Being pregnant is A LOT of work and is physically very hard on mares. Imagine having a 100lb person on her back 24/7. (the weight of the baby, placenta, amneiotic fluid, and extra blood weighs over 100lbs) Could she take that? If not, then I wouldn't breed her. If she could, then I would be very cautious and watch her. You wouldn't want to have to put her down and lose them both.