Hello - I posted the same question on the horse and hound website when i was P with DD1 (knew I'd get reasonable pro riding answers). I rode 1st Pregnancy until 4 months, horse was a little unpredictable and I got thrown off at 4 months at high speed, luckily all was fine. That experience scared me and with my 2nd pregnancy I didn't ride, I was really careful and all went horribly wrong at 17 weeks, and that was that.
My 3rd P I rode until 5 months. I made the choice that what will be will be, and my body was well used to it. My horse is a tall boy, and spooks and larks about, but not nasty, or in the habit of dumping me. I cantered for the last time at 3 months though, and from then went on walking hacks. I mucked out 2 stables every day until 9 months. My doc said this would all do me good as it would keep me fit.
My 1st labour was 6 hours after being induced, 2nd was 1.5hrs and I felt as fit and strong as ever - i think this helped me deliver, and cope in labour as i felt so physically capable. If it hadn't have been for breast feeding I would have been back on the next day.
You have to trust the horse, and accept that IF you came off, you could cope with the thought COULD you have prevented it. Also, riding and doing exercise is good for you in pregnancy, and riding was the onlyt hing that stopped my morning sickness - I would feel bad, then the second I was on board, I felt fine - weird.
You will know when it is right to stop, i pulled a ligament in my lower abdomen and it hurt! That's when I stopped. Good lcuk and enjoy your quiet rides and your pregnancy