When to Replace a Cot Mattress (The Honest Checklist)

These are condition failures, and no amount of flipping, airing or hopeful rotation fixes them:

• It fails the firmness test. The Lullaby Trust's test is beautifully simple: when your baby lies on the mattress, their head should not sink in by more than a few millimetres. A surface that cradles or moulds is a suffocation risk, not a comfort feature. Press the mattress with a flat hand; if it dents and recovers slowly, or stays dented, it has gone soft.
• It sags or dips. A visible hollow where your baby sleeps means the core has fatigued. Sleeping in a dip also tilts a baby towards a fixed position, which undermines back-sleeping.
• The cover is ripped or torn. Any tear breaches the waterproof layer and exposes the core. The Lullaby Trust's reuse checklist requires no rips or tears, full stop.
• Water damage, staining into the core, or any smell of damp. Once moisture has reached the filling, mould and bacteria follow and cleaning the surface does nothing about it.
• The waterproof layer has failed. If night leaks are reaching the core through an intact-looking cover, the membrane is finished even if the fabric is not.
• There is no fire-resistant label. A UK cot mattress should carry one; a mattress that has lost it fails the Lullaby Trust's second-hand checklist and cannot be passed on.

The Lullaby Trust's position, in their own words: "Generally speaking, we recommend buying a new mattress for each baby, although we know this isn't always possible."

The reasoning is worth having in full rather than as a scare line. Their product guide notes that research has suggested an increased risk of sudden infant death syndrome when using a second-hand mattress brought in from outside the family home, although this is not yet proven. So the strength of the advice scales with the mattress's history: a mattress from your own home, used by an older sibling and kept in known-good condition, is a much smaller question mark than one of unknown provenance from a marketplace listing.

If you do reuse a mattress for a second child, it must pass the full checklist at the bottom of this guide, every line. A mattress that has spent two years in a loft, or whose storage you cannot vouch for, does not pass.

No safety body publishes a cot mattress lifespan. In practice, lifespan is a function of the core material, how long the mattress is in service and how well its waterproofing held up, and the honest answer runs on the bed it serves:

• A standard cot (120 x 60 cm) is typically in use for up to around two years. A decent mattress should see one child through that comfortably.
• A cot bed (140 x 70 cm) runs from birth to approximately four years on manufacturers' own claims, with a growing toddler jumping on it by the end. Reaching year four with a budget foam core intact is optimistic; the firmness test, not the receipt date, tells you whether it made it.

Check the mattress properly every few months: strip the sheet and protector, look at the cover, press the sleeping area flat-handed, and check the underside for damp or mildew. Two minutes, four times a year.

Every move up a bed size is a new mattress by definition: moses basket to crib, crib to cot, cot to cot bed. A smaller mattress can never ride along into the bigger bed, because a mattress that does not fill the frame leaves gaps, and the cot standard BS EN 716-1 caps the gap between mattress and frame at 3 cm. The Lullaby Trust's version is simpler still: no gaps.

The reverse error also appears in the wild: trimming or folding a too-large mattress to fit a smaller bed. A cut foam core loses its structure and its cover integrity at a stroke; do not do this.

The one honest continuity is the cot bed conversion, where the sides come off and the same 140 x 70 cm mattress carries on under the same child. Buy the exact size for whatever frame you have; the fitment search gives it to you by brand and model, and our sizes guide covers every size in use.

A cot mattress dies from moisture and fatigue, and you can slow both:

• Run a washable waterproof protector over it from day one, under the fitted sheet. The protector takes the leaks, the mattress keeps its membrane for backup, and you wash the protector instead of scrubbing the mattress. Our protector guide covers choosing one.
• Deal with leaks when they happen. Strip everything, wipe the cover with mild soapy water, and let it dry completely before remaking the cot. Never remake a damp mattress.
• Air it. When you change the sheets, leave the mattress bare for an hour. Babies are damp sleepers and the underside needs to breathe, especially on solid-base cots.
• Rotate it end to end monthly so the sleeping zone moves, and flip it only if the maker says it is double-sided; many cot mattresses have a designated baby side and are not flippable.

Before any mattress goes under any baby for a second tour, it needs a yes to every line. This is The Lullaby Trust's second-hand standard:

• A well-fitting, fully waterproof cover
• No rips or tears in the cover
• Good condition throughout, with no water damage
• Firm and flat, with no sagging, passing the head-sink test
• Fits the destination cot snugly, with no gaps
• Carries its fire-resistant label
• Was stored somewhere clean, dry and smoke-free in between

One miss, and the answer is a new mattress. Given what a correctly sized cot mattress costs against everything else on the nursery list, it is the cheapest place in the room to refuse to compromise.