The Simmons Beautysleep 3 inch Foam Mattress" firm foam provides ideal support for baby"s rest. Best used for Portable Cribs and Day Cribs. Easy to clean hypoallergenic water resistant vinyl cover repels wetness. For a proper fit, please check the crib manufactures instructions for mattress sizing and maximum mattress thickness. When the mattress support is in its lowest position, the height of the sides of the crib must be a minimum of 20 inches from the top of the non-compressed mattress pad to the top of the side rail when the side rail is in its highest position or the gate rail is in the fully latched position.