The AAP recommends that only mattresses designed for the specific product be used. The mattress for a portable crib, bassinet, or play yard must be sold with the item. Mattresses should be firm and should maintain their shape even when the fitted sheet designated for that model is used.
You should check the fit of the mattress regularly. There should be no covering on the mattress when checking the fit. The mattress, when inserted in the center of the crib, portable crib, bassinet, or play yard, must not leave a gap of more than 1/2 inch at any point between the edge of the mattress and the side of the crib, portable crib, bassinet, or play yard. When the mattress is placed flush to one side and the end of the crib, portable crib, bassinet, or play yard, the gap on the other end/side must not exceed 1 inch.
Pillows or cushions should not be used instead of mattresses or placed on top of the mattress. Memory foam mattresses and mattress toppers should not be used for babies.
Many crib mattresses have an infant side and a toddler side. Make sure you use the infant side for children younger than 1 year.
Firmness of the Mattress
Firm means the mattress maintains its shape and will not indent or conform to the shape of the infant’s head when the infant is placed on the surface. A test to determine a suitable mattress is to press on it. A firm mattress should immediately bounce back to its original shape and should not conform to your hand.