Although I wouldn't compete in a CDE with a pregnant mare as there is too much risk of her getting hurt in an accident what I think is pretty funny about this rule is that the first trimester is when the mare is most likely to lose a foal! I also wouldn't begin a mare driving when she is pregnant but if she has been driving all along and she is gotten pregnant there is really nothing wrong at all with continuing to drive her as long as she is comfortable and you take her condition in consideration. I drove my mare when she was pregnant with her first foal until 3 weeks before she was due. We of course slowed down quite a bit towards the end, did a lot more walking, but the only reason I quit driving her was I couldn't get the girth done up any longer. She was in terrific condition and foaled a lot easier than most maiden mares. She also got her figure back immediately and was back driving when the foal was only a couple months old. This rule is such old thinking, along the lines of pregnant women not being allowed to exercise. The main reason I wouldn't take a pregnant mare to any show would be the risk of infection and the hardship of trailering a pregnant mare.