Use this mare gestation calculator to turn a single breeding date into a clear foaling plan. Enter the mating date and you’ll get an estimated foaling date, a practical foaling window, and an easy timeline of veterinary checks and barn tasks. You’ll also find science-backed tips that help you prepare with confidence.
What this Mare Gestation Calculator does
The calculator estimates your mare’s foaling date from a mating date and shows a practical window that better reflects real-world biology. It also lays out checkups like early ultrasound, heartbeat confirmation, and when to set up cameras or stall mats. You get a ready timeline you can follow without guesswork.
Featured quick answers
- Average gestation length: about 340 days for mares. A normal range spans roughly 320–370 days. Sources below.
- Early pregnancy check: ultrasound around day 14–16 to confirm pregnancy and rule out twins.
- Heartbeat visible: about day 25.
- Vaccinate pre-foaling: core vaccines 4–6 weeks before the due date support colostral antibodies.
- Milk calcium strips: rising hardness often signals foaling within 24–72 hours.
How to use the calculator (step-by-step)
- Enter the mating date. If you have multiple cover dates then use the last successful cover.
- Optionally adjust the breed bias in your head. Large pony or donkey crosses can sit longer.
- Read the estimated foaling date and the practical window. That window is the one you post on the tack room wall.
- Print the timeline and pencil in appointments for scans, vaccines, and farrier work.
The formula, simply
We base the estimate on a 340-day average from the breeding date:
Estimated foaling date = Mating date + 340 days
We then display a practical window centered on that estimate. Many mares foal between 335 and 345 days. The window in the on-page calculator is set to ±7 days unless you select a breed note that calls for a wider range.
Equine pregnancy timeline at a glance
This overview gives you the big rocks first. It pairs clinical checks with barn tasks so nothing sneaks up on you.
| Day (approx.) | What to do | Why it matters |
|---|---|---|
| 14–16 | First ultrasound | Confirm pregnancy. Check for twins which need early management. |
| 25–30 | Heartbeat ultrasound | Viability check. Plan next scan if needed. |
| 55–70 | Mid-gestation review | Discuss nutrition. Some vets sex the fetus around this time. |
| 270–300 | Start foaling prep | Order foaling kit. Check cameras. Deep clean foaling stall. |
| ~300 | Udder watch begins | Note changes. Save a daily photo. Optional milk test strips later. |
| ~310–320 | Vaccinate 4–6 weeks pre-foaling | Boost colostrum. Confirm your deworming plan. |
| 330–345+ | High alert | Waxing teats, softened ligaments, and restless nights signal go time. |
Why mares don’t read the textbook
Mares carry a foal on their own timetable. Weather, daylight length, nutrition, and the mare’s history all play roles. Individual mares often repeat a pattern across years. Track it and you’ll forecast better than any generic chart.
Good to know: A healthy foal can arrive at 320 days. Another healthy foal can appear at 360 days. The number alone doesn’t define normal. Behavior, comfort, and monitoring fill in the picture.
Prep like a pro: foaling supplies checklist
- Clean towels, tail wrap, and disposable gloves.
- Navel dip (chlorhexidine 0.5–1%).
- Small scales for colostrum if you have them or a colostrometer.
- Sterile scissors and umbilical tape only if the vet recommends.
- Thermometer, stethoscope, and a watch with seconds.
- Emergency contacts posted on the stall door.
Add a whiteboard with times and notes. Small detail yet invaluable when everyone is tired.
Signs foaling is close
- Udder enlarges. Teats fill. Wax beads appear.
- Tail-head ligaments soften. Croup looks jello-soft.
- Restless pacing at night. Less interest in grain. More tail lifting.
- Milk calcium tests climb. pH drops toward 6.2–6.4. Many mares foal within 24–72 hours after that drop.
Stages of foaling (know the timing)
- Stage I: 30–90 minutes. Uterine contractions ramp up. The mare looks uncomfortable and may sweat. The water breaks at the end of this stage.
- Stage II: 10–20 minutes. Active delivery. White amnion appears then the foal’s feet and nose. Delivery should be smooth once the shoulders pass.
- Stage III: Up to 3 hours. Placenta passes. Save it intact in a clean bag so your vet can evaluate the horns and body.
Vaccinations, nutrition, and parasite control
Keep core vaccines current. Many barns give boosters 4–6 weeks before the due date. That timing supports strong antibodies in colostrum that protect the foal in the first weeks of life. Keep salt and fresh water available at all times. Adjust calories slowly as the mare’s body condition changes. Deworm with products that your veterinarian recommends and weigh the mare to dose accurately.
Breeding method and ovulation tracking
Live cover or artificial insemination may shift your best counting point. If you know the exact ovulation time then you can anchor your estimate to that moment and calculate 340 days ahead. You’ll usually get a narrower window. If you only know cover dates then use the last successful cover. The calculator still gives a reliable plan.
What can extend or shorten a pregnancy?
- Season: Mares bred early in the year can carry longer. Mares bred late can carry shorter.
- Breed: Donkey crosses and some pony breeds may go long.
- Individual history: Your mare may repeat her pattern from previous years.
- Nutrition: Under-conditioned mares sometimes carry longer.
- Fetal sex: Some data suggest colts arrive a shade later on average. The effect is small.
Troubleshooting: when to call the vet
- No placental passage within 3 hours.
- Red bag presentation. You see a red velvet-like membrane at delivery. That is an emergency.
- Thick, foul discharge before due date or fever.
- Lack of vigor in the foal. No standing by 1 hour or no nursing by 2 hours.
Owner-friendly schedule you can print
- Day 14–16: Ultrasound #1.
- Day 25–30: Heartbeat check.
- Day 55–70: Mid-gestation review. Optional sexing.
- Day 270–300: Order supplies. Test cameras. Bank frozen colostrum if needed.
- Day 300: Daily udder photos. Start pH notes later.
- 4–6 weeks pre-foaling: Vaccinations. Deworm plan.
- Week of due date: Barn ready 24/7. Night checks. Keep the vet looped in.
Frequently asked questions
How accurate is the estimated foaling date?
It’s a solid target. The practical window captures most normal foalings. Track udder changes and behavior to tighten the final 72-hour call.
Can I predict sex by belly shape?
No reliable science backs that idea. Ultrasound sexing around day 59–68 works when a skilled vet performs it.
Should I stall my maiden earlier?
Yes. Give her time to relax in the foaling space. Good sleep helps everyone.
Suggested Calculators
- Dog Pregnancy Calculator — great if your barn has canine helpers too.
Further reading
- Merck Veterinary Manual — Normal Parturition in the Mare
- AAEP — Pregnancy, Foaling, and Neonatal Care
- UC Davis CEH — Foal Watch
- Merck Veterinary Manual — Overview of Gestation
A due date is a compass not a stopwatch. This calculator gives you that compass and the route markers that keep you on track. Enter the mating date. Print the timeline. Watch your mare’s signals and keep your vet in the loop. You’ll welcome a healthy foal with a barn that’s calm, stocked, and ready.
