I love this brand (ruffle butts = TOO STINKIN CUTE), but was a little disappointed that the bottoms were too small to accommodate a swim diaper underneath, and we even ordered up a size thinking it might be an issue (ordered 12-18mo size for our petite 9mo daughter). The rash guard top fit our daughter perfectly, though, so it's hard to say definitively that ordering up 2x is the way to go. I wish the smaller sizes (i.e. typically worn by babies still in diapers) factored in some extra room to wear a diaper underneath.