How long are horses pregnant?
About 340 days on average [5][6], or about 11 months[1][6]. Mares vary more than most animals: the sources call anything from about 310 to 370 days normal [4][5][6], and some mares go past 400 days[7][6]. Minnesota Extension says most pregnancies last 335 to 342 days [7].
| Source | What it says |
|---|---|
| Purdue vet school [5] | Average 340 days (315–365) |
| Penn State Extension [6] | 330–370 days, most around 340 |
| Minnesota Extension [7] | Usually 335–342 days; occasionally 315 to over 400 |
| Merck Vet Manual [4] | Normal 310 to more than 370 days |
Because the range is so wide, the due date is the middle of a two-month window, not a deadline. A foal born before 320 days counts as premature [6][4]; the calendar marks that day.
Worked example
A mare last bred on April 20, 2027 is due about March 25, 2028. The normal window runs fromFebruary 24, 2028 to April 24, 2028, and the premature line falls on March 5, 2028. Her udder will usually start to fill between February 26, 2028 and March 11, 2028.
Use the last breeding date if she was covered more than once, or the ovulation date if your vet scanned her, since that is when the pregnancy can begin.
Pregnancy checks and getting ready to foal
| When | What |
|---|---|
| Day 14–18 | First ultrasound and twin check. Vets usually scan at days 14–18 to confirm pregnancy and look for twins. [3][7] |
| Day 24–25 | Heartbeat visible on ultrasound. A heartbeat should show by about day 24–25. [3][7] |
| Day 320 | Foals born before this are premature. Birth before 320 days of gestation counts as premature. [6][4] |
| 7–42 days before the due date | At the foaling site. UMN Extension says 4–6 weeks before the due date; Penn State says at least a week before. [7][6] |
| 14–28 days before the due date | Udder starts to fill. Usually 2–4 weeks before foaling. [5][7][6] |
- Days 14–18: the first ultrasound confirms pregnancy and checks for twins, which rarely survive to term [3][7][2].
- Days 24–25: a heartbeat should be visible [3][7].
- Foaling site: Minnesota Extension suggests having the mare there 4–6 weeks before the due date[7]; Penn State says at least a week before [6]. The calendar shows that whole span.
- Udder and wax: the udder usually fills 2–4 weeks before foaling [5][7]. Beads of wax on the teats usually appear 1–4 days before [5], or about a day before[7].
- Milk tests: test strips for the pH or calcium of udder secretions can show when foaling is near. Purdue gives pH 6.4 or lower, or calcium above 200 ppm, as a 97% chance of foaling within 72 hours[5]; Penn State says pH below 6.5 usually means within 24 hours [6].
Common questions
How many months is a horse pregnant?
About 11 months: 340 days is a little over 11 calendar months [1][6].
Can a mare be pregnant for 12 months or more?
Yes. Pregnancies past 365 days are within or near the top of the normal range, and some mares carry for more than 400 days [7][6]. If your mare is past the latest date in the calendar, ask your vet to check her.
Will my mare foal at the same time as last year?
She may, but the sources here don’t give a figure for how closely a mare repeats herself, so the calculator sticks to the published averages. Compare its dates with her past pregnancies.
Does this work for ponies and draft horses?
The sources here give one range for mares in general, so the calculator uses it for all horses. Donkeys and mules aren’t covered.
These dates are planning estimates from published averages, not veterinary advice. Your vet can confirm the pregnancy, check for twins and advise on foaling care.
Sources
- Merck Veterinary Manual, table “Approximate Gestation Periods”Gestation by species; dog 62–64 days from ovulation, 58–72 days from breeding
- Merck Veterinary Manual, table “Features of the Reproductive Cycle”Estrous cycle lengths (cattle 18–24, sheep 14–20 days); dog puberty 6–24 months
- P. L. Sertich, “Pregnancy Determination in Horses,” Merck Veterinary ManualTwin check at 14–18 days; heartbeat at 24–25 days
- A. S. Munsterman, “Intensive Care and Emergencies in Neonatal Foals,” Merck Veterinary Manual (reviewed 2025)Normal gestation 310 to more than 370 days; premature before 320 days
- J. H. Koziol, A. C. Farr, “What to Expect When… (mares),” Purdue University VA-26-WAverage 340 days (315–365); udder fills 2–4 weeks before; waxing 1–4 days before
- O. Watson, “Foaling Season: Tips for Delivery,” Penn State Extension (2025)330–370 days, most near 340; premature before 320; move to foaling area a week or more before
- S. Madill, E. Malone, C. Ward, “Caring for your mare during breeding and foaling,” University of Minnesota ExtensionUsually 335–342 days, occasionally 315–400+; exams at 14–16 and 24–25 days; foaling site 4–6 weeks before