Congratulations on your pregnancy!!
I disagree with your teacher. I don't think cobra is a good idea. For one thing, it'll start to get uncomfortable. For another, it puts too much pressure on your abdominal area, and just isn't a good idea. Backbends in general aren't advised for pregnancy, and that's pretty standard thinking.
As far as shoulderstand, if you have practiced yoga for many years and are in good physical condition, Geeta Iyengar's book says it's okay for short periods. I say avoid it if you have any question or concern about it.
Also avoid: double leg lifts, boat, deep twists, camel, cobra, upward-dog, bow, bridge, and plow.
In the final weeks of pregnancy, you may want to avoid the warriors and triangle. The baby at that time is large and heavy, and wide-legged stances can put too much pressure on the pelvic area.
I would recommend getting a pregnancy DVD. Shiva Rea has a great one. You're right, it's not going to be "power" yoga, but it'll really help prepare you for labor and show some great modifications for as you get larger.