Occaisonally a mare will give birth at a shorter period than 11 months long, which can be at 10.5 months. Anytime before that foals that are born are deemed premature. On average, 340 days but a healthy gestation can be as few as 315 days and as much as 370. The gestational period of a horse is 11 months. 11 to 12 months all deppeand on the mare I had a mare go to 13 month before dropping. + 5 others found this useful.