Unless a mare has been bred in the pasture, the exact breeding dates are usually known, writes Dr. Nancy S. Loving in the December edition of The Horse
"Normal gestation length in a mare ranges from 335-345 days," says Dr. Lisa Metcalf, a board-certified reproductive specialist in Oregon. "Foals delivered prior to 325 days gestation are considered premature. If a foal is delivered prior to 300 days gestation, it is rarely compatible with life, even with the support of our advanced neonatal intensive care units."
According to Metcalf, it is suspected that fillies are carried two days longer than colts on average, although in general, there is little difference in gestation length based on fetal gender.
Regarding whether a mare tends to follow a gestation length comparable to previous pregnancies, Metcalf says, "Normal gestation lengths can vary by two weeks in the same mare." According to Dr. Liz Scott of the Idaho Equine Hospital, "Mares are usually true to themselves, following a similar pattern in each pregnancy."
Mammary development is a key feature that correlates with a mare's hormonal changes associated with impending foaling. Metcalf says udder development usually begins four to six weeks before foaling, although maiden mares may not show mammary development until just before parturition.
"The mammary secretions normally turn from a sticky, serous secretion to a watery, 'skim-milk' consistency, to a thicker, somewhat yellowish colostrum just before foaling," she says.
A month before foaling, the mare's abdomen will enlarge and become more pendulous (hanging downward), according to Metcalf.