Horses are unique in that they almost never have any birthing problems. If the foal gets too big in the mare then she will get rid of it. I read about a study at a university where shetland mares were inseminated with draft semen. Every mare foaled a healthy colt that grew like a weed but stayed nicely healthy. Don't worry about your mare.
There's nothing like the Rockies in the springtime... Nothing like the freedom in the air... And there ain't nothing better than draggin calves to the fire and there's nothing like the smell of burning hair. -Brenn Hill