We used dental alginate, reinforced with plaster bandages, to make the belly casts. Here I am at 34 weeks covered in minty fresh (and COLD) dental alginate.
Here you can see the dental alginate, with cotton applied. The cotton will help the plaster bandages to stick to the alginate.
There are some more photos here of the making of the 34 weeks belly cast.
Finished Product: 34 Weeks Belly Cast
There were a few imperfections (possibly due to the burlap having been chemically treated), so we decided to do another.
Here's the 38 Weeks Belly Cast (MUCH bigger!)