Horse Gestation Calculator

How to Use This Calculator

Enter the date of the last breeding or artificial insemination. Select the breed — Draft breeds and Miniature Horses have notably different average gestation lengths. Click Calculate Due Date to see the expected foaling date and a full milestone timeline from first heartbeat to foaling.

Because of the wide natural variation in horse gestation, the calculator shows both the average due date and the full 320–362 day window so you can plan your foaling watch accordingly.

Horse Gestation Period — What to Expect

The horse average gestation period is 336 days — approximately 11 months. The gestation period for horses is among the longest of common domestic animals, reflecting the advanced development required for a foal to stand and run within hours of birth. The gestational period for horses also varies more than any other domestic species — a range of 320–362 days is considered normal.

First trimester (days 1–100): The embryo is detectable by rectal ultrasound as early as day 14. A heartbeat is visible by day 25. Spontaneous early pregnancy loss is most common during this period. Twin pregnancies (problematic in horses) should be identified and managed before day 30.

Second trimester (days 100–225): The fetus is fully formed and grows steadily. The horses gestation period during this phase is relatively quiet — fetal movement can be felt and occasionally seen from the outside. The mare's nutritional requirements increase moderately. This is when fetal sex determination is most reliable via ultrasound.

Third trimester (days 225–336): The foal gains most of its birth weight. Colostrum production begins in the udder around day 270. The mare's energy requirements increase significantly, particularly in the final 90 days. Move the mare to the foaling area 2–4 weeks before the expected date so she can adjust to the new environment.

Most mares foal at night, typically between 10 PM and 4 AM. Foaling alarm systems and video monitoring are strongly recommended for valuable mares, as Stage 2 labor should complete within 30 minutes — prolonged delivery requires immediate veterinary intervention.

Signs Your Mare Is Pregnant

Veterinary ultrasound is the only reliable way to confirm pregnancy in the first two months — it can detect an embryonic vesicle by day 14–16 and a heartbeat by day 25. Once pregnancy is established, these physical and behavioral changes become useful reference points as the mare approaches foaling.

Early signs (months 1–3): the mare fails to return to heat around 18–22 days after breeding. Some mares become noticeably calmer and less receptive to a stallion or teaser. Coat condition often improves as hormone levels shift.

Mid-pregnancy (months 4–8): the abdomen gradually enlarges, most visibly along the lower barrel. Appetite increases steadily. Fetal movement can occasionally be seen or felt against the mare's right flank in later mid-pregnancy.

Pre-foaling signs (final 2–4 weeks): the udder begins to fill, teats develop “waxing” (small beads of dried colostrum) 1–4 days before foaling, the muscles around the tailhead relax and soften, the vulva lengthens, and the mare becomes restless, paces, and may repeatedly lie down and stand in the hours before delivery.

Breed-Specific Horse Gestation Periods

How Horses Compare to Other Animals

At 336 days, horses carry foals longer than almost any other common domestic animal — only donkeys and llamas gestate comparably long. The table below compares horse gestation to other species.

Frequently Asked Questions

Sources & References

• USDA National Agricultural Statistics Service — Horse inventory and production data
• Penn State Extension — Equine reproduction management
• University of Minnesota Extension — Mare gestation and foaling
• Merck Veterinary Manual — Equine pregnancy management

Related Calculators

Related Reading