C.W.
I only have two boys, but I think if it was me, I'd put the "big brother" shirt on my second child and send that picture. I thought of sandwich kind of shirt, to show he'd be in the middle,but I don't know how it would work out. I also once saw a shirt on an obviously pregnant woman that had a baby drawn on the shirt, and it read "Coming soon to a hospital near you." It was hand made, actually, maybe you could put a pillow under your shirt if it's big and actually wear it later on, or maybe just make a little baby in a smaller shirt that you can wear now.
Just an idea on how I'd do it if I was to have another baby. Good luck and congratulations!