The standard mare gestation length is from 320 to 362 days after the date of breeding, and on average, most mares will foal, or give birth, 330 to 345 days after a successful breeding. However, it is common for a mare to successfully give birth in a later period than the average range.
Mares have their own individual gestation length tendencies. A pony will likely have a shorter gestation period than a full-size horse, but there can be exceptions. As a rule of thumb, ponies should be watched closely around day 300, as they commonly foal after 320 days. Gestation periods are also different in other equids, like donkeys, which gestate for 360 to 375 days. On average, donkeys gestate for 368 days. Goats gestate on average for 140 days.
Research has also tested the average gestation period range to see if certain variations can exist based upon various factors. For example, gestation periods of standardbred horses have been found to be longer when they occur earlier in the breeding season and much shorter when they occur more toward the end. Standardbred mares tend to have gestation periods of around 347 days, and these periods can fluctuate based upon the aforementioned factors.