As I mentioned on another topic on this site.. For me, the back bending poses such as swan and seal became too much of a stretch on the front body at around 3 months pregnant with twins. Saddle with a bolster I enjoyed for longer, 1-3 minutes maximum though however, as from about 6/7 months pregnant lying on my back for more than 5 minutes was very uncomfy and still is (at 8 months).
I had heart burn (acid reflux) quite badly for the middle of my pregnancy, thankfully it seems to have eased a bit
Standing facing a wall with your feet about 2-3 ft away and walking your hands up the wall to stretch then your upper back and shoulders is credited with helping. I found that Vinyasa style practice was not suitable for me during this pregnancy, it exacerbated a lot of symptoms including heart burn. So I would say move slowly and gently and be carful not to led the head hang below the heart in a forward bend.. Try to keep it up even when lying flat, as this seemed to help me.
I'm afraid that just trying to eat more raw food as much as possible (fruits and veges) , nothing fried, no bread and staying away from acidic or cooked foods as much as possible seemed to be the best thing. I also found that this helped with morning sickness. Definitely staying away from processed foods as much as possible. When I ate badly, my sickness was much worse the next day.
Another thing worth mentioning is that during pregnancy my legs have become stiff very easily.. to avoid cramping etc try to keep the fluids moving, rotate ankles after coming out of yin poses, shake them out etc. The yin stretches for the legs have really helped prevent the stiffness, So I have been focusing more on the lower body than yin on the spine. I find Yang such as cat/ cow variations work well for keeping my spine mobile. I will return to the deeper yin backbends sometime after pregnancy I am sure...
As for forward folding and compressing the belly.. As Bernie says, it will get to a point where your belly is too big to allow such a thing. I have never 'hurt' myself by compressing my belly.. It's size means that I just can't forward fold in the same way as without it.. I just meet with it earlier. So taking the legs wide has become the only option. I think that using a chair is a good suggestion.
Cooling breath can help with feeling hot (I feel the same!) and gentle Yin stretches are cooling. Constipation; cat/ cow, side bends, gentle twists (a good one whilst preg is standing and letting your arms swing by your sides as you rotate left and right, like on Simon Low's Yin/ Yang DVD. In the YAng portion. Moving with the breath slowly, deep breathing with a long slow exhale, drinking lots of water and again, raw fruit and veg as much as possible. Urinating.. You can (and I reccommend that you) strengthen your pelvic floor muscles daily.. I know it may sound boring but I have found it really essential. That will hold it in, but you will still need to go when you have the chance!
Hope that this helps