I have just remembered two incidents.
A cousin if mine had had several miscarriages and had trouble conceiving.
She started riding when on holiday with us and fell in love with it so continued when she got home.
She was 6 months pregnant before she knew she was and swears that it was because of the riding. I know that she stopped when it became uncomfortable. She has had three children with no problems and kept riding all she could.
The other was a friend who had two children and was having threats of a miscarriage with her third. She was climbing the wall with having to be careful and when she found the girl riding her husbands horse was allowing him to nap and get away with it, she started riding again and all the miscarriage symptoms disappeared.