Is a Breathable Cot Mattress Safer?

A safe cot mattress is firm, flat, waterproof and the right size

The checks that matter are few, and you can do most of them with your hand and a tape measure. Professor Peter Fleming CBE, Emeritus Professor of Infant Health and Developmental Physiology at the University of Bristol, answered our questions about cot mattresses in November 2026. He has worked with The Lullaby Trust for many years, and he started exactly where the charity does:

Each of those is something you can check yourself. The Lullaby Trust's guide to baby mattresses and bedding says your baby's head should sink no more than a few millimetres into the mattress, so firm here means properly firm.¹ Press the middle hard with the flat of your hand in the shop: you're feeling for a surface that barely gives and comes straight back. The same guide asks for a mattress that is completely flat, with no raised or cushioned edges or sides, and not sagging in places. The waterproof cover is there so you can wipe it and keep the mattress clean and dry, and intact means no rips or tears.

Size is the one I'd measure rather than trust. Cots aren't all one size, and the mattress has to fit the one you've got, so measure inside the cot, end to end and side to side, and hold those numbers up against the mattress size before you pay. Once it's in, run your hand round every edge and feel for any space between the mattress and the cot sides. The charity's wording for a good fit is "with no gaps", and that's the standard to hold it to. It also suggests checking the mattress meets the British Standard for cot mattresses, BS EN 16890, while being clear that a British Standard covers general safety, such as not falling apart or catching fire easily, and isn't a SIDS standard.

"Breathable" tells you how the cover handles air and moisture

"Breathable" describes the fabric of the cover, and Professor Fleming's answer on it was the most direct part of his reply. He sets out what the word does describe, then what it can't promise:

So the word describes the cover: air and water vapour can pass through it, and liquid usually can't. The walking-jacket comparison is the clearest way I've seen it put. A good waterproof jacket lets the damp from your skin out on a hill walk, and you still couldn't breathe through it if you tried. Professor Fleming adds that the word only really matters for spotting materials that let sweat evaporate, and that this isn't an issue for a sleeping baby in an appropriate thermal environment. Keeping a baby the right temperature is down to the room and the bedding. The NHS guide to reducing the risk of SIDS puts the room at about 16 to 20C, and says to take off a layer of clothing or bedding if your baby's skin is hot or sweating.² Feel their chest or the back of their neck to check, and don't worry if their hands or feet feel cool, which the NHS says is normal.

Where your baby's breathing does come into it, the advice is about how they lie and what's around them. On their back, in a clear cot, with nothing soft that could end up over their face: that's what both the NHS and The Lullaby Trust put first. A breathable cover sits on top of all that and can't stand in for any of it. Whatever the label says, check the description also says the cover is waterproof, because that's the property on the safety list.

A second-hand cot mattress has to pass the same checks, and a few more

I'd buy a new mattress for each baby if you possibly can, and The Lullaby Trust recommends the same while saying plainly that it isn't always possible. Its reason is that some research has found a higher risk of SIDS with a second-hand mattress from outside the family home, although the link hasn't been proven. I slept for years on a hand-me-down that was already twenty years old when it reached me, so I understand the pull of a free one from family. A cot mattress for a baby is the one place I wouldn't take it on trust.

If you do use one, Professor Fleming's advice is that it must meet the same standards as a new mattress, with no tears or damaged areas on the cover. He also said how it has been kept matters, and that it should have been stored somewhere clean and dry. Ask whoever is passing it on where it has been, because a mattress that has sat in a damp garage or a loft can look fine from the top. The Lullaby Trust's page on second-hand baby products has its own list for a used mattress, and adds that second-hand items should come from a smoke and pet free home.³ Take the mattress out of the cot, put it somewhere with good light, and go through the list on both sides. Search "product recalls" on GOV.UK as well, which the charity suggests for any second-hand baby item.

• The cover is whole, with no rips or tears
• The cover fits the mattress well and is waterproof all the way round
• No water marks or other water damage
• Firm: pressed hard in the middle, it barely gives
• Flat, with no sagging and no raised or cushioned edges
• It fits the cot with no gaps at the sides or ends
• It has a fire-resistant label on it
• It has been stored somewhere clean and dry
• It comes from a smoke-free and pet-free home

Nothing else goes in the cot: no nests, bumpers, pillows or wedges

A bare cot can look a little plain after months of picturing the nursery, and it's still the right way to set it up. The mattress, a fitted sheet and your baby is the whole of it. Professor Fleming was direct about sleep nests:

The Lullaby Trust's guide to baby sleeping products gives the reason. Pods and nests are soft with raised or cushioned sides, and a sleeping baby shouldn't lie on or have anything soft around them, particularly around their head, because it can make them overheat and increases the risk of SIDS.⁴ Soft material can also cover a baby's mouth and nose if they're pressed against it. The charity points out that there's a British Standard for cot mattresses and none for pods or nests as a place for a baby to sleep. If you've been given one as a present, keep it out of the cot. Professor Fleming went further on some of the other things that end up in cots:

Each of those has its own reason on The Lullaby Trust's pages. Bumpers become a risk once your baby starts to roll and move about, because babies can get tangled in the ties and fabric, or fall after pulling themselves up on them. Pillows can increase the risk of SIDS, and the charity lists pillows and duvets among the bedding to avoid. Wedges and sleep positioners are on its list of things to avoid, and it says there's no need for any equipment to keep a baby in one position unless a health professional has advised it for a specific medical condition. I know how many lovely things get given at a baby shower. Most of them are for the shelf, and that's fine. This is how the cot is set up for every sleep, day or night:

1. On their back. Feet at the bottom of the cot, which is the feet-to-foot position.
2. Bedding tucked in. Sheets and any blanket tucked in firmly and no higher than their shoulders, or a well-fitted baby sleep bag instead.
3. Nothing soft or extra. Leave out pillows, duvets, quilts and sheepskins, and bumpers, pods, nests, wedges and sleep positioners.
4. Toys out. Take soft toys out of the cot before each sleep, because they can cover a baby's mouth.

Keep the cot flat, even for reflux or a cold

Leave the cot level and the mattress flat, and don't prop up the head end. It's one of the first things people try when a baby is snuffly or bringing milk back up, and the kindness behind it is easy to understand. Professor Fleming's answer on it was clear:

The Lullaby Trust's page on the safest sleeping position says the same in its own words: don't incline, tilt or prop the mattress, the cot or your baby, because it doesn't help with reflux and is unlikely to improve cold symptoms.⁵ Its products guide adds that babies shouldn't sleep in anything that keeps them seated or inclined, as this can restrict their breathing. So keep books out from under the cot legs, and don't fold anything under the mattress to lift one end. The charity warns against putting folded blankets or a quilt under a baby on a thin travel cot mattress too, which is the same temptation in a different room.

If your baby has reflux or any other health condition, the charity's advice is to speak to your doctor about the best care for them. If your baby's consultant or specialist neonatal team has told you to sleep them in a different position for medical reasons, follow their advice. Otherwise it's flat, on their back, every time. The NHS list below is the one I'd stick inside a kitchen cupboard door for the first few months:

Questions people ask about breathable cot mattresses