The covers of our mattresses are quilted with fleece on the inside. After initial use, this fleece loses a little of its volume. If your child always sleeps in the same place on the mattress, a depression may appear to form where its head contacts the mattress. If you lay your child in different places on the mattress, or when the child later moves around more on the mattress, the fleece is compressed across the entire cover, and the "depression" disappears again. This compression of the cover does not represent a disadvantage for your child.