You don't need to stop riding your mare just because she's in foal.
In fact, exercise can be good for pregnant mares. It just has to be done correctly.
Exercise, even strenuous exercise, is not usually harmful during the first part of the pregnancy. While you should not start a mare on a strenuous training regimen after being bred, a mare that has already been conditioned should have no problem continuing her routine for most of her pregnancy.
As Suzan Phillips, a breeder and trainer near Helena, Montana, said in an interview with The Horse, a mare that receives regular exercise is in better shape to foal.
"It's like a pregnant woman," Phillips said. "If you are always exercising, there's no reason to stop just because you are pregnant. A mare should be able to keep doing whatever she's been doing."
There are a few cautions to keep in mind, though. Don't ride or otherwise exercise your mare to the point where she gets overheated, and avoid stressful situations, such as trailering, during the first two months of the pregnancy, as both dehydration and stress can harm the fetus. Finally, you may want to back off the mare's training around the eight-month mark, as it is during the later part of the pregnancy that the foal does most of its growing.
With these guidelines, there is no reason you have to give up riding just because your mare is in foal. You can have the best of both worlds -- a horse to ride, and the excitement of a new foal!