The moment the pregnancy is confirmed is the moment to stop laying on your belly. The information given for the first trimester is faulty.
I am almost 27 weeks in a few days and I am naturally flexible and have been all my life, so I feel that some things I am able to do have no strain on me or barely anymore than originally did, however, many women may feel that so if they feel such strain or pressure, they should listen to their bodies. I am able to do a backbend but it's not in my routine at all, however I was curious to see if it was possible without discomfort and it really wasn't bad at all but it's not something I will do often. However, it does help my back. The weird part is I can't bend forward very well, that is what feels uncomfy and I can't breathe well. So listening to your body, I believe, is the best thing you can do, but don't overdo anything. Just sayin!
At 8 months, Urdva-danurasana still feels blissful (it has been part of my practice for a long time), but then this pregnancy I haven't experienced the common complaint about lying on one's back--unless acid reflux decides to make a visit, then all bets are off. After closed twists, I found I had to give up closed-leg forward folds (too squishy).
Everyday is a new adventure as to where my body is. I've learned to trust it.
this is bullshit! pregnancy is not the time to be backbending or pushing your body at all. go to prenatal class and honor your body! it really pisses me off that yoga teachers who are not educated say it is ok to do all the advanced poses during their pregnancy. any prenatal teacher with proper skill and education will tell you to avoid excess pressure on the abdomen. Geeta Iyengar is a master. Read her book and do not listen to the half assed info received from novices.
I love this advice--I am 8 months pregnant and still practicing regularly 2-4 times per week. I've enjoyed modifying my practice in the comfort of a competent teacher who doesn't mind at all that mid-way through the class I may or may not be doing what he is instructing. My practice continues to feel wonderful, but only through listening to what my body needs.