Gerber tends to run small, so knowing that, I ordered a size up for my 17 month old (size 24 months), but these seem to be much more true to size than, say, Gerber onesies which are notoriously small. That said, I like them and they were a great price. They do NOT have that little snap over the top of the zipper like in the picture. I wish they did - my son can unzip them.