Horses are mammals, and like all mammals, give birth to live offspring who are nourished for the first part of their life by their mother's milk. A mare (a female horse) can only produce one foal per year. A mare is capable of producing a foal at about 18 months of age, but it is healthier for mare and foal if the mare is at least four years old, as by this time, the mare has reached her full size. A mare may continue having foals until she is in her late twenties. A stallion (a male horse) may continue breeding mares into his twenties as well, although the quality of his sperm may decline as he ages.
Foals can stand about 30 minutes after birth. They may nibble grass, concentrate or hay within a few days after birth, although their mother’s milk will be the main source of nutrition. They may be weaned from their mothers as early as three months after birth, although many breeders choose to leave mares and foals together longer. Although feral horses mate and give birth without the attention of a veterinarian, many problems can be circumvented by having the stallion checked before breeding, and mare checked and cared for properly during the gestation period.
Average Gestation Period
The gestation period in horses is typically between 330 and 345 days, or 11 months. Some mares will be inclined to foal earlier or later than the average, and breeders will get to know these tendencies. Ponies usually have a shorter gestation period than horses. In a natural environment, the stallion will breed the mare in the summer, and foals will be born the next year, in spring and early summer. This ensures that the foals are born when pasture is abundant and the weather is mild.
Mares are considered seasonally polyestrus, which means they go into heat (estrus) and are receptive to a stallion at regular periods during the spring and summer. These seasonal estrus cycles are approximately every three weeks. However, breeders who wish to manipulate the breeding cycle, so foals are born earlier in the year (commonly done in the Thoroughbred racehorse industry) will use artificial lighting to simulate the longer days of spring and summer. The artificial daylight stimulates the mare's brain to produce the reproductive hormones needed to induce estrus. This allows mares to be bred earlier and in turn have a foal earlier the following year.
Checking For Pregnancy
Beyond the absence of an estrus cycle, mares may not show any visible signs of pregnancy for the first three months. Pregnancy can be confirmed by ultrasound after approximately two weeks after the breeding took place. Blood and urine testing can be done two to three months after conception. Alternatively, a veterinarian may be able to manually feel the small embryo in the mare's uterus approximately six weeks into the pregnancy via rectal palpation.
It's important to have the mare checked by a veterinarian early in the pregnancy for her health and the health of her foal. Horse twins are rare but can lead to the mare aborting. If the twin foals are carried to term, there is a possibility of losing both. For this reason, it's often recommended to "pinch off" one embryo. This is done very early in pregnancy. It's not unusual for a mare to lose a pregnancy, so it's recommended to ultrasound, blood or urine test again after about three months. Things like checking how a mare shakes her head, the look in her eyes or which way a needle moves when held over her belly are not accurate methods of determining if she is in foal.
Later Stages of Gestation
After about three months the foal will be developing rapidly and start to look like a small horse. After about six months, the mare may start to be visibly pregnant. Mares that have foaled before may show an expanding belly sooner than a maiden mare. Over the remaining months, the mare's belly will continue to grow as the foal approaches its foaling date. About two weeks before the due date, the mare's udder will start to expand and start producing sticky yellowish fluid.
After about 315 days of pregnancy, an owner should watch the mare closely for impending signs of foaling. For example, the yellowish fluid will turn into the first milk or colostrum. The udder may drip, and the muscles around her tail head will become more relaxed. Her belly may appear to drop, as the foal positions for birth. At this point, birth is imminent, and the mare must be checked frequently for signs of foaling. Shortly before birth the mare will appear restless, may paw the ground and check her sides (similar to colic symptoms). She should be stalled in a large, clean stall, preferably bedded with straw. The mare may lie down and get up repeatedly, but will give birth lying down. First, the amniotic sac may be visible, and then the foal's front hooves and nose. The foal is normally birthed within a few minutes at this stage.
Occasionally, a foal is 'breech' or born hindquarters first, or one or both forelimbs may be bent back. Sometimes the mare or foal is injured during the birth process or has other issues that require professional attention. Your veterinarian should check mare and foal carefully shortly after the foal has arrived.
The Birth Of A Foal: What We Look For And What We Do. University of Pennsylvania, School of Veterinary Medicine
Equine Reproduction From Conception To Birth. American Association of Equine Practitioners
Reproductive Cycle In Horses - Management And Nutrition - Veterinary Manual. Veterinary Manual, 2020
Pregnancy Determination In Horses. Veterinary Manual
The Expectant Mare and Foaling. Canberra Equine Hospital, 2020