Cot Mattresses: What to Look For and When to Upgrade
A cot mattress needs to be firm, flat, close-fitting and new, and those four words cover nearly all the safety guidance that matters for the first year. Everything else, from breathability to washable covers to the dual-sided designs that carry a child into toddlerhood, builds on that foundation. This guide works through what to look for at each stage, and the signals that it's time to move on.
Safer sleep guidance for babies in this article follows Lullaby Trust and NHS advice, which are the authoritative sources for infant sleep safety.
The First-Year Rules
Safer sleep guidance for babies under 12 months is specific about the sleeping surface. Firm means the mattress shouldn't mould around the baby: press it and it should spring back promptly, with no lasting impression, because a surface soft enough to cradle an adult is a suffocation risk for a baby who can't move themselves yet. Flat means exactly horizontal, with no incline and no raised sections. Close-fitting means the mattress fills the cot to within about 3cm of the frame on all sides, so no gap can trap a small limb or face.
New matters for two reasons: a second-hand mattress may have softened unevenly in ways that aren't visible, and research associates used cot mattresses from outside the family home with increased risk. When reusing within a family, the mattress must be clean, dry, firm and undamaged, with its waterproof layer intact the whole time.
The cot stays bare beyond the mattress and fitted sheet: no pillows, duvets, bumpers or soft toys before 12 months. For the first year, the mattress is the whole sleeping surface, which is why it deserves this much attention.
What to Look For When Buying
- Fit before features. Measure the cot's internal dimensions and match them. The common UK cot bed size is 70 x 140cm, standard cots run 60 x 120cm, and the 3cm rule applies whatever the frame.
- A removable, washable cover. Babies test every mattress thoroughly, so a cover that zips off and goes in the machine is the feature parents end up using most, and a water-resistant layer underneath keeps the inside of the mattress clean through the inevitable accidents.
- Breathability. Babies regulate temperature poorly, so airflow through the cover and upper layers helps keep the sleeping surface fresh and reduces overheating risk alongside correct room temperature, which guidance puts at 16 to 20°C.
- Hypoallergenic materials. These are a sensible default for baby skin and airways.
- Dual-sided designs, for the long view. Some cot bed mattresses are built with two distinct faces: a firmer, flatter side specified for the first year, and a softer, more cushioned side for toddlerhood. Our kids' range takes this approach, and a dual-sided cot mattress that grows with them means the toddler upgrade is a simple flip of the mattress, with the firm infant side doing the safety-critical first year and the cushioned side taking over from 12 months.
When to Upgrade Beyond the Cot
Cot beds with removable sides typically serve until age three or four, and the mattress question follows the child. The clear signals:
- Climbing out. A toddler scaling the cot side has outgrown the cot's safety design, whatever their age, and the conversion or the toddler bed is due.
- Size. A child touching both ends, or clearly cramped, needs the length of the next stage.
- The mattress's own condition. Softening, visible dips, a waterproof layer that's failed, or a hygiene problem that washing can't fix all mean the mattress is done.
Simba's pick: for a raised or bunk-style first bed, the Hybrid® Kids Mattress, a shallower 16cm UK single built to keep guard rails effective.
The next step is either the cot bed's toddler configuration on the same mattress, or a first single bed. For raised and bunk-style first beds, mattress depth becomes a safety issue of its own, and shallow designs around 16cm keep the guard rails doing their job. Otherwise, a standard single at 90 x 190cm takes over as the size that lasts through childhood.
Caring for a Cot Mattress in Service
You should aim for weekly fitted-sheet washes at 60°C, prompt attention to accidents using the washable cover, a monthly airing of the bare mattress, and a regular check that the fit is still close as the mattress ages. That's the whole maintenance routine. Rotate the mattress head to foot monthly, since babies sleep in remarkably consistent spots and even a firm mattress wears better when it wears evenly. A cot mattress kept this way comfortably serves the full cot years, and hands over to the toddler side, or the next bed, still doing its job.
The Room Around the Cot
The mattress does its job best in a room set up to match. Guidance puts the temperature at 16 to 20°C, judged by the back of the baby's neck, since hands and feet always run cool, with the cot positioned away from radiators, direct sunlight and window blind cords. The safest sleeping arrangement for the first six months is the cot in the parents' room, which decides where the cot lives at first, whatever the nursery plans were. Check the cot base as part of the setup too. It should be sound and correctly assembled at whichever height suits the child's stage, and lowered promptly once sitting and pulling up begin.
FAQs
Firm enough to spring back immediately when pressed, leaving no lasting impression, and to feel notably firmer than an adult mattress. Softness that would read as comfort for an adult is a risk for a baby, and the firm specification applies for the whole first year.
Guidance advises against mattresses from outside your family, which are associated with increased risk. Reusing your own between siblings is acceptable when the mattress has stayed clean, dry, firm and undamaged with its waterproof layer intact throughout.
From 12 months at the earliest. The firmer side is the specified safe surface for the first year, and it remains fine to use beyond that. Flip when you're ready after the first birthday, and note which side is which.
The common UK cot bed takes 70 x 140cm, and standard cots take 60 x 120cm. Measure your frame's internal dimensions and choose a mattress that fits within about 3cm on every side, since a close fit is part of the safety specification.
Typically between 18 months and three years, with climbing out of the cot as the decisive signal whenever it comes. Convert the cot bed or move to a low first bed at that point, and carry familiar bedding across to ease the change.
