The snaps are the key to this being a great swim diaper. Who wants to pull a swim diaper down your child's legs that is full of poop?! This swim diaper is easy to get on and off and fits true to size. See the iplay website or other reviews for the sizing. We ordered the medium (18-22 lbs) which is a 12-month size and fits my son who is 20.5 pounds. We have not had to test its ability to hold in the poop yet but we love it so far. It fits well under my son's swim suit. I love that you can toss it in the wash. I am going to order another one since we plan on swimming everyday this summer.