In the beginning it shouldn't matter much. I'm not 100% on what the verdict is of mares potentially slipping a foal in hard work at the beginning, but fairly normal exercise for typically the first half of the pregnancy is alright. Once she starts getting bigger, she's obviously dealing with a lot more strain and you should adjust your rides accordingly. But yes, mares can be ridden lightly right up to pregnancy. In fact, it's highly advised to keep them in work as a fit and conditioned mare foals 10x easier then a mare who's been allowed to go soft and fat.