Does anyone know how mini/baby Boden runs, lets say compared to babygap? DD wears 12-18 months in babygap but can wear 18 month things too.

I want to order this swim outfit (http://m.bodenusa.com/en-US/Baby-0-3yrs-Swimwear/71271/Baby-0-3yrs-Surf-Suit.html) for our beach trip in may but the 12-18 is back ordered. Wondering if I can get away with next size up. I figured a swim diaper might fill up some space?