Safe Cot Mattress Fitting Guide for Your Baby

A cot can look perfectly prepared, yet a mattress that is even slightly too small, too soft or poorly supported can create a space that is not suitable for safe sleep. This safe cot mattress fitting guide gives you a clear way to check the sleep surface before your baby settles down, whether you are preparing the nursery before birth or moving on from a Moses basket.

For parents concerned about comfort, reflux or head shape, it can be tempting to add cushions, wedges or extra padding. But a well-fitted mattress and an uncluttered cot are the safer starting point. We all want the very best for our little ones, and small fitting checks can bring real peace of mind.

Start with the cot, not the mattress label

Do not assume that a mattress labelled “cot mattress” will fit every cot. Cots vary in internal length, width, corner shape and base construction. Measure the inside of the cot at its widest and longest points, rather than relying on the dimensions printed on an old mattress or remembered from when it was bought.

Then compare those measurements with the mattress dimensions. The mattress should lie flat against the cot base, with its corners sitting neatly into the cot corners. It should not need to be bent, forced into position or compressed at the edges to make it fit.

A mattress that is too large may buckle or lift at the sides. One that is too small can leave a gap around the edge. Both issues matter because your baby could become trapped between the mattress and cot side.

The safe cot mattress fitting guide: check every gap

Once the mattress is in place, run your hand around all four sides and each corner. A properly fitted mattress should sit snugly in the cot, with no substantial spaces at the head, foot or sides.

As a practical UK safety check, the gap between the mattress and cot side should not exceed 3 cm at any point. If you can see a significant opening, or can easily push your hand down between the mattress and cot frame, stop using that mattress in that cot. A gap can become wider when a baby moves, when bedding is tucked in, or when the mattress shifts during everyday use.

Check the fit with the mattress pushed towards each side in turn. Some mattresses seem snug only when centred, but slide enough to create an unsafe space at the opposite edge. Also inspect the corners closely. Rounded cot corners paired with square mattress corners, or the reverse, can leave gaps that are easy to miss.

If you are using a travel cot, use the sleep base supplied or specifically approved by its manufacturer. Adding a separate mattress, folding blankets underneath, or layering mattresses may raise your baby too high and alter the fit against the mesh sides.

Test the mattress support

A cot mattress needs more than the right dimensions. It should be firm, level and fully supported by the cot base. Press gently across the centre and around the edges. The mattress should return to shape and should not sag, dip or feel unstable at the sides.

Lift the mattress and inspect the cot base too. Make sure every slat is secure, the base is correctly fitted at the chosen height, and there is nothing beneath the mattress that could cause an uneven surface. A damaged base can make even a good mattress unsafe.

Be cautious with hand-me-down mattresses. A used mattress may have hidden dips, weakened edges, damaged waterproofing or a shape that no longer matches the cot. It may also have been stored in a damp loft or garage, where mould can develop. If there is any doubt about its condition, replacement is usually the safer choice.

Choose the right depth for the cot

Mattress depth is often overlooked. A very deep mattress can reduce the effective height of the cot sides, particularly once your baby can sit, pull up or stand. A mattress that is unusually thin may not sit at the intended height within the cot frame or provide adequate support.

Use the mattress depth recommended in your cot instructions. Check the minimum distance from the top of the mattress to the top of the cot side for the base position you are using. As your baby becomes more mobile, lower the cot base in line with the cot manufacturer’s guidance.

Never compensate for a thin mattress by placing another mattress, a foam topper, folded towels or blankets underneath it. These additions can move, create a slope, or affect how securely the mattress sits in the cot.

Keep the sleep surface clear and level

For routine sleep, your baby needs a clear cot with a firm, flat and level sleep surface. The fitted sheet should be designed for the mattress dimensions and pulled taut, with no loose fabric gathering at the corners. If the sheet repeatedly comes loose, it is the wrong size or the mattress depth is incompatible with it.

Avoid pillows, duvets, cot bumpers, nests, positioners, loose blankets and soft toys in the cot. While they may look cosy, they can introduce a risk of overheating or restricted breathing. A well-fitting baby sleeping bag, used in the correct size and tog for the room temperature, can be a practical alternative to loose bedding.

If your baby has reflux, congestion, torticollis or a visible flat spot, seek advice from an appropriate healthcare professional before changing their sleep set-up. Do not create an incline with wedges, rolled towels or books under the cot legs. Babies should not be left to sleep on an improvised slope.

What about specialist infant mattresses?

Parents looking for support with plagiocephaly or brachycephaly may consider a specialist mattress. The same fitting principles still apply: it must be made for your baby’s age and sleep environment, fit the cot securely, and be used exactly as directed by the manufacturer.

Do not assume that a specialist shape or therapeutic feature makes fit less relevant. In fact, the cot dimensions, mattress orientation and correct cover are especially important. A clinically informed solution should sit securely within the wider foundations of safer infant sleep, not replace them.

SleepCurve was developed by a leading UK Paediatric Cranial Osteopath for parents seeking a clinically proven approach to infant head shape. If you choose any specialist product, read its instructions carefully and check that it is intended for the specific cot or sleep space you plan to use.

Recheck after every change

Mattress fitting is not a one-off nursery task. Recheck it when you move the cot, wash the cover, change sheets, lower the cot base or switch to another mattress. This is particularly worthwhile after travelling, redecorating or taking the cot apart and rebuilding it.

Look for signs that the fit has changed: a mattress that shifts more easily, a new gap at one corner, a loose sheet, damaged stitching, cracking waterproof material or a sagging area where your baby usually lies. These are reasons to pause and assess the sleep space rather than hoping the issue will resolve itself.

If the cot and mattress are from different brands, take extra care. Compatibility is never guaranteed simply because the measurements appear close. The manufacturer instructions for both products should always take priority.

A final check before bedtime

Before you place your baby down, look into the cot from their level. The mattress should be flat, firm and snug, the sheet should be smooth, and the cot should be empty apart from your baby and suitable sleepwear. That simple check takes seconds, but it helps create the secure sleep space your baby needs while giving you one less thing to worry about overnight.