I am an instructor as well and while I am not teaching while pregnant (totally unrelated) I am still taking my normal yoga classes at 32 weeks pregnant. I would advise you not to take hot yoga or be in any heated room first and foremost. I always snag a spot by the window which they let me keep open to get some air. You really, really don’t want to overheat when you are prego, not good for you or baby. Take breaks and drink lots of water during your practice too. Most of the do-nots have been covered: inversions, deep twists, ab work, jumping back into chatarunga, deep backbends (though some are ok and feel great!) and pretty much anything on your stomach but here are some ideas for what to do when everyone else is doing those things:
1) During ab work I stand up and do squats to strengthen my legs.
2) During inversions, use a bolster/blankets and blocks under your spine (so you aren’t flat on your back) and put your legs up the wall. You want your body to look like a V. You get the benefits of going upside down without flipping over.
3) With twists, you can often open yourself up the opposite direction to keep the twist more open and focused on your spine rather than your pelvis (like in twisting triangle, if everyone is opening to the right, you do a baby twist to the left instead).
4) Instead of upward dog and cobra I just lower in a push up/chatarunga until just before the belly touches (knees on the floor at this point) and come back up and then back to downward dog. Works the arms and abs safely. If you really need the backbend that you can get in cobra, come up to your knees and gently do the same motion you would if you were on the floor being really careful not to bend too far back. Kinda like a mime, just pretend the floor is in front of you and act accordingly.
Also, really listen to your body, you will know what still feels good and appropriate for you and what is too much. Any good instructor will know quick modifications that you can make to still have a great class and reap all the benefits. Also, you have to be able to not worry about what anyone else might be thinking (ie – why is that chick doing squats when I am doing all this ab work) and just make your practice yours. That’s part of yoga and I’m not going to lie, it can be the biggest challenge when you feel all big and pregnant and you can’t do all the things you used to do so easily. Gotta love the human ego.
And I feel you on prenatal all being during the day, in my area the only class that I can fit in is Saturdays at noon (not so easy with summer weddings, etc) so it was really important to me to be able to maintain my regular practice as much as possible.
Feel free to PM me if you have questions about specific poses and other ways to modify your practice!