The best mattress for a bedside crib: height, edge, and the gap that matters

A bedside crib (a side-sleeper cot, the kind that straps to the frame of your bed with one side down) is only as safe as the fit between it and your mattress. The crib has to be at the same height as your mattress top, with no gap, and your mattress edge has to stay put where the crib is strapped. Here is what that means for the mattress you sleep on.

First and above everything: follow the crib manufacturer's instructions and the NHS and Lullaby Trust safer-sleep guidance. This guide is about the adult mattress, not about how a baby should sleep.

What the crib needs from your mattress

Height. Bedside cribs adjust in steps, and each has a stated range for the height of the parent's mattress top from the floor, typically somewhere between about 45 and 65 cm depending on the model. Your bed base height plus your mattress depth has to land in that range. A 25 cm mattress on a 35 to 40 cm divan or frame is in range for almost every crib; a 38 cm mattress on a tall divan may be over it.

A firm, stable edge. The crib straps to the bed frame and its open side sits against your mattress. If your mattress edge is soft or collapses when you lean on it, a gap opens between the crib and the mattress, which is the thing the instructions warn about. A mattress with a foam-encapsulated rail of 50 kg/m³ or a steel edge keeps the line straight. A cheap mattress with no edge support, or an old one with a rounded edge, does not.

Level. The mattress has to be level at the edge, not sloping away, so it needs to be the right size for the frame (a mattress that is narrower than the base leaves a gap; one that overhangs can be tipped).

Firmness at the surface. The crib's own mattress is firm and flat by design; the adult mattress being medium rather than very soft helps keep the two surfaces aligned.

Which mattress depth

Most parents are fine with anything from 22 to 30 cm, given a normal base. The check is the total: measure from the floor to the top of your mattress and compare with the crib's range. If you are buying a new mattress with a crib in mind, 25 cm is the safe middle.

The other thing the mattress has to do

You will be getting in and out of bed several times a night, sitting on the edge to feed, and trying not to wake the person on the other side. So the mattress that suits a bedside crib also wants:

• Low motion transfer: a foam core or a pocket-spring unit with a thick memory foam top, so one of you moving does not wake the other. See mattresses for couples who move differently.
• An edge you can sit on, again.
• A cover that washes. Things happen. A zip-off cover that goes in the machine at 40 °C saves the mattress.

What not to do

• Do not use a mattress topper on the adult bed while a bedside crib is attached unless the crib instructions allow it; toppers raise the height and soften the edge.
• Do not use an old mattress with a rounded or soft edge with a bedside crib.
• Do not rely on the crib's straps to pull the mattress edge straight; the mattress has to hold its own shape.

How we built ours

The New Parent is 25 cm deep, sits in range for most bedside cribs on a standard base, has a 50 kg/m³ firm rail you can sit on to feed without the edge moving, a foam core with micro-springs so one of you getting up does not wake the other, and a zip-off cover that washes at 40 °C. Check your crib's stated height range against your base before ordering; the page says so too.

The short version

• Base height plus mattress depth has to be inside the crib's stated range; 25 cm on a normal base usually is.
• A firm edge (50 kg rail or steel) keeps the gap closed; a soft or old edge does not.
• Low motion transfer, an edge you can sit on, and a washable cover for the rest of the year.
• The crib instructions and safer-sleep guidance come first.