My son was 3 months old, I want to say about 12 lbs at the time, but has really chunky legs and these fit very well on him. He didn't have any accidents that I recall, so I can't really speak on their ability to hold in #2.  Although when he was sitting with my wife at the small wading pool of the indoor water park we were at she said she noticed the water get a little warm, so they moved to a different spot. :-)