My mom rode her horses while pregnant with me. Her doctor told her to ride until it was uncomfortable. I do belive there is a connection. I love to ride horses. Whats interesting is that she rode while pregnant with my oldest sister, and me. We both love to ride. She dednt ride with my other older sister because it was a touchy pregnancie and she didnt ride with my younger sister because my oldest sister was always on the horse!!!
anyway, those two don't like horses. I belive theres a connection.
My opinion would be to ask your doctor and really think about wether or not your horse is "safe". I would ride my gelding pregnant because I ca truly trust him. I wouldnt even think about riding my mare though, she is too unpredictable.
Just my thoughts