Symptoms and Stages of Pregnancy in Horses

Some basics of equine reproduction and horse pregnancy include mating, the gestation period, and foaling. A mare (or female horse) can typically produce one viable foal per year. A mare is capable of producing a foal at about 18 months of age, but it's healthier if the mare is at least 4 years old, as she will have reached her full size. A mare may continue to have foals until she is in her late 20s.

Although horses can mate and give birth without the attention of a veterinarian, many problems can be circumvented with a veterinarian's help. The doctor can check the stallion before breeding and check and care for the mare properly during the gestation period.

Average Gestation Period

The gestation period in horses is typically between 330 and 345 days, or 11 months.1 Some mares will be inclined to foal earlier or later than the average, and breeders will get to know these tendencies. In a natural environment, the stallion will breed the mare in the summer, and foals will be born the next year, in spring or early summer. This ensures that the foals are born when pasture is abundant and the weather is mild.

Mares are considered seasonally poly-estrous, which means they go into heat (estrus) and are receptive to a stallion several times at regular periods during the spring and summer.2 These seasonal estrus cycles are approximately every three weeks. However, breeders who wish to manipulate the breeding cycle, so that 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 approximately two weeks after breeding.3 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 via rectal palpation. This can be done at approximately six weeks into the pregnancy and sometimes even earlier.

It's important to have the mare checked by a veterinarian early in the pregnancy to assess her health and the health of her foal. Horse twins are rare but can lead to spontaneous abortion. 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 the pregnancy. It's not unusual for a mare to lose a pregnancy, so it's recommended to do an ultrasound and test the blood or urine again after about three months.

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 abdomen sooner than a maiden mare. Over the remaining months, the mare's abdomen will continue to grow as the foal approaches the foaling, or due date. About three to six weeks before the due date, the mare's udder will start to expand, and a few days before giving birth, the teats would produce a sticky yellowish fluid.