Don’t move them to a different barn/pasture when they are in foal. Especially not during the last 3 months as they develop antibodies protecting the foal in the environment the mare is kept in. We always let our mares give birth in the field as the weather is nice enough during spring/summer in the Netherlands. Also, the field is a clean, non slippery environment in which a horse is naturally at its best.
No stressors, plenty of food and a comfortable spot to lay down once her belly gets heavy enough that she wants off her feet. No places where she could slip or get cast