The Mattress Mattress cover for Bassinet Pop-Up & YPSI Bassinet is made from medically tested Tencel material, which keeps baby cool in the summer and warm in the winter. Not only does this innovative microfiber aid in temperature regulation, it is also of botanic origin, since it is extracted from raw material wood. The Mattress Cover fits snugly over the Pop-Up Bassinet & YPSI Bassinet mattress, allowing a safe and comfortable rest for baby.