When my mare was in foal, I rode her on trail rides and showed her up until the last 2 months of her pregnancy. She was in a large pasture and was herded down by the gate each day. I would ride her (bareback) in from pasture to the ranch. Less than 1/2 mile. Based on normal gestation I had determined that I should stop doing any riding at all and led her back to pasture. She foaled early the next morning, 2 or 3 weeks early (maidens!!!) The vet said she was in perfect condition and that my riding her did not contribute to her foaling early. Her colt was full grown and perfectly healthy.

If my recollections seems vague... this was around 1971 LOL