I bought 12-18mo for my 13mo baby boy who is 75th percentile in both height and weight, and the size is perfect. It is a little loose but that is what I was looking for since it is still May and we can use it all summer for the fast growing baby.

My only concern was the neckline. My baby has a big head and I was worried his head might not go through it. But the material is stretchy enough that he has no problem putting his head through. Overall, this is a high quality great product.