Not sure, its different with every horse. There was a horse a rescue around here recently rescued, an older Arabian mare who was pregnant, very skinny, was being run off her food by the other horses, and the day after they trailered her to the foster home, she gave birth to a very healthy baby boy. You have to keep in mind, that in the wild there isn't any "special" foal food, or pregnant mare food, the only thing that a pregnant mare really does different, is just eat more. I wouldn't be too concerned, I would just wait and see what the ultra sound shows, and then talk with your vet about options ect. if she's pregnant.