I bought this for my 8 month old son to wear in the pool.  I bought the large size, for 18 month olds because that is the size he wears normally, and it fits perfectly.  The legs and waist are snug elastic, and they do leave a mark, but I expect them to be snug to contain waste.  He hasn't pooped in them, and I can imagine that it would be a pain to take off and clean, but I think it would contain waste in the pool  better than disposables!!!  He has worn them alone and under a bathing suit and it works and looks great.  I'm sure these will fit him for a year of pool use.  I would DEFINITELY buy these again.