I bought two sizes ultimately, I purchased a 6-12 mo aqua diaper for my kiddo when I knew we'd be swimming around 11 months. At the time it fit fine, but a little small. I figured since he was skinny I'd go smaller... Not so much. I purchased 12-18 months next and that fit perfectly but not large. He's wearing size 4 disposable diapers. I'm a little concerned that it won't fit much longer. He's only 50% for weight but 85% for height. Will see. I'd recommend sizing up if you're in between sizes.

I really quite enjoyed the diaper though, it stayed put beautifully and didn't seem uncomfortable but at the same time very snug. I went back and forth between this and an adjustable size diaper but those just seemed to cut into his legs and snap oddly. I'll buy next size up when it's time!

Caveat: We didn't have to deal with any poo, I know a lot of reviewers have and said it went fine...so I'm hoping for the same!