When my first dc was born, I think I said one of those cheezy lines like “so you’re the one who’s been kicking me…”

. Then after a minute, when I had composed myself, but when the baby was still crying, I tried to comfort him by singing a song I had sung to him while pregnant. The song was “All through the night.” Not the cyndi lauper one, but the other one (“Sleep my love and peace attend thee all through the night. Guardian angels God will send thee, all through the night.” Etc…) It was a really sweet moment, and was actually a little startling to the midwife (who was sewing up my tear at the time) because from outside the birthing room, my mother (who had been with me during labor but was kicked out for the delivery) sang harmony through the door.
I think with the other 2 babies, my words were something like, “Hi baby! what a beautiful little baby you are.” And then, because they’ve all cried on my belly while the docs take care of business after the fact, I’ve simply tried to comfort them with “It’s ok. Mommy’s here.” etc.
I personally like the idea of saying or singing something that the baby has heard before (so it is familiar and possibly comforting) and something that you will repeat many times in your child’s life. So that one day you can tell your child, “you know how we always say ___ to you? Well, I’ve been saying that to you since you were in my belly, and it’s the first thing I said when you were born.” That’s kind of what I did with the song (not counting my cheezy comment).