About Mare Gestation
Eleven months and change: a mare bred in spring foals the following spring, and the whole breeding calendar — nutrition changes, vaccination boosters, moving the mare to her foaling location, night watch — counts backward from the expected date. The catch is equine biology's spread: gestation length varies more in horses than most livestock.
Enter the cover or insemination date and a gestation length (340 is the standard planning number; use your mare's own history if she runs consistently early or late). You get the expected date, a ±10-day window, days in foal, and days to go — with the reminder that the window, not the date, is the real prediction.
The cattle version — tighter windows, different math — is the Cow Gestation Calculator.
The Foaling-Date Math
One addition and honest uncertainty:
Foaling date = breeding date + gestation length (≈ 340 days) Window = date ± 10 days (mares normally span ~320–370)
Worked example: a mare covered May 1, 2026 at the 340-day average is due Tuesday, April 6, 2027, with a realistic window of late March to mid-April. Known tendencies: colts tend to run a day or two longer than fillies, and mares foaling early in the season often carry a bit longer.
Breeding Month → Foaling Month
First-of-month cover dates at the 340-day average:
| Bred | Foals (~340 d) | Bred | Foals (~340 d) |
|---|---|---|---|
| Feb 1 | Jan 7 | Jun 1 | May 7 |
| Mar 1 | Feb 4 | Jul 1 | Jun 6 |
| Apr 1 | Mar 7 | Aug 1 | Jul 7 |
| May 1 | Apr 6 | Sep 1 | Aug 7 |
The pattern: subtract about 25 days and go to the next year — an easy check on any date this calculator gives you.
Foaling Watch: the Signs
The calendar says when to start watching; the mare says when it's happening:
- 2–6 weeks out: udder begins filling; the belly drops as the foal positions.
- 1–2 weeks out: teats fill toward the ends; tailhead muscles soften.
- 24–48 hours: “waxing” — colostrum beads on the teats — is the classic short-notice sign; milk calcium testing narrows it further.
- Labor: mares foal fast and mostly at night — active labor should deliver within roughly 20–30 minutes, and trouble at that stage is an immediate vet call, not a wait.
- Post-foaling markers many barns use: foal standing within ~1 hour, nursing by ~2, placenta passed by ~3 — deviations warrant the vet.
These are husbandry conventions, not veterinary advice — your vet and breed association's guidance govern the real decisions.