Gestation Stages of the Mare

As discussed several times in this space, mares seasonally polyestrus, which means that their estrous cycles depend on the season. A mare will undergo regular estrus cycles during late Spring, Summer and early Fall, and none during Winter. It is largely accepted that this is a survival mechanism which came about in order to prevent the arrival of foals during the inclement weather of Winter.

The timing of the mare’s estrous cycle governs when she is able to conceive. Common terminology referencing the estrous cycle is as follows:

• Estrous refers to the entire cycle
• Estrus refers to the "heat" stage of that cycle when the mare is receptive to the stallion's advances
• Diestrus refers to the period in between the estrus phases when the mare is not receptive to the stallion
• Anestrus refers to the complete absence of estrus¹

The normal estrous cycle in the mare is 21 to 22 days long, as defined by the intervals between ovulation. The duration of estrus, however can vary, so the most consistent period in the estrous cycle is the length of diestrus.² Given the close ties between the natural light cycle and that of the mare, horse breeders and owners wishing to breed their mares have long employed manipulation of the light cycle in order to coordinate when their mares come into cycle for convenience purposes in breeding programs. Many breeders of performance horses use artificial light to stimulate the longer days of Spring and Summer. This causes the mare to go into heat earlier, which allows the foal to be born earlier in the year, an advantage for the owners and managers of performance breeds.

Stages of Gestation in the Mare

Gestation is the time span from conception to birth. For the horse, it averages 340 days, but may range from 320 to 370 days; mares have even been known to deliver a healthy foal after 399 days. If a foal is born before 315 days, it is considered premature; if it is born prior to 300 days, it is usually too young to survive.⁴ Most pregnancies and deliveries however, come off without any complications. If a mare fails to show heat at 18 to 22 days after service, it is often because of pregnancy. The veterinarian can diagnose pregnancy using rectal palpation as early as 20 to 30 days into gestation, and transrectal palpation between days 40 to 50 is typically 95% accurate.^1,3

Mares go through three trimesters during their gestation. The first trimester begins with conception and is generally confirmed at two weeks.³ At around 25 days, the veterinarian can perform an ultrasound that can detect the foal’s heartbeat and confirm vitality.

Equine Gestation Timeline

First Trimester

• Day 0: Date of Ovulation
• Days 14-16: Initial pregnancy ultrasound and evaluate for twins
• Day 18: Additional ultrasound examination may be needed for evaluation of twins
• Days 25-30: Second pregnancy ultrasound (heartbeat), evaluate for twins
• Days 45-60: Third pregnancy examination with ultrasound to confirm fetal heartbeat and viability of pregnancy
• Days 60-90: First deworming for mare, fetal sexing can be performed via ultrasound examination

Second Trimester

• Day 114: Second trimester begins
• Days 120-150: Fetal sexing can be performed via transabdominal ultrasound examination
• Day 150: First vaccination for EHV-1, second deworming
• Day 210: Second vaccination for EHV-1

Third Trimester

• Day 226: Third trimester begins
• Day 270: Third vaccination for EHV-1
• Day 300: Increase mare’s nutrition in preparation of foaling, third deworming for mare, open caslicks, administer additional vaccination according to veterinarian recommendations (EHV1/4, EEE, WEE, Tetanus, Flu, West Nile Virus & Rabies, in endemic areas: Potomac Horse Fever, Strep. equi, Botulism, Rotavirus), move mare to area where she will foal to stimulate immune system and develop antibodies
• Days 320-365: Normal foaling date range
• Day 340: Average foaling date

Fertilization and Implantation

Fertilization occurs in the oviducts, also known as the fallopian tubes. The stallion’s sperm is deposited directly into the uterus by the stallion (or during the process of artificial insemination). Once this occurs, the process is identical to that of other mammals, in which the spermatozoa propel themselves toward the oviducts, where they will encounter an ovulated egg (ovum). Each oviduct is a coiled tube which connects the tip of a uterine horn with an ovary. The portion of the oviduct nearest the ovary is called the infundibulum. It is the infundibulum’s function to catch the egg when it is released from the ovary and to conduct it down the oviduct toward the uterus.⁵ In short, the spermatozoa move upward as the ovum moves downward. Both are aided in their travels by cilia, hairlike projections that line the oviducts.

Once fertilization has been accomplished, the egg continues downward toward the uterus, passing through the isthmus, that portion of the oviduct where it joins the uterus.⁵ Hormones govern the process of implantation, in which the now-fertilized egg embeds itself into the uterine lining (as opposed to unfertilized eggs, which are expelled during the normal cycle). The uterus is then home for the embryo until it reaches maturity and parturition (foaling, or birth) occurs.

Embryonic Loss

The mare’s uterus is singularly remarkable among mammals in that it has the ability to defend itself against bacterial contamination extremely effectively. It also has the ability to supply the developing embryo with glandular secretions to nourish it during the first trimester. This is necessary because placental development and function in the horse begin relatively late in gestation.

Despite these unique evolutionary developments, mares still abort their embryos with more frequency than other mammals. In the field of animal medicine, horses are thought to have generally poor reproductive performance (or ability to maintain pregnancy).

There are many causes of early embryonic loss. Stress, fever, uterine infections, hormone abnormalities and twins can all cause a mare to spontaneously abort. The egg is fertilized in the fallopian tubes and does not enter the uterus until it about Day 6 of gestation. This is important to remember since you can manipulate the uterine environment up until this stage. Once the embryo descends into the uterus, it has to traverse the entire uterus to be recognized by the mare. If the embryo does not touch all portions of the uterus by day 16 of gestation, the mare will reject the embryo, and begin showing signs of estrus.⁶

Maintenance of Pregnancy

Prior to the advent of ultrasound technology, many breeders relied on the fact that most mares will come back into heat 17 to 20 days after breeding if they have not conceived. In some regions, veterinarians would use a sterile speculum to see if the mare’s cervix was tightly closed (indicating pregnancy) or relaxing (indicating the beginning of another heat cycle).⁶ Back then, the experienced equine veterinarian would be able to detect a bulge in the mare’s uterus by Day 30 to 35 of gestation.

These days, the intervals for checking mares are (days are approximate):

• Day 14 to 16 – Confirm initial pregnancy and determine if twins are present.
• Day 26 to 30 – Confirm heartbeat and fact that fetus is alive.
• Day 45 – Elective examination.
• Day 45 or > – If the mare aborts her pregnancy at this point or later, it is unlikely that she will be able to get pregnant again during the same breeding season.
• Day 60 – Elective examination; fetal sexing may take place.⁶

As far as exercise goes, pasture turn out should provide all the necessary exercise the mare will need. A regular diet should maintain a mare in the correct body condition, even if the mare is in mid-gestation. Water supply is critical because of the increased fetal fluids and milk production.^1,3

Herpesvirus (EHV-1) can cause late-term abortion and mares should be vaccinated for this disease preferably at five, seven, and nine months of gestation. Thirty days prior to foaling, Caslick’s procedures (if the vulva is sewn partially closed) should be opened or the patency of the vulvar lips should be confirmed.⁶ All recommended vaccinations for diseases against which the foal needs to be protected should be administered to the mare at this point. Bear in mind that vaccinations given to the mare at foaling do not protect the foal, since colostrum does not have adequate time (from generation to consumption) to manufacture the correct antibodies.

Since ascarid larvae can be passed in the mare’s milk, the mare should be dewormed with a product containing ivermectin.⁶ Ascarid impaction was a common cause of colic and death in foals before the use of ivermectin became common. Neonatal Isoerythrolysis (NI) screens are done within two weeks of foaling. These are blood tests to determine if the mare has hypersensitized herself to the blood-type of the foal. Older mares are more prone to being hypersensitized to a foal’s blood-type. If the mare is NI (+), the foal must be muzzled for the first 24 to 36 hours of life, and a colostrum donor sought.^3,6