Cot mattress safety is not a matter of opinion. The Lullaby Trust and the NHS are clear: the mattress must be firm, flat, and waterproof. Those three words are the foundation of every purchasing decision on this page, and everything else - material, brand, price - is secondary to getting those requirements right. A cot mattress that fails on any one of the three creates a risk environment that no amount of quality or cost can compensate for.
I've tested cot mattresses by pressing firmly into the centre and edges - the surface should resist and spring back quickly. If your hand sinks in and leaves an impression, the mattress is too soft for safe infant sleep. That test takes five seconds and is the most important quality check you can do before buying.
This page references safe sleep guidance from The Lullaby Trust (lullabytrust.org.uk) and the NHS. If you have concerns about your baby's sleep safety, contact The Lullaby Trust helpline or speak to your health visitor. Safe sleep practices reduce the risk of sudden infant death syndrome (SIDS).
The three requirements: firm, flat, waterproof
Firm means the mattress doesn't conform to the baby's face or body when pressed. An adult mattress moulds around pressure points for comfort. A cot mattress must not do this, because a soft surface that moulds around a baby's face creates a suffocation risk. Press your hand firmly into the centre. It should resist and spring back immediately. If your hand leaves a visible impression, the mattress is too soft.
Flat means no raised areas, cushioned zones, shaped headrests, or contoured surfaces. The entire sleeping surface must be uniformly level. Products marketed as "ergonomic" or "contoured" for babies are not recommended by The Lullaby Trust. A flat surface is the safest surface for infant sleep.
Waterproof is more important than breathability for cot mattresses, and this is the opposite of adult mattress advice. The Lullaby Trust specifically recommends waterproof over breathable because a waterproof barrier prevents bacteria, mould, and bodily fluids penetrating the mattress core. Bacterial buildup inside a mattress is a factor linked to increased SIDS risk. A waterproof cover or an integral waterproof layer is essential, not optional.
Fit: no gaps between mattress and cot
The mattress must fit the cot with no more than a 2 cm gap on any side. A gap wider than two fingers between the mattress edge and the cot frame is too large - a baby can become trapped or wedged in the space. Measure your cot's internal dimensions before ordering, and check the specific mattress dimensions rather than relying on "standard size" labels. Manufacturing tolerances vary between brands.
UK standard cot internal dimensions are 120 x 60 cm. Cot beds are 140 x 70 cm. Some cots use non-standard dimensions, particularly imported or vintage models. If your cot is non-standard, a made-to-measure mattress is the safe option.
New vs second-hand
The Lullaby Trust recommends using a new mattress for each baby where possible. A second-hand mattress may have internal damage, bacterial buildup, or loss of firmness that isn't visible from outside the cover. If you must use a second-hand mattress, it should still be firm and flat with no tears or damage, the waterproof cover should be intact, and you should know the full history of where it was stored and how it was used.
Materials
Foam core cot mattresses are the most common and affordable. The key is foam density - a high-density foam core stays firm for the full usable period. Low-density budget foam can soften over months of use, losing the firmness that makes it safe. If you press the surface and it doesn't spring back as quickly as when new, the foam has degraded and needs replacing.
Spring core cot mattresses use pocket springs or open springs underneath a firm comfort layer. They hold firmness longer than foam and handle the weight transition from newborn to toddler without losing structural integrity. More expensive than foam but worth the step up if the mattress will be used for multiple children across several years.
Natural fibre cot mattresses (coconut coir, wool, cotton) are breathable and chemical-free, suiting parents who want to avoid synthetic materials entirely. Make sure any natural cot mattress still meets the firm, flat, waterproof requirements. "Natural" does not automatically mean safe for infant sleep, and some natural fibre mattresses are softer than the safety guidance requires.
When to move to a toddler bed
Most children transition from cot to toddler bed between 18 months and 3 years. The trigger is usually the child climbing out of the cot or outgrowing the cot bed length. The toddler bed mattress has different requirements because the suffocation risk profile changes once the child can roll, reposition, and get out of bed independently.
Verdict
Firm, flat, waterproof. New for each baby where possible. Proper fit with no gaps wider than 2 cm. Everything else is secondary. Use The Lullaby Trust guidance as your primary reference. The safest cot mattress is the one that meets the safety requirements consistently, not the one with the most features on the label.
