What is the normal foaling window for a pregnant mare?

A mare's normal foaling window ranges from 320 to 365 days past confirmed ovulation. Deliveries before Day 320 are considered premature and typically require intensive neonatal veterinary care, whereas pregnancies extending beyond 365 days should be evaluated via transrectal ultrasound to assess placental health and fetal well-being.

When should a pregnant mare receive EHV-1 (Pneumabort-K) vaccines?

Equine Herpesvirus-1 (EHV-1) is the leading infectious cause of viral abortion storms in horses. Pregnant mares should receive killed EHV-1 vaccinations (Pneumabort-K +1b or Prodigy) at 5, 7, and 9 months of gestation. In the 10th month (4 to 6 weeks before foaling), administer annual core booster vaccines (Tetanus, EEE/WEE, West Nile, and Rabies) so protective maternal antibodies concentrate in the colostrum.

What are the earliest signs that a mare is pregnant?

The earliest clinical sign is failure to return to estrus (heat) 14 to 21 days after breeding. A veterinarian can confirm pregnancy via transrectal ultrasound as early as Day 14–16 post-ovulation (which also checks for twins). Palpation per rectum reliably confirms pregnancy by Days 30 to 45.

What are the physical signs that a mare will foal within 24 to 48 hours?

Key indicators of imminent foaling include: (1) "Waxing" of the teats—clear or yellowish drops of dried colostrum forming on teat tips 24–48 hours prior; (2) Extreme softening and relaxation of the sacrosciatic pelvic ligaments around the tailhead; (3) Lengthening and edema of the vulva; (4) Teat milk calcium levels spiking above 200 ppm (tested with water hardness test strips); and (5) Restless pacing, sweating along the neck, and flank-watching.

Horses • pregnancyVerified Formula

Horse Pregnancy & Foaling Due Date Calculator

Computes estimated foaling due date, trimester milestones, and pre-foaling vaccination deadlines for pregnant mares.

Parameters

Calculated Output

Estimated Time to Foaling Due Date

160Days Remaining53% Completed

What This Result Means

On Day 180 of ~340 (First Half of Gestation).

Calculation Details & Equivalents

Gestation Progress

53%

Current Stage

First Half of Gestation

Normal Foaling Window

Days 320 – 365

Veterinary & Animal Science Advisory

This calculator provides evidence-based estimates derived from published veterinary equations (NRC, WSAVA, FAO). Outputs are intended for educational and decision-support guidance only. Always consult a licensed veterinarian or qualified specialist for clinical diagnosis, treatment planning, or prescription drug administration.

Understanding the Horse Pregnancy & Foaling Due Date Calculator

What This Calculator Does

This horse pregnancy calculator estimates your mare's foaling due date and maps out essential gestational milestones based on the date of ovulation or breeding. Equine gestation averages 340 days (roughly 11 months and 10 days).

How to Use This Calculator

Enter the number of days elapsed since breeding or confirmed ovulation. The calculator displays the gestation completion percentage, estimates days remaining, and highlights critical management deadlines such as pre-foaling vaccinations and nutritional adjustments.

Key Factors That Affect the Result

Broad normal gestation window: Horse pregnancy is uniquely variable; healthy foals are safely born anywhere between 320 and 365 days.
Late gestation growth surge: Foals accumulate over 60% of their body weight during the final 3 months (months 9 to 11); mares require increased digestible energy and trace minerals during this period.
Herpesvirus (EHV-1) protection: Pregnant mares must receive Pneumabort-K vaccinations at 5, 7, and 9 months of gestation to prevent viral abortion.
Signs of imminent foaling: Look for udder enlargement 2–4 weeks prior, teat waxing (calcium-rich colostrum beads) 24–48 hours prior, and relaxation of the pelvic ligaments.

Why This Matters for Your Horses

Tracking your mare's gestation timeline ensures you administer vital pre-foaling booster vaccines in time for maternal antibodies to concentrate in the colostrum, prepare a clean foaling stall, and attend the delivery.

Veterinary Advisory: This calendar provides educational timeline estimates. Foaling dates vary naturally between individual mares and seasonal day-length. Always partner with an equine veterinarian for reproductive ultrasound confirmation and foaling attendance.

Reviewed & verified by Dr. Imad Khan, DVM, MSc · Doctor of Veterinary Medicine & Masters in Poultry Science · View editorial credentials →

Formula & Mathematical Methodology

Standard Mathematical Formula

Horse Gestation = 340 Days Average from Ovulation (Normal Range: 320 to 365 days)

Formula Variables Defined

DaysDays Since Breeding(days)

Elapsed days

Step-by-Step Calculation Example

Example: 340-Day Horse Gestation

340 days - 180 days = 160 days remaining until estimated foaling

Final Output: Day 180 of 340 — Mid-Gestation (160 Days Remaining)

Key Assumptions

Single foal gestation.

Biological Limitations

Mares have broad normal gestation variation (320-365 days).

Verified Scientific & Veterinary Sources

Version: 1.2.0•Methodology Reviewed: 2026-09-23

1. The Merck Veterinary Manual (11th Edition)textbook

Aiello, S.E. and Moses, M.A. (Eds.) (2020). Merck & Co., Inc., Kenilworth, NJ

Access Source Authority

2. Relationship between condition score, physical measurements and body fat percentage in marespeer-reviewed

Henneke, D.R., Potter, G.D., Kreider, J.L., and Yeates, B.F. (1983). Equine Veterinary Journal, 15(4), 371-372

Horse Pregnancy & Foaling Due Date Calculator — FAQs

The average gestation period for a horse is 340 days (roughly 11 months and 10 days). However, equine pregnancies have wide biological variation: healthy, full-term foals can arrive safely anytime between 320 and 365 days. Thorough-breds and warmbloods often carry 340–345 days, while pony breeds frequently foal closer to 330–335 days.