Looks like it depends.
Mares doing fast work or jumping might have problems if you continue to compete in the last portion of gestation. Robershaw points out that during the last trimester, the weight of the fetus might cause a uterine torsion if the mare is carrying it low and she’s doing active work, such as jumping. A twist in the cervix and connective support structures might occur if the heavy uterus flips over.
Callahan said some mares, especially maiden mares, don’t develop much belly and could probably be ridden up until they day they foal and you’d hardly know they are pregnant.