Horse Gestation Period: How Long Is a Horse Pregnant?

A horse is pregnant for about 11 months, which works out to about 340 days. Most mares foal between 335 and 342 days after breeding, according to University of Minnesota Extension, and the University of Nebraska-Lincoln (UNL) counts anything from 320 to 370 days as normal.

So your mare doesn't really have a due date. She has a due window, and it can be several weeks wide. That is why the watching starts several weeks before the date on your calendar.

How Long Is a Horse Pregnant?

The published figures agree closely. UNL gives an average of 335 to 340 days, and Ontario's agriculture ministry says about 340 days, plus or minus 20, from breeding. A study of 433 Thoroughbred pregnancies dated by ultrasound found an average of 344 days, with live foals born anywhere from 315 to 388 days. A few pregnancies run past 400 days.

"Eleven months" is the breeder's shorthand, and it runs a little short. UNL notes that most mares carry longer than that, so a mare who passes the 11-month mark without foaling is usually right on schedule.

What shifts the length

• Colts tend to arrive 2 to 7 days later than fillies.
• Late spring and summer foalers usually carry for less time than January or February foalers. Artificial lighting in the last third of pregnancy can bring foaling about 10 days sooner.
• Thin mares tend to carry slightly longer.
• Each mare tends to follow her own pattern year after year, so her past foaling dates are one of your best guides.
• Grazing endophyte-infected tall fescue late in pregnancy can make gestation run long (more on that below).

How to estimate a due date

Count about 340 days (11 months and about 5 days) from the last breeding date, or from the ovulation date if your vet tracked it. For a mare last bred on May 1:

If February 29 falls inside the pregnancy, each date moves one day earlier. Treat the result as a starting point. In the last few weeks, your mare's body tells you more than the calendar does.

Horse Pregnancy Month by Month

Most of the vet work happens early, when twins and early losses are caught. Most of the watching happens late. The exam schedule below follows the Merck Veterinary Manual. The month labels are a handy guide, not strict clinical stages.

Is she pregnant? The early clues

A mare who doesn't come back into heat two to three weeks after breeding may be pregnant. Under the influence of the pregnancy hormone progesterone, a pregnant mare usually will not show estrus (heat). That is a clue, not proof. Only your veterinarian can confirm it, usually by transrectal ultrasound (a probe passed into the rectum to image the uterus).

A slim belly rules nothing out either. Maiden mares (mares in their first pregnancy) often barely show. If words like maiden, filly and colt are new to you, our guide to horse terms covers them.

Why the first scan comes so early

Twins. Twin pregnancy is the most common noninfectious cause of abortion in mares, because the uterus usually can't carry two foals to term. When the vet finds twins early, ideally before about day 30, they can reduce the pregnancy to one embryo, a procedure the early scan makes possible.

Rechecks matter for another reason too: some pregnancies are lost. Minnesota Extension puts the chance at about 6% by day 40 and about 8% after day 40.

Vets can also feel the pregnancy by rectal palpation, sometimes from 25 to 30 days in experienced hands, more typically around six weeks. Blood and urine tests that detect pregnancy hormones are another option.

For the techs: around day 35 the embryo forms endometrial cups, which produce equine chorionic gonadotropin (eCG). A blood eCG test works between about 40 and 120 days after ovulation, but it gives a false positive if the pregnancy is lost after the cups form. Such a mare may not cycle normally for 3 to 4 months. Estrone sulfate, made by the fetus and placenta, can be measured in blood after about 60 days and in urine after about 150 days. Our horse anatomy guide covers the horse's main organ systems.

Caring for a Pregnant Mare

Pregnancy care is a plan you build with your veterinarian. The main pieces:

• Vaccines and deworming. Ideally these are up to date before breeding. Your vet sets the vaccination plan for pregnancy, including protection against equine herpesvirus type 1 (EHV-1), timed so antibodies pass to the foal through colostrum (the first milk).
• Nutrition. Her needs rise gradually in the last three months, not the first. A 1,000 to 1,200 lb mare typically gains 150 to 200 lb in that stretch, and a body condition score of about 6.5 to 7.5 (on the usual 1 to 9 scale) is the target at foaling.
• Exercise. Pregnant mares need daily exercise, such as turnout in a paddock or pasture. Long stretches of stall confinement before foaling may make impaction colic and leg swelling more likely. Ask your vet about riding.
• Fescue. Tall fescue infected with a fungus called an endophyte can cause serious trouble late in pregnancy. Penn State Extension lists prolonged pregnancy, abortion or stillbirth, a thickened placenta, a difficult birth and little or no milk. The standard advice is to move mares off infected fescue 60 to 90 days before foaling.

Signs That Foaling Is Near

From about day 315, start watching. The signs usually build in this order:

• The udder starts to develop 2 to 6 weeks before foaling.
• The area around the tail head and pelvis relaxes over the last 7 to 14 days.
• The teats fill about 4 to 6 days before.
• The vulva relaxes and lengthens in the final days.
• Waxing, small beads of dried colostrum on the teat ends, often shows up 1 to 2 days before. Some mares wax early, and some never wax at all.

Mares don't read the textbook. Maiden mares in particular may show every sign or none of them. Milk calcium test kits can help, and they are most reliable at telling you when a mare will not foal in the next 24 hours, which at least tells you which nights you can sleep.

Plan for night duty anyway: 75% to 85% of foals are born between 6 p.m. and 6 a.m.

Get ready by day 315

• Save your vet's emergency number in your phone and post it in the barn.
• Print the AAEP foaling monitor sheet and keep it with her.
• Decide who is on night watch.

The Three Stages of Foaling

Most mares foal without trouble: UNL puts difficult births (dystocia) at fewer than 10% of mares. When something does go wrong, though, minutes matter.

Stage 1: getting ready (about 1 to 4 hours). She gets restless. She may paw, switch her tail, get up and down, sweat on her flanks and behind her elbows, and urinate often. It can look a lot like colic. Stage 1 ends when the water breaks.

Stage 2: delivery (usually 10 to 30 minutes). A white, shiny membrane (the amnion) appears first. Inside it you should see two front feet, one slightly ahead of the other, soles down, then the nose resting on or near the knees. It moves fast.

Stage 3: the placenta. The placenta (afterbirth) normally passes within about 3 hours. Don't pull on it. Save it so your vet can check that it came out whole, because placenta left inside can lead to a uterine infection and laminitis.

Then comes AAEP's 1-2-3 rule, worth taping to the barn wall: the foal should stand within 1 hour and nurse within 2 hours, and the mare should pass the placenta within 3 hours.

When to Call Your Vet

Call your vet now if:

• Stage 2 goes 20 minutes with no progress. That is the call point on the AAEP foaling monitor sheet. If you aren't sure she is progressing, call anyway.
• The foal is coming any way other than two front feet and a nose.
• A red, velvety bag appears instead of the white membrane. This is a "red bag" delivery: the placenta has separated too early and the foal is losing its oxygen supply. This is an emergency. Call your vet now.
• The placenta hasn't passed within 3 hours.
• The foal comes early. UNL considers foals born before 326 days premature, and they usually need a vet straight away.

See your vet soon if:

• A mare already confirmed pregnant starts showing heat behavior. It can mean the pregnancy has been lost, so she needs a recheck.
• She develops an udder early, or you see any vaginal discharge. Both can be warning signs of a problem with the placenta or the pregnancy.

What about a mare who just keeps going? Past 370 days isn't automatically trouble, but fescue exposure is one known cause of long pregnancies. Your veterinarian decides when a long pregnancy needs a closer look.

A Few More Facts About Horse Pregnancy

• Mares cycle during the long days of spring and summer, so foals usually arrive the following spring.
• A mare usually has no more than one foal a year.
• Many mares foal into their 20s, but losses rise with age. Minnesota Extension reports about a 21% chance of loss by day 40 for mares around 20, against about 6% overall, so a pre-breeding vet check makes sense.

Want to work with broodmares and foals yourself? Read how do you become an equine veterinarian? on our blog.

Originally written by Amber LaRock. Updated by the I Love Veterinary editorial team.

Sources

• University of Minnesota Extension: Caring for your mare during breeding and foaling
• University of Nebraska-Lincoln: The Foaling Mare (NebGuide G1874)
• Ontario Ministry of Agriculture, Food and Agribusiness: Anatomy, physiology and reproduction in the mare
• Animal Reproduction Science (PubMed): Factors affecting gestation length in the Thoroughbred mare
• Merck Veterinary Manual: Pregnancy Determination in Horses
• Merck Veterinary Manual: Reproductive Cycle in Horses
• Merck Veterinary Manual: Abortion in Horses
• Merck Veterinary Manual: Parturition in Horses
• Merck Veterinary Manual: Perinatal Mare and Foal Care
• AAEP: Foaling Monitor Sheet
• Penn State Extension: Tall Fescue, a Potential Problem for Pregnant Broodmares

This article is for general information and is not a substitute for advice from your veterinarian.