How long is a horse's gestation period?

The average horse gestation period is 336 days (about 11 months), with a normal range of 320–362 days. Most mares foal between days 330 and 345. Colts (males) tend to be carried slightly longer than fillies (females) on average.

How do I know if my mare is pregnant?

The most reliable early methods are rectal ultrasound (detects pregnancy from day 14–16) and blood testing for equine chorionic gonadotropin (eCG, formerly PMSG) from day 40–120. A mare that returns to estrus 18–21 days after breeding is likely not pregnant. Abdominal distension becomes visible by month 8–9.

Do colts really gestate longer than fillies?

Yes — studies have consistently shown that colts are carried approximately 2–3 days longer than fillies on average. The difference is real but small, and there is significant overlap between the distributions, so breed and individual variation are much larger factors in any given pregnancy.

How can I calculate my mare's foaling date?

Use our horse gestation calculator — enter the breeding date and select the breed (Thoroughbred, Quarter Horse, Draft, Miniature, etc.) for a breed-adjusted estimate. You'll get the expected foaling date, the full normal range, and key milestones.

What are signs a mare is about to foal?

Signs of imminent foaling include: waxing (colostrum droplets on the teats, 12–24 hours before), softening of the muscles around the tailhead and vulva, udder filling (weeks before), restlessness, sweating, and frequent lying down. Most mares foal at night, usually between 10 PM and 4 AM.

What should I prepare for foaling?

Prepare a clean, large (14×14 ft minimum), well-bedded foaling stall. Have available: clean towels, iodine for the navel, a halter and lead, a foal coat for cold weather, a stomach tube and colostrum for supplementation if needed, and your vet's emergency number. Install a foaling camera for overnight monitoring.

Horse Gestation Period: Complete Guide

The horse gestation period averages 336 days — approximately 11 months. For horse breeders and owners, understanding the mare's pregnancy timeline is essential for planning foaling preparation, vaccination schedules, nutrition management, and veterinary care. Use our horse gestation calculator to calculate your mare's expected foaling date.

How Long Are Horses Pregnant?

Mares are pregnant for an average of 336 days, with a normal range of 320–362 days. This wide range — over 6 weeks — means that due date prediction in horses is less precise than in other livestock. A foal born at day 320 is considered full term; one born before day 300 is premature and requires intensive neonatal care.

One well-documented finding in equine reproduction is that colts (male foals) are carried slightly longer than fillies — on average 2–3 days. Draft breeds also tend to carry longer than lighter breeds. Season matters too: mares bred in early spring (when days are short) tend to have slightly longer gestations than those bred in summer.

Horse Gestation by Breed

Breed	Avg. Gestation (Days)	Notes
Thoroughbred	336	Standard reference breed
Quarter Horse	336	Similar to Thoroughbred
Arabian	337	Marginally longer
Draft Horse	342	Larger breed, longer gestation
Miniature Horse	320	Shorter gestation than full-size breeds

The Risk of Twins in Horses

Twin pregnancies are rare in horses but highly dangerous. Unlike cattle or sheep, mares are not physiologically capable of carrying twins to term successfully — the placenta cannot support two foals, and twin pregnancies almost always result in abortion, premature birth, or the death of one or both foals.

When twins are detected on early ultrasound (day 14–16), one embryo should be manually reduced by a veterinarian. This is a routine procedure done early in pregnancy and dramatically improves the survival of the remaining foal. For this reason, a day 14–16 ultrasound is standard practice in managed breeding programs.

Signs a Mare Is About to Foal

The most reliable signs of imminent foaling are:

Waxing — small droplets of colostrum (first milk) appear on the teat ends, typically 12–36 hours before foaling
Udder filling — the udder becomes full and engorged over the final 2–4 weeks
Ligament relaxation — the muscles and ligaments around the tailhead and hindquarters become visibly sunken
Restlessness — the mare gets up and lies down repeatedly, looks at her flank, sweats
Milk pH change — commercial test kits can detect electrolyte shifts in the milk that indicate foaling is within 24 hours

Most mares foal at night, typically between 10 PM and 4 AM, and most do so rapidly once active labor begins — usually within 15–30 minutes. A mare in active labor for more than 30 minutes without progress requires immediate veterinary attention.

Red Bag Delivery: A Foaling Emergency

A red bag delivery (premature placental separation) is one of the most time-critical emergencies in equine reproduction. It occurs when the red, velvety chorionic surface of the placenta — rather than the white, shiny amnion — appears at the vulva. The placenta has detached from the uterus prematurely, cutting off oxygen to the foal.

If you see red velvet at the vulva, immediately cut or tear open the membrane and extract the foal — every second counts. Call your vet simultaneously. This is the one situation where a bystander must intervene rather than allow the mare to deliver unassisted.

How to Calculate Your Mare's Foaling Date

Add 336 days to the breeding date for the expected foaling date. Our horse gestation calculator shows the full 320–362 day range and breed-adjusted estimates. For comparison with other animals, see our full gestation periods chart.