Hi Ashley. Since the RPE is a “subjective” measure that’s based on how a client ‘perceives’ the intensity of the exercise I’m guessing that with her pregnancy, her PERCEPTION of how intense a specific exercise is will change as her pregnancy progresses. This means that what she may perceive to have been a 6-8 BEFORE pregnancy is probably a higher intensity of what she perceives to be a 6-8 WHILE pregnant. That’ being said, this is exactly why (in my opinion) you should first discuss continued training with your client’s physician, and if the two of you decide (together) that using the subjective RPE is no longer appropriate, you may jointly decided to move to a more objective measure such as HR or something else.
I hope that this helps.