While you don't want them to get fat, you also want to be sure to feed enough to keep them in condition. So, are you feeding at least 1.5-2% of her bodyweight in forage? 250# mini that would be 3.75-5# forage daily, more when it's cold and as pregnancy progresses. adding a commercial feed of some sort as needed? If she tends toward easy keeping, a ration balancer is a great choice for most minis; maintenance is about 1/4-1/3# depending on size of mini and particular ration balancer, double it for late pregnancy and early lactation.