Mares can be pregnant for an entire year, but often will deliver their foals at 11 months. It's usually in the final three or four months of the pregnancy that the foal will show and the mare will be too encumbered by pregnancy to be ridden or exercised at all. According to "Blessed are the Brood Mares" (M. Phyllis Lose, VMD, 1978), mares can be exercised normally for their first six months.
Day 1 to 15
At this stage, according to "Horse Owner's Home Veterinary Handbook" (Thomas Gore, DVM, et al; 2008), the foal is referred to as a "conceptus." It drifts from the uterus to one of the uterine walls. It grows about four millimetres per day. The mare will not look as if anything is different. She may, however, refuse the advances of a stallion. It is still too early to do any testing to positively diagnose pregnancy.
Day 16 to 120
By day 16, the conceptus should have attached to the uterine wall and is now called a foetus. The earliest a pregnancy test can be done is 21 days after mating or artificial insemination. By this time, a vet can feel the foal and a gradual change in the shape of the mare's uterus through a rectal exam. To be absolutely sure, a vet will perform an ultrasound. By day 30, the mare may get moody and her appetite will begin to increase. Meanwhile, the foetus will become a tiny hairless horse weighing just over one pound, complete with hooves and genitalia.
Day 120 to 240
The foal's legs will grow longer and it will put on weight. The mare's activity should be gradually reduced. By day 100, hair will begin to grow around the lips. Day 150 to 180 is the time of most dramatic growth, as a foetus usually quadruples in weight in just 30 days. By day 180, the neck will have lengthened. The mane and tail will begin to grow.
Day 241 to 399
Mares will usually foal by day 365, but "Horse Owner's Home Veterinary Handbook" notes that mares can have healthy foals up to day 399. Any foal born before day 350 is classified as premature. Any foal born before day 300 will die. This is when the mare's abdomen will become enormous. Her appetite will increase along with her abdomen. By day 270, the foal will have mane, tail and body hair. By day 270, the mare's feed should be increased by 15 per cent to help the foal's growth spurt.
24 Hours Before Foaling
The mare will prefer to be by herself and avoid the company of people, horses and other animals. Her udder will bulge and milk may begin to drip from her teats. It may also dry to a golden coloured bead on the end of the teat. If the milk begins to stream out, it should be collected for the foal because it contains valuable antibodies.