Horse Gestation Calculator
This horse gestation period calculator finds your mare's expected foaling date instantly. The horse average gestation period is 336 days but varies widely by breed and season. Enter the last breeding date to get the full gestation range and pregnancy milestones.
Horse Gestation Calculator
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How to Use This Calculator
Enter the date of the last breeding or artificial insemination. Select the breed — Draft breeds and Miniature Horses have notably different average gestation lengths. Click Calculate Due Date to see the expected foaling date and a full milestone timeline from first heartbeat to foaling.
Because of the wide natural variation in horse gestation, the calculator shows both the average due date and the full 320–362 day window so you can plan your foaling watch accordingly.
Horse Gestation Period — What to Expect
The horse average gestation period is 336 days — approximately 11 months. The gestation period for horses is among the longest of common domestic animals, reflecting the advanced development required for a foal to stand and run within hours of birth. The gestational period for horses also varies more than any other domestic species — a range of 320–362 days is considered normal.
First trimester (days 1–100): The embryo is detectable by rectal ultrasound as early as day 14. A heartbeat is visible by day 25. Spontaneous early pregnancy loss is most common during this period. Twin pregnancies (problematic in horses) should be identified and managed before day 30.
Second trimester (days 100–225): The fetus is fully formed and grows steadily. The horses gestation period during this phase is relatively quiet — fetal movement can be felt and occasionally seen from the outside. The mare's nutritional requirements increase moderately. This is when fetal sex determination is most reliable via ultrasound.
Third trimester (days 225–336): The foal gains most of its birth weight. Colostrum production begins in the udder around day 270. The mare's energy requirements increase significantly, particularly in the final 90 days. Move the mare to the foaling area 2–4 weeks before the expected date so she can adjust to the new environment.
Most mares foal at night, typically between 10 PM and 4 AM. Foaling alarm systems and video monitoring are strongly recommended for valuable mares, as Stage 2 labor should complete within 30 minutes — prolonged delivery requires immediate veterinary intervention.
Breed-Specific Horse Gestation Periods
| Breed | Avg. Gestation (Days) | Notes |
|---|---|---|
| Thoroughbred | 336 days | Average light breed range |
| Quarter Horse | 337 days | Average light breed range |
| Arabian | 335 days | Average light breed range |
| Warmblood | 340 days | Sport horse breeds run slightly longer |
| Draft Horse | 342 days | Heavy breeds tend to gestate longer |
| Tennessee Walking Horse | 338 days | Average light breed range |
| Paint Horse | 337 days | Average light breed range |
| Appaloosa | 336 days | Average light breed range |
| Pony | 330 days | Slightly shorter than full-sized horses |
| Miniature Horse | 320 days | Shortest of horse breeds |
How We Calculate Horse Gestation
The horse gestation calculator uses the formula: Foaling Date = Breeding Date + 336 days. The 336-day average is based on data from the American Association of Equine Practitioners, University of Kentucky Equine Research, and the Merck Veterinary Manual. Because horse gestation varies significantly, the calculator also shows the complete foaling window (day 320–362) so you know when to begin and end your foaling watch.
Frequently Asked Questions
Sources & References
- USDA National Agricultural Statistics Service — Horse inventory and production data
- Penn State Extension — Equine reproduction management
- University of Minnesota Extension — Mare gestation and foaling
- Merck Veterinary Manual — Equine pregnancy management
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GestationCalc Editorial Team
Our editorial team includes animal husbandry specialists, veterinary consultants, and agricultural extension educators. Content is reviewed against peer-reviewed research and guidance from USDA, Penn State Extension, and the Merck Veterinary Manual.
Last reviewed: January 25, 2025