I have an 11 month old named Arthur. One of the reasons we choose the name is because we liked the full name, as well as the potential nicknames Art and Artie.
We've always called him Arthur...in the beginning I think it was to get used to his name (instead of saying "the baby"), and also so he would learn his own name. I know people that start using their LOs nickname right away, but to me, using his nickname just feels odd right now. Is this something that might naturally happen when he's older, or will it just get more difficult to start calling him by a nickname.