Cot mattress buying guide UK: fit & safety

What kind of mattress should a baby sleep on?

A firm, flat, waterproof mattress that fits the cot or basket so there are no gaps. Soft, sagging or “cloud” surfaces are the opposite of the advice. The Lullaby Trust’s sleep space is a clear cot with that mattress and lightweight bedding only. If you remember one sentence, remember firm, flat, waterproof, in your room.

Quote the source, not a brand: “Use a firm, flat, waterproof mattress” and “the safest cot is a clear cot” — The Lullaby Trust safer-sleep overview, last reviewed 1 February 2025, checked 13 August 2026. Room-share for at least six months. Back to sleep. Smoke-free.

Source: The Lullaby Trust — Safer sleep advice (checked 13 Aug 2026).

Should you buy a second-hand cot mattress?

Only if you know its full history, it is clean, firm, unstained, and it still fits with no gaps. A mystery mattress from a marketplace is a skip. Frames travel better second-hand than mattresses. If in doubt, buy a new mattress for an old cot — cheaper than guessing.

The Lullaby Trust discusses second-hand products in its baby-product advice. We will not pretend a stained mattress is “fine if you air it”. Waterproof covers help cleaning; they do not unsag a foam core.

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Do you need a mattress topper or baby pillow?

No. Pillows, toppers, nests and wedges are what a clear cot is trying to not be. A waterproof sheet or a fitted sheet is enough. Head-shaping pillows sold for plagiocephaly are a clinical conversation, not a newborn gift. Do not put them in the overnight sleep space because an advert said “safe”.

If a product exists to keep the baby in one position, The Lullaby Trust’s list already said no. That includes rolled towels and “anti-roll” cushions.

How often should you replace a cot mattress?

When it sags, smells of damp you cannot explain, or no longer fits with a snug edge. There is no honest universal “every two years” figure we will invent. A second child can reuse a firm, clean, known-history mattress. A stained mystery from a marketplace cannot. If you are unsure, replace the mattress and keep the frame.

Waterproof covers make cleaning possible; they do not unsag foam. If the surface hammocks when you press the middle, it is done. Pair with Moses basket vs next-to-me so you do not buy a second sleep product to compensate for a bad mattress. The Lullaby Trust’s product pages are the deeper read on second-hand kit (checked 13 Aug 2026).

The NHS recommends buying a new mattress where possible. Its guidance also explains when an existing mattress can be reused: it must be firm, flat, fit without gaps, be clean, dry, waterproof and undamaged, and have a fire-resistant label (checked 9 September 2026). A waterproof cover and two fitted sheets are the rest of the bed. Pillows, toppers and “cloud” memory foam are how a clear cot stops being clear. If money is tight, spend it here and take clothes second-hand.

Which cot mattresses are worth comparing in the UK?

Compare size, firmness and a waterproof surface — buy new. Little Green Sheep and generic firm foam listings are common. We have not sag-tested them. Skip memory foam and mystery second-hand. The Lullaby Trust wants firm, flat and snug, not a brand story.

Ad · affiliate link Compare cot mattresses at John Lewis*

How snug should a cot mattress be?

Snug enough that there are no gaps around the sides the baby could get trapped in. The Lullaby Trust wants a good fit. We will not invent a finger-count their page does not print. If you can see a gap, the mattress is wrong. Buy the size the cot maker states.

Measure the inside of the cot, then the mattress, before you keep a sale item. An inherited frame with a random foam slab is how gaps appear. Replace the mattress; keep the frame if it is sound. See Moses basket vs next-to-me so you do not buy a second sleep space to hide a bad fit.

Is a waterproof cover enough, or does the mattress have to be waterproof?

The Lullaby Trust wants a waterproof mattress. A removable waterproof cover helps cleaning; it does not unsag a sponge core. Do not use a pillow-top protector that makes the surface soft. Two fitted sheets are enough rotation. Skip a “cloud” topper sold as comfort. Firm and flat still win after the cover goes on.

Worked example: inherited pine cot, mystery foam, a pretty quilted protector. Strip it. If the core hammocks, replace the mattress new. Keep a wipe-clean cover that does not add loft. The NHS Best Start in Life page says the cot mattress should be new (checked 13 Aug 2026). That is the same logic as the car seat.

Source: NHS Best Start in Life — What to buy for your newborn baby (checked 13 Aug 2026).

Is a travel-cot mattress the same product as a home cot mattress?

No. A travel cot has its own board or foam that must stay firm and flat in that frame, with no extra cushions. Do not drop a home mattress into a travel cot if it creates gaps. Do not add a pillow to soften hotel travel. See travel cots. The home cot still needs its own snug mattress.

A second mattress is not a reason to buy a second nest. Clear space, firm surface, in your room at home. Travel is a different frame with the same rules: no extras. If the travel mattress is a hammock, that travel cot is the wrong product.

How do you compare cot mattresses without relying on marketing claims?

Check the exact cot dimensions, firmness, flatness, waterproof protection and the manufacturer’s instructions before comparing extra features. A product name such as breathable or premium does not establish suitability. Check how covers are cleaned and whether replacement covers are approved. Keep the sleep surface clear rather than adding comfort layers.

When comparing prices, include the cover that is needed for normal use. Look at the care instructions before buying: a cover that takes a long time to dry may affect how you manage an overnight leak. Do not solve that inconvenience by layering unapproved bedding or another mattress on top. Keep the cot instructions available to check that a replacement product has the intended dimensions.

Source: The Lullaby Trust — Keeping a clear cot (checked 9 September 2026).

What should you check when setting up a cot in your bedroom?

Check that you can assemble the cot as instructed, reach your baby comfortably and keep the sleeping space clear. Consider the location of windows, blind cords and heat sources before choosing a position. Follow the manufacturer’s instructions for mattress height and adjustments as your baby grows. Leave safe access around the cot.

A full-size cot may work from birth if it fits your room and is suitable for your baby. A Moses basket or crib can be a practical alternative when space is limited, but it will have its own use limits. Measure both the available floor space and how you will move around the bed. Buying a bedside crib does not remove the need to follow its attachment and use instructions.

Make setup a shared task for the people caring for the baby. Show each person how the cot is used, which bedding belongs with it and where the instructions are kept. Keep the Moses basket versus next-to-me guide for furniture comparisons and the TOG guide for sleeping-bag questions.

Source: NHS — Baby bedding and cot safety (checked 9 September 2026).