The average gestation period for horses (including miniatures) is 340 days, give or take ten days. In warm weather, the mare will likely foal on the early side, whilst in a cold climate, she could carry a little longer.
Here’s a handy Horse Due Date Calculator to help you estimate when your mare is due. (Or you can try our 340-Day Horse Gestation Tables)
Please note, this is just an estimate (even, “guesstimate”) of your mare’s due date. Don’t rely on it, as a mare may foal early or late regardless of the time of year. Keep an eye on the mare at least 10 days before she’s due to foal and talk to your vet or equine expert, as your mare’s circumstances (like her age and the number of foals she’s carrying) may be different, or there may be medical complications.