Glossary
Crib mattress firmness
The requirement that infant mattresses be firm and flat, not soft or contoured, and fit the crib with no gaps.
Crib mattress firmness refers to how little a mattress should compress under a sleeping infant. Unlike adult or even toddler mattresses, an infant mattress needs to stay firm and flat so the baby's face cannot sink into it. A simple check many guidelines describe is pressing on the mattress, it should not deeply cup around your hand.
Fit matters as much as firmness. The mattress must match the crib or bassinet it is paired with so there is no gap around the edges, a common rule of thumb is that you should not be able to fit two fingers between the mattress and the crib frame. A mattress that is too small for the frame, even from a reputable brand, recreates the gap hazard the crib design was meant to prevent.
Shoppers sometimes assume a plusher, more cushioned mattress is more comfortable for the baby, but pediatric sleep guidance treats extra softness as a risk factor rather than a comfort upgrade. Waterproof covers are useful for cleanup, but avoid stacking extra pads or toppers on top of the mattress, since that changes the firmness and the fit. Always buy the mattress sized for your specific crib or mini crib model.