Cot mattresses and safer sleep in the UK

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.

See cot mattresses at John Lewis*

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 Best Start in Life page says the cot mattress should be new (checked 13 Aug 2026). That is the same rule as the car seat: you cannot see a previous sag or a previous crash. 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.

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.