I bought the small/6 month size for my 6 month-old boy, who is skinny according to those baby charts, and it barely fits.  I'm sure we will need the next size up in a month.  So, if you have a 50th percentile or better baby I'd say size up.  We haven't had any pool poopies yet so I can't say if it really 'works' or not but so far it seems to do a decent job as a diaper for the pool.