You ordered the cot from John Lewis, assembled it in the nursery, and then discovered the mattress sold separately costs almost as much as the cot itself. You looked online, found options ranging from £30 to £300, and immediately became confused by terms like pocket sprung, open coil, foam density, and coir fibre. Your mum helpfully added that “we just used whatever came with the cot and you were fine” — which was reassuring but not useful. Cot mattress safety standards have changed since then, and getting this one right matters more than almost any other baby purchase.
In This Article
- Why the Cot Mattress Matters More Than the Cot
- Safety Standards and Certifications
- Types of Cot Mattress
- Best Cot Mattresses in the UK
- Firmness: Why Firmer Is Safer
- Size and Fit
- Breathability and Temperature Regulation
- Waterproof Covers: Essential or Optional
- When to Replace a Cot Mattress
- Should You Buy a Second-Hand Cot Mattress
- Frequently Asked Questions
Why the Cot Mattress Matters More Than the Cot
Safe Sleep
A baby spends up to 16 hours a day sleeping. The mattress surface is the single most important factor in safe infant sleep — more important than the cot brand, the bumper (which you should not use anyway), or the bedding. The Lullaby Trust — the UK’s leading safe sleep charity — is clear: the mattress must be firm, flat, and fit the cot snugly with no gaps.
SIDS Risk Reduction
Research links soft, sagging, or ill-fitting mattresses to increased SIDS (Sudden Infant Death Syndrome) risk. A soft mattress can conform around a baby’s face if they roll, potentially restricting airflow. A firm mattress keeps the sleeping surface flat and prevents the baby sinking into it. This is not marketing — it is evidence-based safety guidance from the Lullaby Trust, the NHS, and every paediatric sleep authority in the UK.
Comfort Over Years
A cot mattress is used from birth until the child moves to a bed — typically 2-3 years. Some cot bed mattresses last until age 4-5. The mattress needs to maintain its firmness, support, and integrity over thousands of nights. A cheap mattress that sags after six months is not just uncomfortable — it becomes a safety concern.
Safety Standards and Certifications
BS 1877 and BS 7177
All cot mattresses sold in the UK must comply with BS 1877 (dimensions) and BS 7177 (fire safety). These standards ensure the mattress meets minimum size requirements for standard UK cots and passes flammability tests. Any mattress sold by a reputable UK retailer will carry these certifications — if a mattress does not mention BS compliance, do not buy it.
OEKO-TEX Standard 100
This voluntary certification tests for harmful substances (pesticides, heavy metals, formaldehyde, phthalates) in textiles. A mattress with OEKO-TEX certification has been tested to ensure none of the materials in the finished product exceed safe limits for baby skin contact. Most quality cot mattresses carry this certification. It is worth looking for, especially for foam mattresses where chemical content is a common parental concern.
CertiPUR
A certification specific to polyurethane foam, confirming the foam is made without ozone-depleting substances, heavy metals, or prohibited phthalates, and has low VOC emissions. If you are buying a foam cot mattress, CertiPUR or similar certification provides reassurance about the foam content.
Types of Cot Mattress
Foam Mattresses
The most common and usually the cheapest option. A solid block of polyurethane foam (sometimes with a shaped or ventilated surface) wrapped in a removable cover. Quality depends entirely on the foam density — denser foam provides better support and lasts longer.
Pros: lightweight (easy to change sheets at 2am), wide range of prices, good density options available Cons: cheaper foam mattresses lose firmness within 12-18 months, less breathable than spring mattresses, some parents have concerns about foam chemicals (addressed by CertiPUR certification)
Open Coil Spring Mattresses
A traditional spring mattress with a single interconnected wire spring unit inside a foam border. These are the mid-range option and offer good support with better breathability than solid foam — the spring cavities allow air circulation.
Pros: durable (the springs maintain support for years), good airflow, widely available Cons: heavier than foam (harder to lift for sheet changes), springs can be noisy if the baby moves a lot, interconnected springs mean movement transfers across the surface
Pocket Sprung Mattresses
Individual springs wrapped in fabric pockets, each responding independently to pressure. This is the premium spring option and provides the best support because each spring conforms to the baby’s body shape without affecting adjacent springs.
Pros: best support and pressure distribution, very durable, minimal movement transfer, good breathability Cons: the most expensive option (£100-300), heaviest type of cot mattress
Natural Fibre Mattresses
Made from coconut coir, organic cotton, natural latex, or wool — sometimes in combination. These appeal to parents who want to minimise synthetic materials in the cot environment.
Pros: natural materials, good breathability (especially coir), often organic-certified, firm and supportive Cons: expensive (£100-250), coir can be very firm (not a disadvantage for safety but some parents expect softness), natural latex allergies are rare but possible
Dual-Sided Mattresses
A mattress with two different surfaces — a firmer side for newborns (safer sleep) and a slightly softer side for toddlers (comfort). You flip the mattress when the child is old enough. These offer good value because one mattress covers both stages.
Best Cot Mattresses in the UK
Best Overall: Little Green Sheep Natural Twist Cot Bed Mattress
About £180 from Little Green Sheep, John Lewis, or Natural Baby Shower. A dual-sided mattress with a coconut coir fibre layer on the baby side (firm, breathable) and a natural latex layer on the toddler side (supportive but slightly softer). The cover is organic cotton, GOTS certified, and removable for washing.
This mattress hits the perfect balance between safety (the coir side is reassuringly firm), comfort (the latex side is excellent for toddlers), and ethics (natural materials, organic certification, UK-based company). The price is premium, but it lasts from birth to 4-5 years without compromising support.
Best Value: Mothercare Essential Spring Cot Bed Mattress
About £60 from Mothercare. An open coil spring mattress with a foam border and a water-resistant cover. It does not have the natural credentials of the Little Green Sheep, but it is well-made, firm, meets all BS standards, and has been used in millions of UK nurseries. The springs maintain support for years and the breathability is good.
At £60, it is the sensible choice for parents who want a reliable, safe mattress without spending nursery budget that could go elsewhere. The cover is water-resistant (not fully waterproof) — use a separate waterproof mattress protector underneath the sheet.
Best Foam: Silentnight Safe Nights Essentials Cot Bed Mattress
About £50 from Silentnight, Amazon UK, or Argos. A high-density foam core (25kg/m³ density — firmer than most budget foam mattresses) with a removable, washable cover. OEKO-TEX certified and hypoallergenic. Silentnight is one of the most established mattress brands in the UK and their baby range reflects that experience.
The foam is firm enough for safe newborn sleep and durable enough to maintain that firmness through toddlerhood. At £50, it is the cheapest mattress on this list that genuinely delivers on both safety and longevity.
Best Pocket Sprung: John Lewis & Partners Pocket Sprung Cot Bed Mattress
About £120 from John Lewis. 256 individual pocket springs provide excellent support and pressure distribution. The springs are surrounded by a foam comfort layer and wrapped in a breathable cover with a waterproof membrane. Dual-sided (firmer baby side, slightly softer toddler side).
John Lewis bedding carries their standard 2-year guarantee, and the quality control is consistently good. This is the mattress for parents who want pocket sprung support without the price of a natural fibre option.
Best Natural: The Little Green Sheep Natural Coir Spring Cot Mattress
About £200 from Little Green Sheep or John Lewis. A combination of coconut coir and pocket springs with an organic cotton and wool cover. The coir provides natural firmness and breathability, the pocket springs add responsive support, and the wool regulates temperature naturally — warm in winter, cool in summer.
This is the flagship natural option. For parents who prioritise organic materials and minimal chemical exposure, it is the best available without going to bespoke artisan makers.
Firmness: Why Firmer Is Safer
The Safety Logic
A firm mattress keeps a baby’s sleeping surface flat and prevents their face from sinking into the material. The NHS and Lullaby Trust both specify that cot mattresses should be firm — there should be no visible indentation when you press the mattress with your hand.
How to Test Firmness
Press your hand firmly into the centre of the mattress. It should resist your pressure and spring back quickly when you release. If your hand creates a visible indentation that recovers slowly, the mattress is too soft for safe infant sleep.
“But Won’t My Baby Be Uncomfortable?”
New parents worry that a firm mattress means an uncomfortable baby. It does not. Babies have different sleep needs from adults — they need a flat, supportive surface rather than a conforming one. A baby sleeping on a firm, well-fitted mattress sleeps more safely than one on a soft, cushioned surface. Comfort in the adult sense (sinking into a pillow-soft bed) is actively dangerous for infants.
Size and Fit
Standard UK Cot Mattress Sizes
- Standard cot — 120cm × 60cm
- Cot bed — 140cm × 70cm
- Space saver cot — 100cm × 50cm
The Gap Rule
There should be no more than a 2cm gap between the mattress edge and the cot frame on any side. Larger gaps create a risk of the baby’s limbs or head becoming trapped. Measure your cot interior before buying a mattress — not all cots are exactly standard size, especially if the cot is from a European brand or is vintage/second-hand.
What to Do If the Fit Is Not Right
Some manufacturers offer custom-cut mattresses to fit non-standard cots. The Little Green Sheep, for example, makes mattresses to fit specific cot brands (Stokke Sleepi, SnüzKot, and others). If your cot is an unusual size, check whether a custom option exists before buying a standard size and hoping it fits.
Breathability and Temperature Regulation
Why Breathability Matters
Overheating is a risk factor for SIDS. A mattress that traps heat against the baby’s body increases the risk. Breathable mattresses allow air to circulate through the material, helping regulate the baby’s temperature during sleep.
Most to Least Breathable
- Coconut coir — natural fibre structure with significant air space. The most breathable mattress type.
- Pocket sprung — air circulates between and around the individual spring pockets
- Open coil sprung — good airflow through the spring cavities
- High-density foam — less breathable than spring or natural options. Ventilated or shaped foam improves airflow but does not match spring or coir
- Low-density foam — the least breathable. Traps heat and moisture.
Room Temperature
The Lullaby Trust recommends keeping the nursery between 16-20°C. Even the most breathable mattress cannot compensate for an overheated room. Use a room thermometer and adjust bedding and clothing rather than relying on the mattress alone — our baby room temperature guide covers safe ranges for summer and winter.
Waterproof Covers: Essential or Optional
The Case For
Babies produce bodily fluids. Nappies leak. Milk gets spat up. A waterproof layer between the sheet and the mattress prevents these fluids from soaking into the mattress core, where they create a breeding ground for bacteria, mould, and dust mites. A mattress that has been soaked through with urine and not dried properly becomes a hygiene and potential health risk.
Built-In vs Separate
Some mattresses have built-in waterproof layers. Others need a separate mattress protector. Built-in waterproofing is more convenient (one fewer layer to manage) but if it fails or becomes damaged, the whole mattress is compromised. A separate protector can be replaced cheaply and washed at higher temperatures than most mattress covers.
Breathability Trade-Off
Waterproof layers reduce breathability. A completely sealed plastic liner traps heat — which conflicts with the SIDS prevention guidance about avoiding overheating. Look for waterproof covers that use a breathable membrane (similar to Gore-Tex in concept) rather than solid PVC. Brands like Hippychick and PurFlo make breathable waterproof protectors specifically designed for cot use.
When to Replace a Cot Mattress
Signs It Needs Replacing
- Sagging or indentation — visible dips where the baby sleeps, or the mattress does not spring back when pressed
- Staining that will not clean — deep stains from fluid penetration (the waterproof layer has failed)
- Odour — persistent smell despite cleaning indicates bacterial or mould growth inside
- Damage to the cover — tears, loose stitching, or exposed foam/springs
Between Children
The Lullaby Trust recommends using a new mattress for each baby where possible. If reusing a mattress for a second child, it should be firm, undamaged, clean, and properly fitting. If there is any doubt about its condition, replace it — a new mattress costs far less than the risk of compromised sleep safety.
Should You Buy a Second-Hand Cot Mattress
The Short Answer: No
The Lullaby Trust advises against using a second-hand cot mattress unless you know its full history. Research has found a link between used mattresses and increased SIDS risk — though the mechanism is not fully understood, the association is strong enough that all UK safe sleep guidelines recommend new mattresses.
Why Second-Hand Is Risky
- You cannot verify the mattress’s age, usage history, or whether it has been contaminated with bodily fluids
- Internal deterioration (foam breakdown, mould growth, spring fatigue) is not visible from outside
- The mattress may not meet current safety standards if it was manufactured before the latest BS revisions
- Waterproof layers may have failed without visible external signs
The Exception
If you have stored a mattress from your first child in clean, dry conditions and it is still firm, undamaged, and less than 3 years old, reusing it for a second child in the same household is generally considered acceptable. But if in doubt, spend the £50-60 on a new one.
Frequently Asked Questions
What is the best type of cot mattress? For most families, a pocket sprung or high-density foam mattress offers the best balance of safety, comfort, and value. Natural fibre mattresses (coconut coir, organic cotton) are excellent but more expensive. The most important factor is firmness — the mattress must be firm, flat, and fit the cot with no gaps greater than 2cm.
How firm should a cot mattress be? Very firm. When you press your hand into the centre, it should resist and spring back immediately with no visible indentation. The Lullaby Trust and NHS specify firm mattresses for safe infant sleep. Softness that feels comfortable to an adult is a safety risk for a baby.
Do I need a waterproof cot mattress? A waterproof layer is strongly recommended — either built into the mattress or as a separate protector. Nappy leaks and milk spills are inevitable, and fluid penetrating the mattress core creates hygiene and safety issues. Choose a breathable waterproof option rather than solid PVC to avoid overheating.
Should I buy a new mattress for each baby? The Lullaby Trust recommends a new mattress for each baby where possible. If reusing within the same household, ensure it is still firm, undamaged, clean, and less than 3 years old. Never use a second-hand mattress from an unknown source.
How long does a cot mattress last? A quality cot mattress lasts 3-5 years — from birth through toddlerhood. Foam mattresses may lose firmness faster (2-3 years) than spring or natural fibre options (4-5 years). Replace when you notice sagging, indentation, or the mattress no longer springs back when pressed.