EquiDiaryFoaling date calculator: the due date, the normal window and the pregnancy calendar
Last checked: 13 October 2026
A mare is pregnant for about eleven months — 340 days on average — but the normal range runs from about 320 to 370, so a due date is really a window seven weeks wide. Put in the date of her last breeding and this lays out the window and the dated steps of the pregnancy: the scans, the vaccines, the move to where she will foal.
These are published averages applied to one date, not veterinary advice. Your vet times the scans and the vaccines for your mare, and a mare that seems unwell, or is well past her date, needs a vet, not a calendar. Nothing you type here leaves your browser.
How long a mare is pregnant
The Merck Veterinary Manual gives an average of "about 340 days (about 11 months)". The range around it is wide, and every source draws it a little differently: Nebraska Extension says 320 to 370 days, Oklahoma State 320 to 380 — adding that pregnancies "as long as 365 are not entirely uncommon" — and the University of Minnesota "usually 335 to 342 days", with 315 to 400 or more possible. Below about 320 days a foal is premature (Merck, Oklahoma State; Nebraska uses 326), and before 310 days it counts as an abortion.
Breed moves the average a little. From studies of each:
| Breed | Average gestation | Range in the study |
|---|---|---|
| Thoroughbred (UK) | 344 days | 315–388 |
| Thoroughbred (Germany) | 347 days | 306–390 |
| Standardbred | 349 days | — |
| Arabian | 340 days | — |
| Warmblood | 338½ days | 313–370 |
| Andalusian | 337 days | — |
| Heavy draft | 335 days | 313–352 |
Why one mare's date moves
- The season. Every study agrees that mares bred early in the year carry longer. In Arabians and Andalusians, each month later at breeding took about three days off; in Old Kladrubers, pregnancies conceived in March averaged 343.9 days and those conceived in July 334.9.
- The foal's sex. Colts are usually carried a few days longer: 3.8 days in Thoroughbreds, 2.5 in Standardbreds, 4.4 in heavy drafts — though one study of Lusitanos found no difference.
- The mare herself. Mares tend to repeat their own length: studies put the repeatability between 0.36 and 0.56. Her previous pregnancies say more about this one than a breed average, which is the case for writing each one down.
- Tall fescue. In the US, grazing endophyte-infected tall fescue late in pregnancy can prolong gestation to "13 to 14 months", with a thickened placenta and no milk. The US feeding guide covers it.
Breeding date or ovulation date
Owners count from the last breeding; vets time their scans from ovulation. The two are close but not the same. The Merck manual says a mare ovulates up to 48 hours before the end of her heat, which lasts two to eight days, and that sperm survive for at least 48 hours, so a mare is usually covered every other day until she ovulates. The last cover is normally within a couple of days of ovulation; a first one can be several days earlier. Use the last — or the ovulation date, if your vet scanned for it.
The calendar, step by step
| When | What | Why |
|---|---|---|
| Days 14–16 | first scan | to confirm the pregnancy and catch twins early, while the embryo still moves around the uterus (until about day 16–17) |
| Days 24–25 | heartbeat | "At day 25, a heartbeat should be present" (Merck); if not, a recheck in a day or two |
| Days 30–35 | second scan | about 6% of pregnancies are lost by day 40, and a mare that loses one after day 35 cannot be rebred for three to four months (University of Minnesota) |
| Days 40–60 | later check | part of the schedule the Merck manual gives as an example, with a final check in the autumn |
| Days 57–70 | fetal sex, if wanted | the window studies found best for a scan to tell colt from filly |
| 5, 7 and 9 months | EHV abortion vaccine | the AAEP's schedule, with a vaccine labelled for preventing EHV abortion; the UK's Codes of Practice give the same months, and many vets add a dose at three |
| 90–60 days before the due date | off tall fescue (US) | Penn State and the University of Kentucky advise taking pregnant mares off infected fescue 60 to 90 days before foaling |
| 6–4 weeks before | boosters | the AAEP advises boosting the core vaccines four to six weeks before foaling — in its chart, tetanus, the equine encephalitis viruses, West Nile, rabies and influenza — so that her first milk passes the antibodies to the foal |
| 6–4 weeks before | move to the foaling place | four to six weeks before (University of Minnesota); the Merck manual says three to four, and the UK's Codes of Practice at least 28 days, followed by two weeks' isolation |
| The last 30 days | watch the udder | the University of Kentucky's advice for the end of the pregnancy |
And two things the calendar leaves out on purpose. No vaccines in the first 60 days of pregnancy (University of Minnesota) and no dewormers in the first two months (Merck). And the dewormer many owners give on foaling day against the threadworm Strongyloides westeri: the AAEP's parasite guidelines say "there is no basis for recommending this practice at present".
Signs that foaling is near
- The udder fills over the last two to four weeks (Merck).
- Waxing — beads of dried colostrum on the teats — appears in almost all mares 6 to 48 hours before foaling, but not in every mare, and in some days early.
- Milk tests. Oklahoma State gives the odds for milk calcium: above 200 ppm, 54% of mares foal within 24 hours, 84% within 48 and 97% within 72; below it, fewer than 1% foal within 24 hours. Penn State adds that a milk pH under 6.5 usually means foaling within a day.
When she goes past her date
Day 340 is the middle of the range, not a deadline, and a mare past it is usually just a long one. There is no agreed day at which a pregnancy counts as prolonged: Colorado State describes a mare that foaled at 385 days and advises against inducing on dates alone. What decides it is the mare, examined by a vet — not the number of days.
Donkeys and mules
This calculator uses mares' figures. Jennies carry about a year: studies give averages from 362 to 374 days, with spring-bred jennies about 16 days longer, so a jenny's due date falls three to five weeks after a mare's would. For mules the studies disagree, one finding them shorter than horses and another longer.
Put the calendar where it reminds you
In EquiDiary each date above can go into the care schedule as a one-off — the EHV doses under Vaccination, in its EHV (rhinopneumonitis) type, the scans under Medical — and a morning email arrives as each comes due. Log each scan and dose when it happens, and next time her own gestation length is in her history, which is the number this page says matters most.
Start free for 3 monthsRelated guides
- Horse vaccination schedule — the core vaccines and how often boosters are really due
- Horse feeding plan — the numbers in pounds, and the fescue problem in more detail
- Horse weight calculator — a tape-measure weight, for dosing and feeding
Sources
- Merck Veterinary Manual — Management of reproduction in horses (average length, signs of foaling); Intensive care and emergencies in neonatal foals (range, premature); Breeding (heat, ovulation, sperm survival); Pregnancy determination (scan schedule, heartbeat); Parturition (moving the mare)
- Nebraska Extension — Management of the pregnant mare (2020); Oklahoma State AFS-3985 — Foaling management (2024: range, premature, milk calcium); University of Minnesota Extension — Caring for your mare during breeding and foaling (range, exams, losses, moving, no early vaccines)
- Colorado State Equine Reproduction Laboratory — Pregnancy examination and The overdue mare (2024)
- Gestation studies (Europe PMC): Arabians and Andalusians, 2006; Old Kladrubers; Lusitanos; Thoroughbreds, UK; Thoroughbreds, Germany; Standardbreds; Warmbloods; heavy drafts; fetal sexing 2019; donkeys 2022, 2010 and 2021; mules 2019 and 1997
- AAEP — Adult horse vaccination chart (EHV at 5, 7 and 9 months; boosters 4–6 weeks before foaling); EHV vaccination guidelines (the third-month dose); Vaccination and passive transfer; Internal parasite control guidelines (deworming at foaling)
- Horserace Betting Levy Board — Codes of Practice 2026: EHV (UK: vaccination months, arrival before foaling)
- Fescue: Penn State Extension; University of Kentucky ID-144 (also: watching the udder in the last 30 days)
- Penn State Extension — Foaling season tips for delivery (milk pH)
Averages and schedules are revised from time to time; your vet sets your mare's.
Next: horse vaccination schedules — the core vaccines, and how often boosters are really due.