My awesome buddy the Balloon Sage has a great tutorial out for how to make a pregnant dog.
Warning! Making these are addicting!
I am pretty sure there are secret 12 step groups just for people who are too fascinated with making these things.
It is just so cool- and I am always surprised when I manage to make one,lol.
Here is a link to his tutorial, http://www.youtube.com/watch?v=VpOuxdGZkyQ.
He makes one first and then walks you through it as he makes it a second time.
Pictures of my replication below.