Gestation in mares is estimated to last about 340 days, but this is just an average, since mares often foal as much as three weeks earlier or later than this standard time. Mares are notoriously unpredictable, and this is why horse breeders often try to figure out ways to more accurately pinpoint the time of foaling so they can be present at the birth.

Pete Sheerin, DVM, Dipl. ACT (reproduction specialist), of Rood & Riddle Equine Hospital in Lexington, Ky., says typically breeders figure an estimated foaling date using the mare’s last ovulation or breeding date. This just gives a starting point; breeders need to be aware that the mare could foal several weeks ahead of the projected due date. When examining the mare for signs of impending foaling, things they generally look for include udder development, relaxation of the vulva, loosening of the ligaments on each side of the tailhead, and waxing at the ends of the teats.

These changes might take place within just a few days of foaling or transpire over several weeks. A mare can bag up three to four weeks before foaling and might even leak milk for several days before she actually foals. “This is the frustrating thing about foaling one or two mares at home and trying to watch them,” says Sheerin. “The important thing is to know your mare. If it’s a mare you’ve foaled out before, she will usually have a pattern. Length of gestation in an individual mare is often similar with each subsequent foaling.

“Maiden mares are tougher to predict, not only because of the variability of when they might foal, but they also may not build an udder as quickly as a mare that’s had foals before,” cautions Sheerin. A maiden mare might suddenly bag up and foal without much warning, and some might not produce milk until after they foal