Baby shower gift (followup question!)

I asked yesterday about making one of these sweaters as a baby shower gift. I really appreciate all your input! I'm going to make her one, but now I have another question lol. As I mentioned, the idea with this sweater is to get a bigger size and use it for their monthly photos and watch them grow into it until their 1 year photo (if they want to use it that way). Now that I'm shopping for them, I wanted to double check what size would be best. 9-12mo or 12-18mo. (Keep in mind, I don't have kids, so I'm not familiar with sizing and such lol)