The earliest live foal we have had here was 286 days gestation, and the latest was 363 days.
Some of these early foals are truly premature foals, showing classic signs of dismaturity; short velvet coats, low birth weight, needing a bit more time to learn to get around, and sleeping longer than a full term birth. These foals often require an additional heat source and foal blankets to maintain their heat.
Some miniature mares do not get full, hard bags until after they foal.
Mares will show their discomfort by kicking at their bellies and rubbing their tail, and increase in food and water, can also be an indication of their getting ready to foal.
We have seen mares bag up quickly and foal in a few hours. Rarely do ours wax, though we love it when they do.
Past history is great, however mine do not read it no matter how detailed I record it, and seem to have their own ideas about when to foal.
Our Breeder Alert paging system, combined with Mare Stare, has saved more than one foal here.