Did you know you can legally buy a cot mattress that doesn't meet product safety standards? Just because it's on retail shelves doesn't necessarily mean it's safe. A sleeping surface that's too soft increases the risk of fatal sleeping accidents if a baby rolls over face-first and their breathing becomes obstructed due to their nose pushing against any cushioning or undulating surfaces in the mattress. But how do you know which mattresses are safe?

Want to know how we get our review results? Check out how we test cot mattresses.

Looking for the best cot mattress?

See our expert product reviews.

Cot mattress reviews

In this buying guide:

How firm is firm?

Parents are advised by safety experts to always choose a 'firm' mattress to minimise the risk of sudden unexpected death in infancy (SUDI). But until very recently there was no agreed definition of 'firmness'.

The good news is that there is now a standard test method for firmness, AS/NZS 8811.1:2013 Method 1: Sleep Surfaces – Test for firmness. However, it's not mandatory for cot mattress makers (or makers of any other infant sleep surface, such as a bassinet) to meet this standard, and we still aren't seeing many mattresses claim compliance. This means consumers don't know whether a cot mattress is safe.

How can I test cot mattess firmness at home?

Wrapping up CDs out of their cases
Wrap the CDs in protective cling film

To get around the lack of information about compliance, Dr Ron Somers, one of the researchers behind the standard firmness test, has devised a DIY method for parents and carers which mimics the lab-based standard test for firmness. This informal test method is for home use only and is not a replacement for the standard test for firmness (which uses a calibrated apparatus and conditioned lab environment). But it's far better than not knowing how safe your mattress is at all. If your mattress fails this test then it is definitely not safe. You will need:

  • Two unopened 1L milk or juice cartons with square bottoms (not rectangular bottoms)
  • A marker
  • A ruler
  • A stack of 12 DVDs or CDs (not in their cases), protectively and tightly wrapped in cling film
Informal mattress firmness test method
Two milk cartons placed on top of a stack of CDs is a useful way to test for firmness. This mattress is safe because the lower carton overhang does not come into contact with the mattress surface.

Method:

  1. Mark one of the milk cartons with a line 40mm parallel from the base. Mark all four sides this way so they form a ring around the carton.
  2. Place the tightly wrapped discs near the softest part of the sleep surface. Look for a 'worst case scenario', such as a fold or peak on the surface where a baby's nose could be positioned.
  3. Lay the marked milk carton sideways on the stack of discs so that it is centred, and have the marked line match up with the edge of the disc so you get a 40mm overhang. Make sure the overhang sits over the soft part of the sleep surface.
  4. Lay the second carton sideways and stack it onto the first, making the stack as level as possible – this usually means selecting an off-centre position.
  5. See whether the overhang touches the soft part of the sleep surface. If it does then the sleep surface is too soft to be safe.
  6. Test a few locations on the mattress, especially anywhere where the child's head might rest. A mattress should be firm enough in every location on the sleeping surface.

For a practical demonstration, see the video.

Check the recommended dimensions

Check that the mattress fits snugly in the cot and that it meets the cot manufacturer's recommended dimensions. There must never be more than a 4cm gap between the edge of the mattress and the adjacent cot side when the mattress is pushed to the opposite side. Gaps at the sides are a suffocation risk, just as firmness is. If a mattress fits too tightly, on the other hand, sections of the mattress could deform and bunch up along the cot edge, making it easier for a child to climb out of the cot. It could also cause the dropside to not move freely, if there is one.

The thickness of the cot mattress can also be the difference between a cot failing the mandatory standard for cot depth. The distance from the top of the mattress to the top of the lowest side when the dropside is closed should be at least 50cm when the base is in the lowest position, and at least 30cm in the upper position.

What kind of material? And what about comfort?

You'll often find cot mattresses promising "comfort" or "support" for your baby. A baby is not built like an adult, and a firm mattress, not a cushioned one, is safest for your baby. Our tests show that cot mattresses which are flat and do not have undulating or quilted features are less likely to fail our firmness test. 

As for mattress construction, you'll find a huge variety including foam, latex and inner-spring. All of these sorts of mattresses are capable of passing the firmness test, so we can't recommend one type over another, but an inexpensive and very basic mattress is just as likely to be as good as any other.

What about vacuum-packed cot mattresses?

Some cot mattresses are shipped in boxes where they've been vacuum packed and rolled up. In these cases, you will need to allow several days at least for the mattress to fully expand. In our latest round of testing, some cot mattresses remained slightly bent even after several weeks, thus failing the firmness test.

What are some safe sleeping tips?

More tips on keeping your cot safe are on the SIDS and Kids website. The main things to remember:

  • Choose a standards-compliant cot; see our review for recommendations.
  • Never wrap your mattress in plastic or any other unbreathable material.
  • Soft toys and bumpers could pose a suffocation risk and should never be placed in the cot.
  • Always place baby to sleep on his or her back, with feet at the base of the cot.
  • Do not use doonas/quilts, lambswool, thick blankets or pillows in the cot.
  • Buy a firm, safe mattress.
  • Keep baby's head uncovered and away from blankets, which should be tucked in tightly. Alternatively, use a safe sleeping bag.