Only use a thin knit blanket to cover her, and tuck it in at the edges of the mattress so it doesn't come loose and get wrapped around her. (dadsadventure.com)
As you sit on the edge of the mattress it should feel firm and should definitely fall away when you put weight on it. (thefashionablehousewife.com)
Re-enforced edging, which stops the edge of the mattress from caving down under your baby's body weight, can also weaken with time. (thespruce.com)