A mare carries her foal for about 11 months. Unlike many species, the range is wide: anything from 320 to 365 days is considered normal, and a few healthy pregnancies run longer. This calculator gives the average due date, the normal window, and the checkpoints where vets intervene, so the last weeks are not a guessing game.
How it is calculated
- Due date = breeding (or ovulation) date + 340 days, the average gestation for light-horse breeds. Ponies and drafts tend a few days shorter, some breeds and older mares longer.
- Normal window = 320 to 365 days. Foals born before 320 days are premature and need immediate veterinary care.
- If the mare has foaled before, enter her own gestation length: mares are individually consistent to within about a week.
Example
A mare covered on March 1 has an average due date of February 4 the following year, with a normal window from mid-January to the end of February. Ultrasound at day 14 (March 15) confirms pregnancy and checks for twins; the EHV-1 vaccine series runs at months 5, 7 and 9; the mare moves to a foaling area at about day 300.
Why the date is only a guide
Day length affects gestation: mares due in spring tend to carry longer than those due in autumn. Colts are carried slightly longer than fillies on average. Mares grazing endophyte-infected tall fescue can carry well past term and have foaling problems; remove them from fescue in the last 60–90 days. What matters more than the calendar is watching the mare: udder development, waxing of the teats and relaxation of the tail-head ligaments give a few days’ notice.
Frequently asked questions
How long is a horse pregnant?
About 11 months: 340 days on average, with a normal range of 320 to 365. Some mares go a few weeks beyond that without problems.
Can a mare go past 365 days?
Yes, occasionally, and the foal is usually fine if the mare is healthy. Past about 370 days a vet should check the mare, and fescue exposure should be ruled out.
When can pregnancy be confirmed?
By ultrasound from day 14 after ovulation, which is also the best time to detect twins. A heartbeat is visible from about day 25.
What are the signs a mare is about to foal?
Udder filling 2–4 weeks before, waxing of the teats 1–4 days before, relaxed muscles around the tail-head, restlessness, sweating and repeated lying down and getting up in the final hours. Most mares foal at night.
How should I feed a pregnant mare?
Normal maintenance for the first eight months, then a gradual increase in energy and protein; about 60% of foal growth happens in the last three months. Avoid over-conditioning early.
Sources
By Calcelate Team. Formula from the sources above.
- 2026-09-14 · Formula and texts checked