🐴 Horse Pregnancy Due Date Calculator

What Is Equine Pregnancy?

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

1. Enter the breeding or covering date. For pasture-bred mares, use your best estimate and widen your mental window.
2. Read the expected foaling date (breeding date plus 340 days) and the full earliest-to-latest range (plus 320 to plus 360 days).
3. Start foaling watch at day 320 — cameras, alarms, or nightly checks — since mares typically foal at night and problems escalate fast.
4. Share the dates with your vet so pre-foaling vaccinations and the foaling plan are timed correctly.

How the Horse Pregnancy Due Date Works

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: Horse Pregnancy Due Date

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 the Horse Pregnancy Due Date?

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.

Mare Pregnancy Check Schedule: What Happens at Each Vet Visit

Because a mare’s due window is five weeks wide, the pregnancy is managed on a checkpoint schedule rather than a single date. This is the standard schedule breeding farms follow, as set out in the Merck Veterinary Manual — count each checkpoint from your breeding date:

The twin screen at the first check is the appointment breeders never skip: twin pregnancies remain a leading cause of abortion in mares, and reducing a twin vesicle is only straightforward before about day 30. The Merck Veterinary Manual’s parturition guidance also recommends keeping the mare in her foaling location for the final 3–4 weeks, so her colostrum carries antibodies against the pathogens her foal will actually meet.

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 About Horse

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.

Sources and References

• Merck Veterinary Manual. Pregnancy Determination in Horses — the standard checkpoint schedule (days 14–18, 25–30, 40–60, and fall check) used in the table above. merckvetmanual.com — Pregnancy Determination in Horses
• Merck Veterinary Manual. Parturition in Horses — pre-foaling signs including udder development and waxing, foaling preparation, and booster timing. merckvetmanual.com — Parturition in Horses

The 340-day average and 320–360 day window used by this calculator are the standard equine gestation figures used in veterinary practice. Last reviewed October 23, 2026.