How the horse gestation calculator works
A mare's foaling date is the cover date plus the length of her pregnancy. Purdue's College of Veterinary Medicine puts the average at 340 days, with a normal range of 315 to 365 days. That range is wide, so the calculator gives you three things: the 340-day date, the narrower window most mares foal in, and the full normal range.
Want every date at once? Print the horse gestation chart: every cover date of the year with its foaling date, on two pages.
1. Enter the first and last cover
Mares are often covered or inseminated more than once in a heat. The due date counts from the last cover, which is usually closest to ovulation. The window counts from the first cover too, in case she caught early. Vets count gestation from ovulation, so if yours confirmed the ovulation date, enter that date in both boxes.
2. Read the foaling date and the window
The University of Minnesota says a normal pregnancy is usually 335 to 342 days. The calculator shows that window from the first cover's early end to the last cover's late end. Plan to be watching closely across the whole window, not just on the due date.
3. Plan the milestones
- Days 14 to 16: Colorado State's reproduction lab recommends the first ultrasound now. An exam on or before day 16 is the time to find and manage twins. A heartbeat usually shows around day 25, and Minnesota lists a second exam at day 30 to 35.
- First 60 days: Purdue advises little stress, no transport, and no vaccines or dewormers.
- Months 5, 7 and 9: Purdue and the University of Georgia advise a rhinopneumonitis (EHV-1) vaccine to help prevent viral abortion. The calculator counts calendar months from the last cover.
- 90 days out: Georgia advises moving mares off tall fescue pasture, which can cause foaling problems.
- 4 to 6 weeks out: give the pre-foaling vaccines so the colostrum carries antibodies to the foal, and have the mare settled at the foaling site (Georgia and Minnesota).
- 2 to 4 weeks out: the udder usually starts to fill. Teats fill out 4 to 6 days before foaling, and wax beads on the teats show 1 to 4 days before (Purdue).
Worked example
A mare is covered on Fri, May 1, 2026 and again on Sun, May 3, 2026. 340 days after the last cover is Thu, Apr 8, 2027. If she caught on the first cover she would be due Apr 6, 2027. Most mares foal within Apr 1 to Apr 10, 2027, and anything from Mar 12, 2027 to May 3, 2027 is inside the normal range.
| First ultrasound, check for twins | May 17 to May 19, 2026 |
|---|---|
| Heartbeat visible on ultrasound | May 28, 2026 |
| Second pregnancy exam | Jun 2 to Jun 7, 2026 |
| End of the first 60 days: no vaccines or deworming before this | Jul 2, 2026 |
| EHV-1 rhinopneumonitis vaccine, month 5 | Oct 3, 2026 |
| EHV-1 rhinopneumonitis vaccine, month 7 | Dec 3, 2026 |
| Off tall fescue pasture, 90 days out | Jan 8, 2027 |
| EHV-1 rhinopneumonitis vaccine, month 9 | Feb 3, 2027 |
| Pre-foaling vaccines, mare settled at the foaling site | Feb 25 to Mar 11, 2027 |
| Udder starts to fill | Mar 11 to Mar 25, 2027 |
| Start of typical foaling range | Apr 1, 2027 |
| Expected foaling date | Apr 8, 2027 |
| End of typical foaling range | Apr 10, 2027 |
Where the sources differ
The normal range depends on who you ask. Purdue gives 315 to 365 days. Minnesota says usually 335 to 342, but occasionally from 315 to more than 400 days. If you have records of this mare's past foalings, type her usual number in the gestation box.
Vaccine timing differs a little. Purdue lists the core pre-foaling vaccines 30 to 45 days before the due date, Georgia says 4 to 6 weeks, and Minnesota mentions 3 to 6 weeks, sometimes split into two visits. Georgia words the EHV-1 schedule as during the fifth, seventh and ninth months; Purdue says at 5, 7 and 9 months. Your vet will set the exact dates.
Sources
- Purdue University College of Veterinary Medicine: What to Expect When Your Mare Is Expecting (VA-26-W)
- University of Minnesota Extension: Caring for your mare during breeding and foaling
- Colorado State University Equine Reproduction Laboratory: Pregnancy examination
- University of Georgia Extension: Managing the Pregnant Mare (B1465)