This onesie is a game changer! It fits my son perfectly, and I did size up when I initially got it. I got the 9-12M size, and my son is now 10 M. It fits PERFECTLY! Anytime we go outside, whether it's the pool or my other kids' baseball games, I know he isn't sunburned from being outside all day. I don't have to worry about putting sunscreen all over now, which is great because he hates it when I do. It's comfortable, easy to clean, and, of course, ABORABLE. I'm waiting patiently for the baby bucket hats because that is my next purchase 😂