Once a mare is bred, her body begins the 11-month process of gestation in preparation for the birth of a healthy foal. For the first two-thirds of her pregnancy, her body size may not change noticeably but the foal is busy developing within her womb. Anticipating the birth of a foal is an exciting and educational time for everyone involved.
The approximate gestation period in horses is 340 days or 11 months. However, unlike many animals that give birth relatively close to their due dates, in horses, the gestation period is only a guideline and they may give birth as early as 315 days or as late as 370 and still be considered normal.
Sometimes, the most exciting stage in your mare's pregnancy is the anticipation of learning whether the breeding resulted in a pregnant mare. Because a mare retains hormones from her estral cycle for weeks, a urine test for pregnancy is unreliable during the first two months. Veterinarians may perform a urine or blood test between 60 days and 100 days. Ultrasound, performed on your mare will confirm a pregnancy within four weeks but most vets do not carry portable units. You may order a home mare pregnancy test that will give you an accurate reading after 110 days. (See Resources)
The first 100 days of your mare's pregnancy will not produce noticeable physical changes. However, the foal is undergoing rapid development. At 30 weeks gestation, the foal embryo is about 1--inch long with tiny bumps that will develop into legs. At 60 days, the embryo is still very small, less than 3-inches long but it has a definable body with legs and a tail. At 100 days, the foal is 7-inches long, its main features are now formed and hair is beginning to develop.
From 100 to 250 days, the mare enters the middle stage of her pregnancy. At 100 days, veterinarians suggest another pregnancy test, because nearly one-third of all mares "slip" their pregnancy in the Early Term without their owner's knowledge. At 150 days, the foal weighs 0.907 Kilogram and his extremities are fully formed, including tiny hooves. Now, the foal begins an increased growth rate. By 180 days, he weighs nearly 4.54 Kilogram. At the end of the Mid Term, the foal is the size of a small lamb and he has whiskers and eyelids that he blinks. By the end of this period, the mare shows noticeable abdominal weight gain.
After 250 days gestation, the foal continues to gain weight rapidly and his lungs develop, preparing him for life in the outside world. Now the mare begins to experience additional changes. By 300 days, the mare's udder will distend slightly and around 315 days, the udder may produce a sticky yellow discharge that will turn to milk within two weeks of foaling. Her abdomen will grow heavy and her vulva will relax and lengthen within a week of foaling.
By day 315, the mare's owner should be prepared for foaling although it may not occur for as long as 55 more days. Now, observing the mare on a daily basis is the best indication of impending birth. Within one week of birth, the foal will "drop," settling lower in the mare's belly. Her hind end relaxes during the final two weeks and it will appear lower in proximity to her tail. When the mare's vulva relaxes and elongates, foaling is imminent.