100% Polyester ALL-IN-ONE FITTED SHEET & MATTRESS COVER: A soft fabric top surface for infants to sleep comfortably while the waterproof backing helps protect the mattress from moisture and messes. No need for a mattress pad. HELPS MAINTAIN INTEGRITY OF MATTRESS: Maintains the safety and integrity of the mattress by covering the mattress with only a single layer of fabric to ensure the firm side of the mattress stays firm for infants under 12 months. SAFER FOR INFANTS: Helps prevent exhaled air from getting trapped between fabric layers and helps prevent carbon dioxide rebreathing, a phenomenon suspected to be a contributor to accidental suffocation. EASY CARE & DURABLE: Wrinkle-free and shrink resistant polyester microfiber can be laundered time after time without having to worry about fit issues causing unsafe gaps between the mattress and sides. FITS CRADLE MATTRESSES: Continuous extra wide elastic and 4" (10 cm) deep pockets help ensure a secure fit for cradle mattresses measuring 36" x 18" (91 x 46 cm).