I am also an avid yoga practitioner (am now finishing my second trimester) and based on my experience, find that you should avoid postures that put pressure on the tummy such as bow pose and similar poses where you will be lying on your stomach. Also, you should avoid the deep twist poses. These poses you will find difficult to do anyways once your bump gets bigger. Also avoid poses where you will be lose your balance easily and might fall and some of the intense breathing exercises where you use your abdomin. The best solution is to let your teacher know you are pregnant and he/she should be able to tell you which poses and breathing exercises to avoid. Also, I read an article that you shouldn't do jumps (done mostly in ashtanga/power yogas) as it may potentially dislodge the egg in early pregnancy. Hope this helps.