We did one at 34 weeks or so during my second pregnancy. DH is not artistic at ALL and isn't particularly sentimental, so I wasn't even sure he'd be willing to help me (I had actually asked a friend if she might help me do it!)...but one evening he said he was willing. He did a great job! He casted my entire belly and chest, making the top look like a tank top up onto my shoulders. I don't remember it being uncomfortable - I just sat on a stool in the kitchen with a plastic drop cloth on the floor under where we were working. I do remember the vasoline being really hard to wash off afterwards, though - I wonder if olive oil or something like that would work better?? Anyway, it turned out great...when Noah was born I put his little footprints right on the belly.
I meant to get it out and try it on during this last pregnancy, but I forgot!