i was a nanny for a professional horse trainer...HER doctor said she could ride until 36 weeks....but not after....it all depends on what your dr thinks, and how far along you are.....
I don't know what the doctor's say, but my best friend seriously hurt herself at 5 months pregnant. She went with her older daughter, and it was just supposed to be a walk, you know a little scenic thing. Well the horse all of a sudden bolted and bucked her and her 6 year old daughter off. She ended up fracturing her hip
I heard you aren't supposed to after 36 weeks but my Mom's friend rode all through her pregnancy I think you mite be aloud to but ask the doc he always know's
If you are an amateur rider, I'd say HECK NO! Because what if the horse bucked you or ran with you and you lost control of it?
But if you are an experienced rider, or even own your own horse(s) then it's worth asking your doctor. They may give you a limit for how far along you could be to ride. Personally I think it would be really difficult to ride, much less straddle the horse comfortably.