The mattress is the part of the cot your baby actually lies on, yet it is usually the last thing parents think about. It deserves more attention than that: fit and firmness are two of the most important factors in safe sleep.
Fit comes before everything else
There is no universal cot mattress size in Australia. Cots vary, so the mattress must be matched to your specific cot, not bought by name.
Measure the inside of the cot base, length and width, from inner wall to inner wall. Then choose a mattress that leaves a gap of no more than 20mm on any side. If you can fit two fingers flat between the mattress and the cot wall, the mattress is too small.
A mattress that is slightly too large is also a problem: if it has to be forced in, it will dome in the middle rather than sitting flat.
Firm means firmer than you expect
Adult instincts work against us here. A soft, plush mattress feels kind, but a firm, flat surface is what keeps a baby's airway clear. Press the centre with your palm. It should barely give, and spring straight back with no lasting impression.
A simple check: place the mattress on the floor and press down in the middle. If your hand sinks in noticeably, it is too soft for infant sleep.
The main types
Foam
Light, affordable and easy to lift when you are changing sheets at night. Quality varies with density, and a very cheap foam mattress may soften within months. Look for higher-density foam rather than the lowest price.
Innerspring
Contains a spring unit with padding above and below. Generally holds its shape longer than budget foam and feels more substantial. Heavier to handle.
Natural fibre and latex
Coconut coir, natural latex and wool options appeal to parents wanting fewer synthetic materials. They are typically the most expensive, and some are dual-sided with a firmer infant side and a softer toddler side.
Ventilation, covers and thickness
A breathable cover and ventilation holes help with airflow and with drying out after spills. A removable, washable cover is worth paying for, because you will use it.
Thickness usually sits between 8cm and 13cm. Thicker is not safer. What matters is that with the mattress in place at the cot's lowest setting, there is still at least 600mm from the top of the mattress to the top of the cot side.
Protectors and sheets
Use a firm, well-fitted waterproof protector directly on the mattress, then a fitted cot sheet. Avoid loose, thick or padded protectors that can bunch or slide. That is the complete list. Nothing else goes in the cot.
Buy new, not second-hand
Mattresses compress with use, and a mattress that was firm for one child may not be for the next. There are also hygiene considerations that a cover cannot fully address. Given that a cot mattress is one of the less expensive nursery items, buying new is straightforward insurance.
Frequently asked questions
How do I know if the mattress has gone too soft?
Look for a visible dip where the baby sleeps, or press the centre and the edge and compare. Any noticeable difference means it is time to replace it.
Can I use a bassinet mattress in a cot?
No. Sizes and firmness specifications differ, and the fit will not be safe.
Should I air a new mattress before use?
Unwrap it and let it air in a well-ventilated room for a day or two before first use, particularly foam.
Need help matching a mattress to your cot?
Bring your cot's internal measurements to our Peakhurst showroom, or send them through and we will confirm the right size before you order.
General information only. For safe sleep guidance see Red Nose Australia.