I remember my water broke on my mattress lol talk about funny. woke up one more sat up in bed then splash hahah. I just cleaned it once we got back from hospital. but yes put the zipper plastic mattress cover on like @karina809 said. they sell them at Walmart

there's a plastic cover they sell... with zipper... then buy a soft layer to put over if you don't like the plastic feel

Girl or you can put down an old blanket and fold it under you too... That's what I did with my first daughter.