I kind of let my mare tell me when riding was getting hard on her. I think I rode her up until about 1 1/2- 2 months before she foaled. And I quit because she started breathing hard (and she was huge) so I knew it was too much for her to carry me and the baby. Heck, before she foaled she would breath hard just standing around her pen.
But anyway, I would say you have a long way to go before you have to stop riding her. And let her be the judge. If she rides great and has lots of energy, she is probably okay. If she starts getting lethargic or breathing harder than she should, you should probably start taking it easy with her.
But I have only had one mare foal, so I am no expert. But I did try to research it on the Internet when my mare was pregnant, because she was my first pregnant mare and I didn't want to hurt her.
Here is a neat article I saved from when my mare was pregnant and I was wondering when and how much I could ride her: Riding pregnant mares - Balanced Rider Blog