Mare Gestation Calculator: Horse Pregnancy Due Date
Enter the date your mare was bred and her breed to get the estimated foaling date, the normal early-to-late foaling window, and how many days of pregnancy remain. The calculator also shows her current trimester, a month-by-month veterinary care schedule, and lets you adjust the gestation length if your vet has given you a specific figure.
How horse gestation is calculated
A mare's gestation period is the number of days from the date she is bred (or the last successful cover date) to the date the foal is born. The average across all breeds is approximately 340 days - roughly 11 months - but individual mares vary widely. To estimate the foaling date, you simply add the expected gestation length to the breeding date: Foaling Date = Breeding Date + Gestation Days. Because a healthy foal can arrive 10 days earlier or later than this estimate, the calculator also gives you an early foaling window (breeding date plus 330 days for a 340-day mare) and a late window (plus 350 days). Foals born before day 320 are considered premature; foals born after day 370 are overdue, and a vet should be consulted.
How to use this calculator
Enter the date the mare was bred or the date of the last successful cover. Select her breed to auto-fill the typical gestation length for that breed. You can then adjust the gestation length manually if your vet has measured the foal and given a more specific estimate, or if this mare has a history of carrying longer or shorter than average. The calculator shows the estimated foaling date, the early-to-late foaling window, days elapsed, days remaining, and which trimester of pregnancy the mare is currently in. The care schedule below the result lists key veterinary and management milestones for every stage of the pregnancy.
The three trimesters of equine pregnancy
- Trimester 1 (days 1 to ~113): Embryo implantation, organ formation, and the highest-risk period for early pregnancy loss. Confirm pregnancy by rectal ultrasound at 14-16 days, check for twins at days 25-30, and do a final early check around day 45. EHV-1 vaccination is not yet needed, but baseline health should be established.
- Trimester 2 (days ~114 to ~227): The fetus grows steadily. Body condition is easier to maintain in this phase. Schedule EHV-1 boosters at months 5 and 7 to reduce the risk of abortion caused by equine herpesvirus. Nutritional demands increase modestly; keep the mare at a body condition score of 5-6.
- Trimester 3 (day ~228 to foaling): Rapid fetal weight gain. The foal acquires roughly 60% of its birth weight in the last three months. Feed quality matters more than quantity. Give pre-foaling vaccines at 4-6 weeks before the due date so antibody concentrations peak in the colostrum the foal will drink in its first hours of life.
Signs that foaling is approaching
Physical signs are more reliable than calendar math in the final two weeks. Watch for:
- Udder development (bagging up): The udder begins filling 2-6 weeks before foaling.
- Waxing: Beads of dried colostrum on the teats appear 12-48 hours before foaling in most mares.
- Relaxation of hindquarter ligaments: The ligaments on either side of the tailhead soften and sink 1-2 days before foaling.
- Behavioral changes: Restlessness, pawing, and looking at her flank are signs that labor is starting.
- Milk pH changes: Commercial test kits can detect the calcium rise in the milk that precedes foaling by 24-48 hours.
Factors that affect gestation length
Several factors can shift the actual foaling date away from the estimate:
- Season: Mares bred earlier in the year (January to March in the Northern Hemisphere) tend to carry slightly longer than those bred in spring or summer, likely because photoperiod influences hormones.
- Foal sex: Colts are carried an average of 2-3 days longer than fillies, though the difference is too small to be clinically useful.
- Mare age and parity: Older mares and maiden mares (first-time mothers) show slightly more variable gestation lengths.
- Nutrition and body condition: Mares in poor condition may foal early; obesity is associated with more complications than length changes.
- Individual history: A mare's own previous gestation lengths are the best predictor. If she has consistently carried to 345 days, that is a better estimate than the breed average.
Average gestation length by breed
| Breed | Average gestation (days) | Normal range (days) |
|---|---|---|
| Thoroughbred | 340 | 325-360 |
| Arabian | 339 | 320-358 |
| Quarter Horse | 340 | 320-365 |
| Warmblood (Hanoverian, Dutch, Trakehner) | 341 | 325-360 |
| Draft (Clydesdale, Percheron, Shire) | 341 | 330-365 |
| Miniature Horse | 330 | 310-355 |
| Pony breeds | 338 | 315-360 |
| Average (all breeds) | 340 | 320-370 |
Published averages vary by study. Individual mares often carry consistently longer or shorter than their breed average.
Frequently asked questions
How long is a horse pregnant?
The average horse pregnancy lasts 340 days, or roughly 11 months. The normal range is 320 to 370 days, and individual mares often carry consistently shorter or longer than the average. Foals born before day 320 are considered premature.
How do I calculate my mare's due date?
Add the expected gestation length to the breeding date. For example, a mare bred on January 1 with a 340-day gestation would be expected to foal around December 7. This calculator does that arithmetic for you and also shows the normal early-to-late foaling window of plus or minus 10 days.
What is the difference between the due date and the foaling window?
The due date is a single-point estimate: breeding date plus the average gestation length. The foaling window is the range of dates in which it is normal to foal - typically 10 days before to 10 days after the due date. Both ends of the window are healthy; foaling before day 320 or after day 370 warrants veterinary attention.
Do different horse breeds have different gestation lengths?
Breed differences are real but small - usually 1 to 3 days across common breeds. Drafts and Warmbloods tend to carry slightly longer than the average, Arabians tend to carry a day or two shorter, and Miniature Horses average around 330 days. Individual variation within a breed is much larger than the between-breed average.
Does a colt or filly gestation last longer?
On average, colts are carried about 2-3 days longer than fillies, but the difference is so small relative to individual variation that it is not useful for predicting the foaling date before birth.
When should I start watching for signs of foaling?
Begin monitoring closely about 2 weeks before the estimated due date - or the early end of the foaling window. Physical signs such as udder development, waxing of the teats, and ligament relaxation around the tailhead are more reliable predictors than the calendar alone. Most mares foal between 7 pm and 7 am.
What vaccines does a pregnant mare need?
The core vaccination during pregnancy is EHV-1 (equine herpesvirus / rhinopneumonitis), given at months 5, 7, and 9 of gestation to reduce the risk of abortion. Pre-foaling boosters for tetanus, equine influenza, and EHV-1 are given 4-6 weeks before the due date so antibodies peak in the colostrum. Always follow the schedule your own equine vet recommends.
Is it normal for a mare to go over her due date?
Yes. Mares routinely foal anywhere from 10 days early to 10 days late relative to their calculated due date, and some healthy mares carry to 360 days or beyond. The due date is an estimate based on averages. If your mare has not foaled by day 370, or if she shows signs of distress, contact your veterinarian.