Is it safe to ride a mare that is pregnant? Most vets agree it is fine to ride a mare that is pregnant until the last 90 days of her pregnancy, or until the horse's saddle no longer fits comfortably. It is fine to ride your pregnant mare; maintaining physical fitness is good for mare and foal. Pregnant mares should not be worked extremely hard, or overstressed, but there's no harm in moderate riding.
Riding a mare semi-regularly through her pregnancy, will mean she will have more muscle tone for delivery. Mares that are healthy and fit deliver easier and have healthier foals. Your main concern in riding a pregnant mare should be in not overworking her and also in saddle fit.
Some mares as their belly is grow heavy with pregnancy may find the girth very uncomfortable, and the tree of their saddle may no longer fit their widening body. During saddling and girthing up a mare for a ride, watch for these signs which may indicate a saddle no longer fitting: laying the years back, tail swishing, or biting. A mare being ridden with a saddle that does not fit might toss their head, hollow their back, or even try to bolt or buck. If your mare's saddle stops fitting towards the end of her pregnancy you'll either need to stop riding at that point, or just take your pregnant mare for easy bareback rides.
Most vets agree it's a good idea to give a mare at least six weeks off before her foal due date. However, in years past it would not be at all unusual to use a working mare right up to the point she started going into labor. Use discretion, and your personal understanding of what your horse is telling you to determine if your mare is safe to ride through the last trimester of her pregnancy.