Equine Reproduction From Conception to Birth

DVM, DACT, Benjamin Espy

Feed, energy, labor, water bills, barns, employees, stud fees, transportation, and veterinary bills are all expenses associated with equine reproduction. To get the most out of your reproductive dollar, you must first define what your reproductive program’s goal is. Is it to breed horses for performance, display, or pleasure?

Horse embryos are not more fragile than those of other animals; rather, horses have low reproductive performance in general (ability to maintain a conceptus). Early embryonic loss can be caused by a variety of factors. A mare’s spontaneous abortion can be caused by a variety of factors including stress, fever, uterine infections, hormone imbalances, and twins. The egg is fertilized in the fallopian tubes before entering the uterus on Day 6 of pregnancy. This is crucial to know because the uterine environment can be manipulated up until this point. Once the embryo has entered the uterus, it must pass through the entire uterus in order for the mare to identify it. If the embryo does not touch all parts of the uterus by day 16, the mare will reject it and begin to show signs of estrus, signaling the start of the next “heat” cycle. If the mare recognizes the embryo’s presence, the embryo will attach itself to the uterine wall on or around Day 17. Transrectal ultrasound can be used to visualize a heartbeat and confirm embryonic viability as early as Day 26 of pregnancy.

Many people depended on the idea that most mares will come back into heat 17 to 20 days after breeding if they had not conceived before ultrasonography became commonly available. In some areas, veterinarians might examine the cervix with a sterile speculum to see if it was tightly closed (indicated pregnancy) or relaxed (not pregnant) (indicating the beginning of another heat cycle). By Day 30 to 35 of pregnancy, experienced equine vets can detect a mechanical bulge in a mare’s uterus. The following are typical intervals for checking mares: 14–16 days – confirms initial pregnancy and searches for twins. Day 26 to 30 – establishes the fetus’s existence and heartbeat. Day 45 – a routine inspection with no specific cause, as endometrial cups should have formed by this point. If the mare aborts her pregnancy on Day 40-45 or later, it’s doubtful she’ll be able to conceive again during the same breeding season. Day 60 – an optional examination that has grown increasingly relevant since the introduction of fetal sexing.

Thoroughbreds are more likely to have twins (25 to 35 percent of all conceptions). Quarter horses aren’t known for having them (five to 10 percent). Transrectal ultrasonography should be used to check for twins in all horses. Because the twin should be 14 or 15 days old, this author likes to check mares on Day 15 or 16. If you examine mares every day at Day 14, you’ll eventually miss a younger twin that is 12 or 13 days old and too little to see. It is far easier to reduce a twin before they become fixed at Day 17 of gestation, regardless of whichever day of pregnancy you check for twins. Crushing a twin is another term for reducing twins. Only since the invention of ultrasound has this been possible. Previously, vets and owners were typically unaware until the mare gave birth. Usually, the smaller twin gets crushed. The mare is usually evaluated 48 to 72 hours following the treatment to make sure the remaining embryo is still alive. Almost all twins (>90 percent) are aborted. Almost everyone perishes. In the uterus, there is a finite amount of space. Even if twins survive, they are frequently frail and/or nonviable.

Fetal sexing is a reproductive industry innovation that has been perfected in cattle but only recently become available in the equestrian business. The genital tubercle will migrate towards the tail to become the clitoris or towards the prepuce to become the penis starting on Day 58 of pregnancy. Between Days 60 and 70, or between Days 110 and 140, you can have fetal sex. Between Days 80 and 90 and after Day 140, fetal sexing is nearly difficult.


The finest reproductive organ conformation is found in athletic horses, who do not require calorie sacrifice to be alive. They will be able to devote all of their focus to cycling properly and maintaining a pregnancy. Avoid relocating your mare unless absolutely necessary. Ascertain that she is isolated from any newcomers. A mare’s diet does not need to be supplemented until she starts lactating. Obese mares have a hard difficulty getting rid of a fetus.

Drug companies don’t usually spend the time or money to have their medicine “certified for use on pregnant mares.” Pyrantel Pamoate (Strongid ®), Ivermectin, and fenbendazole (Panacur ®) are all routinely used deworming drugs on pregnant mares with no additional side effects than on non-pregnant mares.

Even if a mare is in the middle of her pregnancy, pasture turn out gives all the required activity, and a regular diet should keep her in good shape. Because of the increased fetal fluids and milk production, water supply is crucial. During the winter, keep an eye on automatic water supplies and ice-covered troughs.

Herpesvirus (EHV-1) can induce late-term abortion, hence mares should be vaccinated against it at five, seven, and nine months of pregnancy, if possible. Caslick’s procedures (where the vulva is sewed partially closed) should be opened or the patency of the vulvar lips should be validated thirty days before to foaling. All vaccinations for diseases against which you want the foal to be inoculated should be given to the mare first. Because the colostrum does not have enough time to build the necessary antibodies, vaccinations given to the mare during foaling DO NOT PROTECT the foal.

Because ascarid larvae can be transferred through the milk, the mare should be dewormed using an ivermectin-based solution. Prior to the widespread use of ivermectin, ascarid impaction was a common cause of colic and mortality in foals. Isoerythrolysis (NI) tests are performed within two weeks of birth. These are blood tests to see if the mare has become hypersensitive to the foal’s blood type. Hypersensitization to a foal’s blood type is more common in older mares. If the mare is NI (+), the foal must be muzzled throughout the first 24 to 36 hours of life and a colostrum donor found.

The gestation period of a normal mare varies greatly. Mares typically bear their fetuses for 320 to 380 days. The most generally quoted gestation length is 330 days (11 months). “How long do I wait before I feel worried?” is the most common question I get. Fescue toxicity is the most prevalent cause of delayed gestation and decreased milk production, although it is usually too late to restrict the mare’s fescue grazing before the due date, as it takes 60 to 90 days of restricted grazing to make a difference. I normally ultrasound for placental thickness or palpate for fetal activity if it pleases the owners. I’ve never discovered a dead fetus at term after an owner expressed concern about a long pregnancy. I NEVER recommend inducing labor, even though it is possible and has been done in research or controlled situations. Many competent and well-educated veterinarians have seen horrific side effects and the loss of both the mare and the foal on a frequent basis. Unless the mare’s health is in jeopardy, there is no cause to induce abortion. When considering the hazard, I believe that the owner’s convenience is a terrible excuse to induce labor. I encountered no fetus that benefited from induced foaling during my two years in Lexington dealing with Mare Reproductive Loss Syndrome (MRLS). Foals can survive, but an intensive care unit (ICU) with specialized critical care veterinarians and support workers is required. If a mare does spontaneously abort, the fetal membranes and fetus must be chilled or kept for necropsy, histopathology, or veterinary investigation.

Dripping milk or vaginal discharge could suggest an impending abortion or foaling. The most common occurrence is for a mare to abort without showing any clinical signs of illness. Two to four weeks before foaling, the udder will generally fill. Four to six days before foaling, the teats will normally distend. One to four days before foaling, “wax” will form on the teats. There are commercial tests available to screen for calcium levels in udder secretions. Water-hardness test strips can also be used for this. Calcium levels often rise 24 to 48 hours before foaling. The vulva’s relaxed appearance and motions in the flank “of the foal kicking” are inconclusive and should not be relied upon.

Reduce stress and keep an eye on the mare, as she has showed some capacity to control her labor. “The mare controls the hour, and the fetus determines the day.” For ages, outdoor foaling arrangements have been used. Stalls for foaling should be at least 14 feet long and 14 feet wide. Between deliveries, disinfect the floor. The best surface is straw. Shavings can cling to the eye and cause corneal ulcers in newborns.

LABOR IN THE FIRST STAGE: The majority of mares (>85%) give birth at night. Because the foal should be ready to run with the mare by daybreak, this is regarded to be a survival adaptation. Mare is tense. Kicks in the stomach. Nesting behavior is possible. Continuous up and down movement and profuse urination can be misinterpreted for colic. Within an hour of giving birth, many mares will begin to sweat… “Mare is getting hot,” says the narrator. Clean the perineal area and wrap the tail. This stage lasts around an hour on average. When the chorioallantois ruptures and a flood of fluid appears… The first stage has ended.

LABOR IN THE SECOND STAGE: 15 to 25 minutes is typical. Because many individuals will lose track of time due to the thrill of the event, it could be a good idea to start a stopwatch. Expect to see continued progress with the front hooves, snout, and ears, among other things.


“Breech delivery” vs. “caudal presentation” Check to see if the foal is breathing. Use a blunt tool to stimulate the nostrils. Using a towel, vigorously rub the area. When the foal is born, do not cut the cord as people do. Some studies assume that a small amount of blood passes into the foal via the umbilical artery after birth.

CHLORHEXADINE > IODINE should be used for disinfection.

LABOR IN THE THIRD STAGE: It should be considered an emergency if the placenta is not passed within three hours.


* The foal should be able to stand in one hour.

* By TWO hours, you should be able to nurse. (At this point, Placenta is typically over.)

* By THREE hours, the foal should be actively ingesting colostrums.

To aid in the passing of the meconium, foals should be given an enema (or two). The most prevalent type of colic in a newborn foal is meconium impaction. If the foal does not aggressively devour the colostrum, take it from the mare. You can always have colostrum administered through a nasogastric tube by your veterinarian.

If you’re milking the mare, try to extract 16 to 32 ounces of colostrum out of the udder. The first eight to twelve hours of life account for 80% to 85% of colostrum absorption. Get in the habit of drawing blood for an IgG test six to eight hours after foaling, and you should have enough IgG to test, as well as 3 to 4 hours left to feed colostrum via a nasogastric tube if necessary.

Post-partum care is usually not required for mares. To relieve swelling in their vulva or rectum, they may need phenylbutazone (Bute ®) or flunixin meglamine (Banamine ®). Take her temperature BEFORE you administer any drugs to a sick mare who has recently had a foal before you call your doctor. Fevers can be reduced using Bute ® and Banamine ®, therefore take a rectal temperature before reducing the mare’s fever artificially. It should be less than 101.5 degrees Fahrenheit. In febrile mares, retained placenta or endometritis is prevalent right after foaling. Colonic displacement is common in mares after foaling, and they can also burst their cecum or bladder during the process.

It is common for foals to have fetlocks that are so weak that they touch the ground. These normally correct themselves with age and exercise, and no bandages or splints are required. Thoroughbreds are more likely than other breeds to have fractured ribs. Your veterinarian should treat contracted legs or leg deviations that prevent lactation right away. The majority of foals are intended to be sent out with the mare the morning after they are born. Stall limitation is not required unless the foal has orthopedic issues that need limiting movement and exercise.

In 2016, the original author reviewed the work.

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