It is not only safe to practice yoga while pregnant, but also highly beneficial. Some caution and modifications must be heeded; however, you would be surprised how many postures you can still practice while pregnant.
The best rule of thumb I have found: LISTEN TO YOUR BODY. Do what postures feel good and avoid those that do not.
I am currently 6 and a half months pregnant and have been practicing yoga since Day 1 of my pregnancy. I have a 10-year-plus yoga practice, so continuing to practice yoga seemed natural once I became pregnant.
Not only have I been practicing yoga regularly while pregnant, I also completed a 200 Hour Yoga Teacher Training while 3 months pregnant. Since then, I have been teaching Vinyasa Flow Yoga anywhere from 3 to 6 classes a week (and am LOVING it!)
Some of my favorite pregnancy postures are: pigeon (to open up hips); happy baby (another excellent hip opener); lunges (can't get enough hip openers); feet up the wall and hero postures (both of which increase circulation to tired legs and knees~especially important during the second and third trimesters).
Feel free to email me with any questions regarding my journey through yoga and pregnancy at firstname.lastname@example.org