Mare Gestation Calculator - Horse Gestation

Equine gestation averages 340 days, about 11.2 months, but the normal range is unusually wide, roughly 320 to 370 days, a full 50-day spread. This makes horses one of the least predictable common livestock species for exact foaling timing, even though the average is well established.

Foaling Date = Breeding (or covering) Date + Gestation Length in days. With the default 340-day average, a mare bred on July 12 would be due around June 17 of the following year. Because equine gestation varies so widely, this calculator lets you adjust the gestation length between 320 and 370 days.

Several factors influence the exact length: the season bred (mares bred early in the year, when daylight is shorter, tend to carry longer than mares bred later, when daylight is longer), the individual mare, whether it is a maiden (first) pregnancy, and the foal's sex, with colts typically carried a day or two longer than fillies on average. Together these factors produce the wide 320-370 day normal range.

Transrectal ultrasound can detect an embryonic vesicle as early as 14-16 days after ovulation, making it the earliest reliable method and also allowing early detection of twins for reduction if needed. A fetal heartbeat is typically visible by day 25-30. Blood tests (eCG/PMSG) become useful from about day 40-100.

In the final weeks, the udder develops and enlarges. In the final days, 'waxing' (small drops of dried colostrum on the teat ends) commonly appears 24-48 hours before foaling, though not every mare wax noticeably. Other signs include softening and lengthening of the vulva, relaxation of the muscles around the tailhead, and restlessness or repeated lying down and standing in the final hours.

Yes. Mares bred early in the calendar year (roughly February through April), when daylight hours are still short, statistically tend to carry a bit longer than mares bred later in spring or summer, when daylight is longer. This seasonal photoperiod effect is one of the better-documented reasons two mares bred a month apart can still foal close together, or vice versa.

On average, colt (male) foals are carried a day or two longer than filly (female) foals, a small but consistently observed effect across multiple studies. It is one of several minor factors, alongside breeding season and individual mare history, that contribute to the wide normal range rather than a single fixed number.

Given the 320-370 day normal range, a horse foaling-date estimate is inherently less precise than for species like cattle or goats with narrower ranges. Most owners treat the estimated date as the opening of a multi-week watch period rather than a specific day, and increase monitoring (foaling alarms, night checks) starting around day 320, the early end of the normal range.

Gestation extending past 370 days does happen occasionally and is sometimes called 'prolonged gestation.' While many such pregnancies still result in a healthy foal, a veterinary check is recommended for mares well beyond the normal range, since prolonged gestation can occasionally be associated with placental issues or an unusually large foal.

Twin pregnancies are uncommon in horses, occurring in roughly 1-2% of natural conceptions, and are considered high-risk: the equine uterus generally cannot adequately support two foals to term, so twin pregnancies often end in loss of one or both foals if not managed. This is why early ultrasound (day 14-16) for twin detection, with reduction to a single embryo if needed, is standard practice in managed breeding programs.

Yes, but expect a wider estimate. Enter the earliest date within the breeding window as a starting point, and treat the resulting foaling date as the opening of your watch period rather than a fixed prediction, since the true date could fall anywhere within the breeding window plus 320-370 days.