Baby or belly?

Strangest thing. Since I've been pregnant, but before I knew, JJ has been pointing at my belly and putting his ear on it like he's listening for something. At first I thought "oh gosh, I've gotten fat enough he's looking at it like another breast", but that's not it. He keeps pointing at it and then smiling and putting his ear to it. Then three of days ago, he does this same thing and says "Baby." Could've knocked me over with a feather! Now, we've talked about the pregnancy in front of him and told him he's going to be a big brother, but I don't really think he understands or has any means of comprehension for such an abstract concept at this point. But here he is saying "baby." Then I thought, maybe he is trying to say "Belly" instead. That would make a lot more sense. Two days ago he did it again, except with John in the room. John's jaw dropped - "Did he just say 'baby'?" So, maybe he did. How would he know?