Generally, the rule of thumb is that anything that the mare is doing before she gets pregnant, she can continue to do. For example, if she has been training and competing heavily she can continue. But, you should never start a new discipline or start heavily training for a discipline if she is not used to it.
What I generally do is do light exercise for my mares until the last three months. Then I may or may not do very light exercise the last three months. ie 15-20 minutes at a walk bareback. But, my mares are not in heavy competition, and I have some that are broodmares only. (but most can be ridden) It is perfectly fine to ride a healthy mare, but if she has had problems in the past slipping a foal, I wouldn't.
We've had pregnant mares ride on 12 mile trail rides, but, they were being ridden the entire time. We usually give them 2-3 months off after they foal, mostly because they are so worried about their foals they don't listen well. (totally understandable IMO) Good luck with your new baby!