A mare should come into "foal heat" between 1-12 days post foaling. Even though a wide range of 1-50 days post-foaling has been observed, many mares are in estrus, or in season, on the seventh to ninth day. The foal heat period may be as long as 12 days, but normally it is approximately 3 days in length. As the average length of gestation ranges from 335-340 days, breeding during a mare's foal heat ensures that if a foal is conceived, it will be born approximately the same time as its sibling - only one year later. Many breeders feel that mares are more fertile during their foal-heat than other periods of estrus.