I had a similar experience at yoga the last time I was pg. I told the instructor prior to the class and she would just either give a little shake of her head not to do something, or would just say that if there were any pg ladies, that they should not do the following stretches.

Most of the time, people are on their backs or with eyes closed in yoga, so no-one knew who was and wasn't doing things. At any rate, there are a lot of people that do pilates and yoga for back probs that can't do a lot of the movements.

Hayseed