Calculate your mare’s foaling (birth) date from the breeding date. Average equine gestation: 340 days (~11 months), with a normal range of 320–360 days.
Equine pregnancy — horse gestation — lasts an average of 340 days, roughly 11 months, making it the longest pregnancy of the common domestic animals. A healthy mare can foal anywhere between 320 and 360 days after breeding, a window spanning nearly six weeks. Enter the breeding or covering date and this calculator shows the expected foaling date plus that full normal range.
Unlike many species, equine gestation length shifts with the season. Mares bred in late spring or summer tend to carry for fewer days than mares bred in fall or winter, because day length influences the hormones that time fetal maturation. Colts are commonly carried 3–7 days longer than fillies, and draft breeds trend longer than lighter breeds.
Key veterinary milestones punctuate the long wait: ultrasound can detect the embryonic vesicle around days 14–16, the heartbeat appears around days 25–30, and the critical placental transition completes near day 120. Rapid fetal growth happens in the final two months, which is when the mare’s belly becomes unmistakably large and her nutritional needs peak.
How to Use the Horse Pregnancy Calculator
Enter the breeding or covering date. For pasture-bred mares, use your best estimate and widen your mental window.
Read the expected foaling date (breeding date plus 340 days) and the full earliest-to-latest range (plus 320 to plus 360 days).
Start foaling watch at day 320 — cameras, alarms, or nightly checks — since mares typically foal at night and problems escalate fast.
Share the dates with your vet so pre-foaling vaccinations and the foaling plan are timed correctly.
How the Calculation Works
Expected foaling date = breeding date + 340 days
Normal window = breeding date + 320 days to + 360 days
The 340-day average is the long-standing veterinary standard for horses, and the calculator applies it directly to the date you enter. Because equine gestation varies more than any other farm species’, the wide range is not a footnote — it is the main planning tool. A mare foaling at day 325 or day 355 is within normal limits, while a mare at day 310 or day 370 needs veterinary attention.
Worked Example
Suppose your mare was bred on April 15, 2025:
Expected foaling date: April 15, 2025 + 340 days = Saturday, March 21, 2026
Earliest normal foaling: + 320 days = Sunday, March 1, 2026
Latest normal foaling: + 360 days = Friday, April 10, 2026
Planning notes: book your pre-foaling vet visit for late January (about a month before the earliest date), install cameras or alarms by February 25, and keep the foaling kit packed from March 1 onward. If she is showing no foaling signs by April 10, call your vet for an assessment.
How Accurate Is It?
The 340-day figure is accurate as a population average, but individual mares are famously variable — some foal like clockwork on day 342 every year, while others swing widely between seasons. Pasture breeding with an unknown exact date adds uncertainty on top of that biology.
Veterinary monitoring narrows the window considerably. Early ultrasound confirms pregnancy and dating, while milk calcium testing and udder assessment in the final weeks give much better near-term predictions than calendar dates alone. Use this calculator to bracket the season, then let physical signs and your vet guide the final days.
Planning Your Foaling Watch
Begin active surveillance at day 320. Most mares foal between 10 PM and 4 AM, so a foaling alarm (triggered by the mare lying flat) or a stall camera with phone alerts is worth far more than hourly barn trips. Keep the stall large, deeply bedded with straw, clean, and disinfected — at least 14×14 feet so the mare can move freely.
Feed late-gestation mares well: energy and protein needs rise sharply in the final trimester as the foal grows. Time pre-foaling vaccinations 4–6 weeks before the expected date so antibodies concentrate in colostrum, and have frozen colostrum or a supplement on hand in case the foal cannot nurse promptly.
When to Call Your Vet
Foaling emergencies develop in minutes, so call your veterinarian immediately if:
Stage two labor (visible contractions, water sac) lasts more than 20–30 minutes without a foal.
The presentation looks wrong — anything other than two front feet and a nose first.
A red membrane appears at the vulva (red bag delivery — an emergency requiring immediate action).
The placenta is not passed within 3 hours of foaling.
The foal has not stood within 2 hours or nursed within 3 hours.
The mare is in pain, bleeding heavily, or off feed with a foul discharge after foaling.
Never pull a foal without veterinary guidance on positioning first, and do not break a red bag yourself unless your vet has trained you. Pre-program your vet’s emergency number and confirm after-hours coverage before day 320.
Please note: This calculator provides estimates for educational purposes and is not veterinary advice. Always confirm with your veterinarian.
Frequently Asked Questions
Common questions answered by our medical team
Mares carry foals for an average of 340 days — about 11 months — with a wide normal range of 320–360 days. This spread is the widest of any common domestic animal, so foaling can legitimately happen more than 3 weeks early or late relative to the average. Night foaling is typical.