Enter the date your mare was bred to get her estimated foaling date, the normal 320–370 day foaling window, how far along she is today, and a calendar of ultrasound checks, vaccines and foaling preparations.
Last updated:
Breeding dates
Or the date of insemination / confirmed ovulation.
days
340 days is the usual average. If your mare has foaled before, use her own typical length.
Enter your values to see the result.
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How to use the Horse Foaling Date Calculator
Choose single or range: one cover or AI date, or the first and last dates the mare ran with the stallion.
Enter the date she was covered or inseminated.
Adjust the gestation length if you know your mare's history; otherwise keep 340 days.
Read the due date, foaling window and how far along she is today.
Use the calendar to book vet checks and plan vaccines and foaling preparations.
Horse gestation formula
Due date = cover date + 340 days (about 11 months and 1 week). The normal range is 320 to 370 days. With pasture breeding, the earliest likely date is the first cover + 320 days and the latest is the last cover + 370 days.
Mare pregnancy timeline
Gestation day
Approx. month
What happens / what to do
14–16
Week 2
Ultrasound to confirm pregnancy and check for twins
25–30
Month 1
Heartbeat visible on ultrasound
45–60
Month 2
Recheck; endometrial cups form and early losses become less likely
55–70
Month 2
Fetal sex can be seen by ultrasound
150, 210, 270
Months 5, 7, 9
EHV-1 vaccine doses commonly given
227+
Last trimester
Most fetal growth; energy, protein and mineral needs rise
250–280
Months 8–9
Remove from endophyte-infected tall fescue
298–312
Month 10
Pre-foaling vaccine boosters
310–326
Month 10–11
Move to the foaling area; begin foaling watch
320–370
Month 11–12
Normal foaling window; due date ≈ day 340
This calendar is a general planning guide, not veterinary advice. Work out vaccines, deworming and feeding with your equine vet, and call them right away if the mare shows colic, early milk dripping, vaginal discharge, or has been in labor (water broken) for more than 20–30 minutes without progress.
Foaling kit checklist
Vet's phone number and a plan for after-hours emergencies
Clean, well-bedded foaling stall (straw rather than shavings), good lighting and a camera or foaling alarm
Navel dip supplies as your vet recommends and a watch to note times: the foal should stand within about 1 hour, nurse within 2 hours, and the placenta should pass within 3 hours
Frequently asked questions
How long is a horse pregnant?
About 11 months. The average gestation is close to 340 days, but healthy foals are born anywhere from about 320 to 370 days. Foals born before 320 days are considered premature.
How do I calculate a mare's due date?
Add 340 days to the date of the last cover, insemination or confirmed ovulation. A mare covered on April 15 is due around March 21 of the next year (or March 20 in a leap year).
Why do mares foal earlier or later than 340 days?
Gestation length varies with the individual mare, breed, season and foal sex. Mares bred early in the year often carry longer than those bred in late spring, and colts tend to be carried a few days longer than fillies. Many mares repeat their own pattern, so a mare's past foaling records are the best guide.
When should a mare be checked for pregnancy?
Most vets do the first ultrasound 14-16 days after ovulation to confirm pregnancy and catch twins, then check for a heartbeat at about 25-30 days and again at 45-60 days. Mares can also lose pregnancies later, so a fall recheck is common.
When should a pregnant mare be vaccinated?
A common plan from equine vets is an EHV-1 (rhinopneumonitis) vaccine at months 5, 7 and 9 of pregnancy, and core vaccine boosters 4-6 weeks before foaling so protective antibodies pass into the colostrum. Your vet will tailor the schedule to your area and the mare's history.
What are the signs a mare is about to foal?
The udder fills 2-4 weeks before foaling, the muscles around the tailhead soften, the vulva relaxes and lengthens, and wax-like beads often form on the teats 1-4 days before. Milk calcium test strips can help predict foaling within the next day or two. Most mares foal at night.
When does a mare come back into heat after foaling?
The first heat after foaling, called foal heat, usually starts 5-12 days after the birth. Some breeders rebreed on the foal heat; others wait for the next cycle about 30 days after foaling for better conception rates.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.