They are usually of vinyl material so they are just for covering the mattress; they may be a bit too rough in material for the soft skin of your baby. (pregnancy-info.net)
For example, some of the mattresses are covered in a thin vinyl material that will rip or tear easily. (kaboutjie.com)
Featuring a dusty pink vinyl material with an elasticated waistline and skinny fit, team them with some strappy heel, layered jewellery and a pink bralet to nail the one tone look. (prettylittlething.us)