproof, or take a famous name, high price or "reduces the risk of SIDS" claim as proof of safety. The Lullaby Trust's practical checks are what you need to work through.
Safer sleep advice from The Lullaby Trust and the NHS
The Lullaby Trust's guidance on baby mattresses and bedding says: "Use a firm, flat, waterproof mattress in good condition." Its advice on picking a mattress reads: "Use a mattress with a waterproof cover. This will help keep the mattress clean and dry because you can wipe the cover. Choose a firm, flat mattress with no raised or cushioned edges or sides. Your baby's head should sink no more than a few millimetres into a mattress. Make sure your baby mattress is in good condition, with no rips or tears, and not sagging in places. Check that it fits the Moses basket or cot properly. Don't use a sheepskin to line the cot, Moses basket or other sleep space. Although it might seem cosy, a baby's needs are different to an adult's. Adding a sheepskin can mean the surface is too soft and risks the baby getting too hot. We don't recommend pods or nests. Sleeping babies should not lie on or have anything soft around their heads as this can cause them to get too hot. The soft material can also cover a baby's mouth and nose if they are pressed against it."
On second-hand mattresses, The Lullaby Trust says: "Some research has found a higher risk of SIDS when using a second-hand mattress from outside the family home, although the link hasn't been proven. Generally speaking, we recommend buying a new mattress for each baby, although we know this isn't always possible." Its checklist says a second-hand mattress should:
- have a well fitting, 100% waterproof cover
- have no rips or tears
- be in good condition with no water damage
- be firm and flat (no areas of sagging)
- fit the cot or Moses basket well, with no gaps
- have a fire-resistant label on it
The Lullaby Trust adds: "Note there is no safety standard related to sudden infant death syndrome (SIDS). This means that even if a product meets the relevant British Standards, it won't necessarily be safe when it comes to reducing the risk of SIDS."
The NHS guidance on sudden infant death syndrome (SIDS) says: "There are things you can do to lower the risk of sudden infant death syndrome (SIDS). Follow this advice whenever your baby is sleeping. If your baby was born prematurely, follow the advice for safer sleep for a year from their due date not the date they were born." Its list of things to do:
- always place your baby on their back to sleep in the feet-to-foot position (this means with their feet at the bottom of the cot, Moses basket or pram)
- move your baby onto their back if they roll (until they can roll onto their front and back again by themselves)
- place your baby in a separate cot or Moses basket in the same room as you for every sleep for at least the first 6 months
- use a firm, flat mattress
- keep your baby's cot clear of anything that could cover their face and head: do not use pillows, duvets, cot bumpers, loose blankets, soft toys or anything to keep your baby in place, like pods or nests
- breastfeed your baby if you can: breastmilk protects your baby from infections and diseases
- think about using a dummy to settle your baby to sleep: some research suggests this can help lower the risk of SIDS
- make sure your baby is up to date with their routine NHS vaccinations: these provide protection against childhood illnesses that may increase the risk of SIDS
Among the NHS's list of things not to do:
- do not sleep on a sofa or in an armchair with your baby
- do not smoke when you're pregnant or around your baby after they're born, and do not let anyone smoke in the same room as your baby
- do not let your baby get too hot or too cold: make sure that your baby's room is between 16C to 20C
- do not let your baby stay in a car seat for too long: it's ok for your baby to fall asleep in a car seat while you are travelling, but take your baby out of the car seat as soon as you get to where you're going
For everything else about how your baby sleeps, read the full NHS page and The Lullaby Trust's safer sleep advice.
