How to choose a cot mattress for your baby

When cot mattresses seem to differ mainly by fillings, covers and reassuring-sounding claims, it helps to start with the cot. The instructions for the exact cot, cot bed or bedside crib should identify the mattress size and type it needs. Any mattress you are considering must match those details and fit snugly, with no spaces around the edges.

The next essential check is the sleep surface itself. NHS guidance and The Lullaby Trust advise a firm, completely flat mattress that is clean, dry, intact and in good condition. Waterproofing, materials and labels such as “breathable” are secondary features, not proof that a mattress is suitable.

This order quickly narrows the choice. Rule out anything that fails the compatibility, surface or condition checks, then compare cover care, construction and stated uses with your household routine.

Match the mattress to the cot first

“Cot mattress” and “cot bed mattress” are broad product descriptions, not guarantees of fit. Begin with the current instructions for the cot, cot bed or bedside crib you own or plan to buy. Note the required mattress dimensions and type, along with any named compatible models or permitted configurations.

Compare all of those details with the mattress you are considering. A matching length and width matter, but dimensions alone are not enough if the instructions specify a particular type, supplied component or compatible model. Category labels and familiar-sounding names are useful starting points, but the exact instructions control the decision.

Require a snug fit with no spaces at the edges

The mattress should sit snugly inside the cot, with no spaces around its edges. Both the NHS and The Lullaby Trust advise a snug fit, but their guidance does not set one universal mattress size, depth or permitted gap for every cot.

Before buying, confirm:

•the exact cot, cot bed or bedside crib model;

•the mattress size and type stated in its current instructions;

•the mattress's stated dimensions and compatible products; and

•that the mattress is intended to fit snugly with no spaces at the edges.

Match the stated mattress size and type to the exact cot instructions, and make sure it fits snugly with no spaces around the edges.

Check cot bed and bedside crib compatibility separately

A cot bed may have several permitted configurations, while a bedside crib may be designed around a mattress supplied for that model. Do not assume that two products are interchangeable because their category names or stated dimensions look similar.

If you hope to use a cot bed for longer, check the configurations and use limits stated for that cot and mattress combination. Only count a use as a benefit if it fits your routine. A “multi-stage” or similar label does not promise a particular lifespan, saving or suitability for another child.

Check firmness, flatness and condition

Once compatibility is clear, check the mattress's surface and condition. It should be firm and completely flat, without raised or cushioned edges. Do not rely on a home pressure test, a claimed sink depth or the idea that one filling must be firmer than another.

Rule out damage, dampness and sagging

A mattress should be clean, dry and intact. Any known or visible damage is a reason not to choose it, and a mattress already in use should be replaced if it no longer passes these checks.

Rule it out if you find:

•rips or tears;

•water damage or dampness;

•sagging or an uneven surface; or

•a damaged cover that no longer fits properly.

These checks can identify a clear problem, but appearance alone cannot reveal the internal condition of a mattress or prove that it has passed a safety test.

Choose carefully between a new and second-hand mattress

The NHS and The Lullaby Trust advise using a new mattress for each baby where possible. If reuse is necessary, try to establish the mattress's history and apply the same checks without judgement about the family's circumstances.

A reused mattress must still be the right size and type, fit without gaps, remain firm and completely flat, and be clean, dry and free from damage or sagging. Check that its waterproof protection fits well and look for a fire-resistant label, as the guidance advises. Cleaning cannot repair damage, sagging or an incompatible fit, and passing a visible checklist is not a guarantee.

Check the mattress when it arrives and during use

You can narrow the choice before purchase using compatibility, construction, cover and care information. Physical checks come second, either when you can inspect the mattress or when it arrives.

At that stage:

•compare its label and dimensions with the cot instructions;

•confirm that it sits snugly and completely flat;

•check for damage, dampness or sagging;

•make sure any removable or waterproof cover fits smoothly; and

•follow the stated cleaning, drying and refitting method.

Repeat the relevant checks during use. Stop using the mattress if it develops a gap, damage, dampness, sagging or another problem with its condition. These observations help catch a change, but they do not certify the mattress or prove that it prevents SIDS.

Choose waterproof protection and care for your routine

Waterproof protection has a straightforward practical role. A well-fitting waterproof cover or waterproof surface can be wiped and helps keep the mattress clean and dry. It does not, by itself, prove breathability, temperature control, allergy protection or overall safety.

Check whether the protection is built into the mattress, supplied as a separate cover or needs to be bought separately. If it is separate, confirm that the cover is intended for that mattress and can fit without changing the flat surface or close fit.

Match the care method to everyday life

A removable cover may sound convenient, but the useful question is what the exact care instructions allow. Verify whether it can be removed, washed or replaced, how it should be dried and how it must be refitted. “Washable” does not automatically mean machine-washable, and it is not a hygiene or health guarantee.

Think through the routine before choosing:

•how the waterproof protection is provided;

•whether wiping or washing is permitted;

•how long washing and drying are likely to take;

•whether refitting the cover is straightforward; and

•whether a compatible spare cover would be useful for quick turnarounds.

Choose an option whose permitted care routine you can follow after ordinary leaks and accidents. If two mattresses pass the essential checks, their care routines may help you choose between them, but ease of cleaning cannot compensate for damage, sagging or poor fit.

Check how the waterproof protection is provided, then follow the instructions for washing, drying and refitting the cover.

Look past material and longevity claims

Foam, springs, fibre and named materials tell you something about how a mattress is built. They do not, on their own, establish that it is safer, healthier, cooler, more supportive, more comfortable or more durable.

After the mattress has passed the fit, surface and condition checks, construction can still matter as a practical household preference. You might compare the listed materials, weight, care requirements or cost, but focus on what matters to your routine rather than broad outcome claims.

Ask what each label really means

Words such as “breathable”, “natural”, “hypoallergenic”, “orthopaedic”, “temperature-regulating”, “antibacterial”, “non-toxic”, “eco”, “premium” and “dual-core” need unpacking. Look for the specific feature behind each label. If none is clearly defined and relevant to your needs, do not give the claim extra weight.

Check that each claimed use fits your plan

For a cot bed or multi-stage mattress, match every proposed configuration to the exact cot instructions. Check the stated care requirements and current use limits as well as the dimensions. A feature only adds value if it remains compatible with the sleep product and your intended routine.

Do not assume that a whole construction type has a guaranteed lifespan or replacement date. Condition and compatibility still determine whether a particular mattress remains suitable, and labels such as “long-lasting” do not guarantee shape retention, future reuse or resale value.

Frequently asked questions