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Mare gestation averages about 340 days but 320-370 is normal, and individual mares repeat their own pattern. A guide, not veterinary advice: work with your vet or stud on scanning, vaccination and foaling preparation.
What The Horse Pregnancy Calculator Works Out
- Expected foaling date: the covering date plus 340 days, eleven months. From a 1 September covering that is early August the following year.
- Normal range: 320 to 370 days, an unusually wide window. Foals born before 320 days are premature and need immediate veterinary attention; overdue mares more often need patience than intervention.
- Milestones: the day 14 to 16 scan that confirms pregnancy and, critically, checks for twins (which mares almost never carry safely and which are usually reduced at this stage), the day 25 to 30 heartbeat scan, vaccinations, the move to the foaling environment and the final-days signs of bagging up and waxing.
- Gestation length and previous gestation (advanced): override the 340-day average, or enter how long this mare carried last time. A mare's own history is the best predictor because individuals tend to repeat their pattern, so a previous gestation overrides the generic figure.
- Mare type (advanced): choosing thoroughbred adds a check against the industry's universal 1 January birthday, which drives February-to-May breeding so foals are born early in their official year; a late-summer foal would race against animals up to six months older.
- Today (advanced): the current day, month and trimester of the pregnancy and the days remaining, with a prematurity flag before day 320.
- EHV boosters (advanced): the dates five, seven and nine months after covering for the equine herpesvirus boosters that reduce abortion risk. Flu and tetanus are timed four to six weeks before foaling so the foal inherits immunity through the colostrum.
- Move to foaling environment (advanced): the date to move the mare, four to six weeks ahead by default, so her colostrum matches the bugs where the foal will live.
- Foal heat and weaning (advanced): mares can conceive again on the foal heat barely a fortnight after delivery (about day 9 is typical), and most UK studs wean at five to seven months; set your own weaning age.
- Expected foal birth weight (advanced): about 10% of the mare's normal weight, so a 550kg mare produces a 55kg foal. The healthy aftermath follows the 1-2-3 rule: standing within an hour, nursing within two, placenta passed within three. Once the waters break the foal should arrive within 20 to 30 minutes; red-bag deliveries and slow progress are genuine emergencies.
- Month-by-month schedule (advanced): the start date of each of the eleven months with the scans, boosters, feeding changes and preparation that belong in each, with the current month marked. A guide, not veterinary advice.