I am a student nurse. I just started this fall, so my knowledge in obstetrics is quite limited. I'd say both are equally dangerous. Assuming she falls off a horse, it can cause injuries. And in more serious cases, one of her feet is stuck on the pedal while the horse is running, effectively just dragging her. However, in either instance, I don't think falling would cause that damage to an embryo/fetus unless it causes a major injury in the mother.
Could be as well. I don't think Melissa was pregnant when Haley was announced.