Eleven months and a week. That’s the average time separating the breeding from the foal’s birth. A long stretch, one that requires attention — but surprisingly robust when the mare receives the right care. Equine pregnancy doesn’t need to be a source of anxiety. It does need to be managed.
This article covers how long mare gestation lasts, how it develops across each stage, what care is necessary, and when to call the veterinarian.
How Long Mare Gestation Lasts
A mare’s gestation averages 340 days, with normal variation from 320 to 360 days. Foalings before 320 days produce premature foals with serious survival challenges. Foalings beyond 360 days are rare but occur, and may be associated with larger-than-expected foals.
Several factors influence duration:
Foal sex: mares tend to carry colts slightly longer than fillies — by roughly 1 to 5 days on average.
Season of breeding: matings at the start of the breeding season (spring) result in slightly longer gestations; fall breedings tend to be shorter.
Individual genetics and breed: there is meaningful individual variation — some mares consistently foal close to day 340; others regularly carry to day 355. Knowing a mare’s reproductive history helps estimate her foaling date more accurately.
Nutrition: gestations in mares that are severely underfed or nutritionally deficient tend to run slightly shorter.
Pregnancy Diagnosis
Early pregnancy confirmation is standard practice in well-managed operations. Transrectal ultrasound can detect the embryonic vesicle as early as 10 to 12 days post-ovulation, though exams are most commonly performed between days 14 and 16, when the vesicle is clearly visible and risk of error is minimal.
By day 25, the embryo’s heartbeat is detectable. Around day 40, the embryo becomes properly classified as a fetus.
A second ultrasound between days 28 and 45 is recommended to confirm fetal viability and screen for twin pregnancies — a serious complication in mares, whose uteri rarely support two fetuses to term. When detected early, twins are typically managed by manual reduction (destruction of one vesicle) in the first days after diagnosis.
Stages of Gestation
First Trimester (Days 0–120)
This is the highest-risk period for pregnancy loss. Most equine spontaneous abortions occur here — particularly between days 35 and 45, when the embryo transitions between nutritional support structures, and between days 60 and 90, as the placenta actively develops.
At this stage, the fetus is tiny — by the end of the first trimester, it weighs around 10 oz (300 g) and measures about 8 to 10 inches (20 to 25 cm). The mare shows no visible physical changes. Management can remain essentially normal: moderate exercise, quality pasture or hay, free-choice mineral, and fresh water.
Intense stress events, febrile illnesses, non-steroidal anti-inflammatory drugs (NSAIDs) used without veterinary supervision, and ingestion of pasture with toxic fungi or plants are risk factors for pregnancy loss during this stage.
Second Trimester (Days 120–240)
The fetus grows progressively — by the end of the second trimester, it weighs around 22 lbs (10 kg) and has the recognizable appearance of a miniature horse. The mare begins to show a slight abdominal rounding, more noticeable in mares with looser abdominal muscles.
Gestation is generally stable during this stage. Nutritional requirements increase moderately — the mare needs more energy and protein than when not pregnant, but not dramatically so.
Exercise continues to be beneficial. Mares in good physical condition that work moderately throughout gestation tend to have easier foalings than stabled, sedentary mares.
Third Trimester (Days 240–340)
Fetal growth accelerates markedly in this stage. Roughly 60% of the foal’s total birth weight is gained during the last trimester. The fetus grows from approximately 22 lbs (10 kg) to 100 to 120 lbs (45 to 55 kg) — or more, depending on the breed — in just four months.
This rapid growth places significant nutritional demands on the mare. Energy, protein, calcium, phosphorus, vitamin E, and vitamin A requirements all increase. Feed should be adjusted progressively — usually with the introduction or increase of concentrate beginning around the seventh month, guided by body condition score.
Equal attention must go to colostrum quality: the foal’s passive immunity depends on the quality of colostrum the mare produces, and nutrition in the final weeks directly influences that quality.
Care During Gestation
Nutrition
Nutritional management of the pregnant mare is straightforward to understand but requires consistent monitoring:
- First and second trimesters: maintain body condition score between 5 and 6 (1–9 scale). Quality pasture or hay, mineral supplement, and water are sufficient for most mares.
- Third trimester: gradually increase concentrate, especially in the final 90 days. Mares that are nursing after foaling have even higher energy needs — arriving at foaling in good body condition supports faster recovery.
Avoid obesity: overweight mares have a higher risk of foaling complications and post-partum problems, including retained placenta and metabolic syndrome.
Deworming and Vaccination
Strategic deworming is recommended during gestation — typically with ivermectin or fenbendazole, following the veterinarian’s protocol. A dose of ivermectin in the last 30 days before foaling helps reduce parasite burden that the mare might otherwise transmit to the foal shortly after birth.
The vaccination schedule should be reviewed before breeding and during pregnancy. Vaccines against the primary equine respiratory viral diseases (equine influenza, equine herpesvirus types 1 and 4) and tetanus are priorities. Vaccination during the final trimester — typically at 8, 4, and 2 weeks before the estimated foaling date — concentrates antibodies in the colostrum, directly benefiting the newborn foal.
Exercise
Moderate exercise is beneficial throughout gestation. Mares in regular work can continue to be ridden and worked through months six or seven, with a gradual reduction in intensity. After months seven or eight, intense work should be avoided, but daily movement — grazing, walking — remains important.
Exercise improves placental circulation, maintains the mare’s physical condition, and reduces the risk of limb edema, which is common in late gestation.
Environment and Safety
In the final weeks before foaling, the mare should be housed in a large, clean, well-bedded stall — or in a reserved paddock. The foaling stall should be at least 170 to 215 sq ft (16 to 20 m²), with smooth walls (no projections the mare could injure herself on during labor) and deep bedding.
Avoid abrupt changes in environment or herd composition in the final four weeks before the estimated foaling date — social stress can advance foaling or negatively affect milk production.
Signs of Advanced Pregnancy
As pregnancy progresses, several physical changes become increasingly visible:
Abdominal enlargement: the mare’s belly grows visibly from months five or six, becoming very large in the final weeks.
Udder development: the teats begin filling out in the final weeks. Two to three weeks before foaling, the udder is distended and may show drops of waxy secretion at the teat ends.
Waxing or streaming milk: the appearance of wax-like droplets or streams of milk from the teat ends — called “waxing” — is one of the most widely used signs for estimating foaling proximity. When milk begins flowing regularly, foaling generally occurs within 24 to 72 hours.
Perineal relaxation: sinking of the croup muscles and relaxation of the vulvar ligaments signal the action of the pre-foaling hormones.
When to Call the Veterinarian
Not every pregnancy is problem-free. Situations warranting immediate veterinary evaluation:
- Vaginal bleeding at any stage
- Abnormal vulvar discharge (purulent, foul-smelling)
- Sudden collapse or signs of pain
- Colic in a pregnant mare (any colic episode in a pregnant mare requires urgent evaluation)
- Very violent fetal movements or absence of fetal movement after month six
- Flow of clear liquid from the vulva before the start of labor
