The last month that I rode her, I pretty much stayed at a walk. I did do alot of hill climbing tho. I figured horses were alot like humans. If it is something they are use to doing it's OK. Belle was in extremely good shape. The was so tight and fit that she never really looked all that pregnant even at the end. It wasn't till the last few weeks that you knew she was pregnant.
Becky
I agree, to a very large extent... And pregnant mares will run and play some of their own volition, I'd just be really careful to make sure they really want to go, and not push them beyond what they will do eagerly.
Being in tune to how they are feeling is key, with any horse really, but especially late in pregnancy.