in my yoga class, and on tapes and books I've got, there's downward dog all over the place. Sometimes we do half dog agains the wall.
for downward dog we usually spread our feet wider to make more room for the belly.
also, inversions are great for pg, esp if you are trying to convince the baby to turn. but, if you arne't comfortable w/them before pg, be extra cautious.
also, in my more advanced prenatal classes, we do more lying poses, but if it's uncomfortable, put a blanket under one butt-cheeck to get the pressure off or put a blanket under your whole lower back b/c your back/hips may be sore. if it feels bad, your breath gets weird, or you get numb, definately stop right now
I've heard people say not to do twists, but I still see tons of twists, I think you just have to be gentile, don't twist too far.
Just don't over heat, and don't get out of breath. Not going to the same extent is a good idea, too. Since things are loser it's easier to cause strains, as well.
I find balancing poses really challenging, right now, but worthwhile.