It's really a personal decision between yourself and your doctor.
I rode until 6 months with my last baby - but that was because I didnt know I was expecting!! I stopped when I found out... because quite frankly I developed my bump overnight and with the shock of finding out - my whole body went through a huge amount of change very quickly.
My first pregnancy I stopped right away - I didn't have my own horse and was riding TBs and breakers.... I didnt have a horse that I would trust so stopped.
In between the two I had a baby that I lost... oddly I stopped riding very early on after getting a huge amount of pressure from those around me... to this day I find it amazing the contrast between that and my little boy who is strong, healthy and full of life and I was riding regularly for so long.