Mare Gestation Period Calculator
This mare gestation period calculator finds your mare's expected foaling date by breed. The mare gestation period averages 340 days but varies significantly by breed and season — enter the last breeding date to get the full foaling window, milestone timeline, and breed-specific adjustments.
Quick Answer
The mare gestation period averages 340 days — approximately 11.2 months or 48.6 weeks — with a wide normal range of 320–370 days. Mares have one of the most variable gestation periods of any domestic animal. Enter your mare's breeding date above to calculate the exact expected foaling date by breed.
Mare Gestation Calculator
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How to Use This Calculator
Enter the date of last breeding or artificial insemination using the date picker. Select the breed for a more accurate estimate — Draft Horse mares gestate longer than Thoroughbreds, and Miniature Horse mares have the shortest average gestation. Click Calculate Due Date to see the expected foaling date and the complete 320–370 day foaling window.
Because the mares gestation period is so variable, the calculator shows both the average expected date and the full range so you can plan your foaling watch accordingly. Results are saved in your browser between visits.
How Long Is a Mare Pregnant?
A mare is pregnant for an average of 340 days — just over 11 months. The mare gestation period is among the most variable of any common domestic animal, with a fully normal range of 320–370 days. Season of conception affects the mares gestation period: mares bred in winter and early spring tend to have shorter gestations, while those bred in summer may carry slightly longer.
First trimester (days 1–100): The embryo is detectable by transrectal ultrasound as early as day 14. Heartbeat is visible by day 25. Twin pregnancies — problematic in horses — should be identified and one embryo reduced before day 16 for best outcomes. Early pregnancy loss is most common during this period.
Second trimester (days 100–225): The foal grows steadily. The mare's nutritional requirements increase moderately. Fetal sex determination by ultrasound is most reliable between days 60–100. The mare's abdomen begins to enlarge visibly around months 4–5.
Third trimester (days 225–340): The foal gains most of its birth weight. Udder development (bagging up) becomes visible around day 270. The mare's energy requirements increase significantly. Move the mare to the foaling area 2–4 weeks before the expected date.
Most mares foal at night, typically between 10 PM and 4 AM. Stage 2 labor (from water breaking to delivery) should complete within 30 minutes — foals should be standing within 1–2 hours and nursing within 3 hours.
Mare Gestation Period in Days, Weeks and Months
The mare gestation period in days, weeks, and months is summarized below. The wide range reflects the natural variation in mares gestation period across breeds and individuals.
| Measurement | Average | Normal Range |
|---|---|---|
| Days | 340 days | 320–370 days |
| Weeks | 48.6 weeks | 45.7–52.9 weeks |
| Months | 11.2 months | 10.5–12.2 months |
Mare Gestation Period by Breed
| 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; slightly above average |
| Draft Horse | 342 days | Heavy breeds gestate slightly longer |
| Paint Horse | 337 days | Average light breed range |
| Pony | 330 days | Slightly shorter than full-sized horses |
| Miniature Horse | 320 days | Shortest of all horse breeds |
How We Calculate Mare Gestation
The mare gestation period calculator uses the formula: Foaling Date = Breeding Date + 340 days. The 340-day average is based on data from the American Association of Equine Practitioners (AAEP), University of Kentucky Equine Research, and the Merck Veterinary Manual. Because mare gestation varies significantly, the calculator also shows the full 320–370 day foaling window.
Frequently Asked Questions
Sources & References
- USDA NASS — Horse inventory and foaling data
- Penn State Extension — Equine reproduction and mare 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: March 10, 2025