I would venture to say it's a case by case personal decision. What shape are you in to start? Previous difficulties? What kind of riding? Is your horse a Pokey Polly or Spooky Spunky? Exercise while pregnant is very important - otherwise it's like saying rest up for the big game - for nine months! LOL
I am fully convinced, however, that vacuuming and doing the dishes from 60 days after conception is high risk behaviour that needs to be stopped by all concerned Mommas-to-be. Hmmmm, maybe I could sponsor a ride-a-thon to help spread the word..... need a lot of mounting blocks by port-a-johnnies for all the preggo bladders ....geeze, what a nightmare.