Ultrasound for Pregnancy Diagnosis in the Mare

Ultrasound has become the most common and reliable method for early pregnancy diagnosis in mares. Most equine practices own one of many commercially available portable ultrasound scanners.

For pregnancy diagnosis, real time linear array B-mode portable ultrasound machines that are fitted with a rectal (linear array) scanner or probe are used. For this procedure, the rectum of the mare is emptied of feces, and the lubricated probe is introduced into the rectum with the gloved arm to visualize the ovaries, uterus and the new developing embryo within.

Using ultrasound enables detection of pregnancy as early as 11 days post-ovulation. Most veterinarians, however, prefer to check your mare between 12 and 15 days post-ovulation, when the embryonic vesicle is larger and easier to find. The experienced veterinarian will follow every inch of the uterine horns and body to find the early embryo (which can be as small as a quarter of an inch in diameter) and to rule out the presence of twins for early twin reduction. Ultrasound during pregnancy also allows the veterinarian to monitor for normal growth and health of the embryo. Additionally, if early embryo loss is detected, one can arrange to rebreed the mare before the end of the breeding season.

Estrus (heat) in the mare lasts for an average of 5 to 7 days, and ovulation usually occurs towards the end of behavioral estrus. The egg within the follicle is released into the Fallopian tube at ovulation, where fertilization occurs if the mare has been bred at the appropriate time. The fertilized oocyte or embryo reaches the uterus around 6 to 7 days after ovulation, but at that time is still too small to be seen on ultrasound examination.

A peculiar fact about pregnancy in the mare is that the early embryo travels all around, covering every inch within the uterus, from days 7 to 16. This is a very important event in what we term maternal recognition of pregnancy; basically, the embryo is asserting its presence by preventing prostaglandin release from the endometrium (uterine lining), which would induce the mare to come back into heat, and thus the pregnancy would be lost. This is why when we perform early pregnancy diagnosis (before day 16) the embryo can be located anywhere in the uterus and thus it is very important to be thorough and systematic with the ultrasound examination. At around day 16, the embryo becomes too large for such movement and gets lodged at the base of one of the uterine horns, where development progresses.


Linear array B-mode real-time ultrasound scanners are the most commonly used for examination of the reproductive tract of mares. An ultrasound machine has two important parts: A transducer or probe and a processing computer with a monitor.

The transducer is the instrument that touches the part of the body to be imaged, in this case inside the rectum to visualize the reproductive tract of the mare. Piezoelectric crystals arranged in a linear mode within the transducer vibrate emitting sounds in a frequency above that heard by the human ear. These ultrasounds travel through different body tissue densities at different speeds and then bounce right back to the transducer, producing an image in different shades of gray, from black to white, is processed and seen in the monitor. There is no question that interpreting an ultrasound image requires training and experience.

Ultrasounds travel best through fluids giving rise to a pitch-black image on the monitor. The early equine embryo appears like a small vesicle or bubble filled with fluid (yolk sac stage), and therefore on our ultrasound screen we see a black circumference within the lumen of the uterus. After day 21 of the pregnancy, the embryo proper can be seen within a fluid-filled vesicle. Structures within the fetus appear in different shades of gray and the bones appear bright white.

For ultrasound examination of the reproductive tract, the mare must be properly restrained in stocks or held in hand standing with her rear end against the stall doorway. Most mares will tolerate the procedure very well, although occasionally a nose twitch or chemical restraint may be necessary. Proper restraint is not only important for safety of the veterinarian performing the procedure, but will also ensure that no harm is made to the mare's rectum. Ideally, the tail should be wrapped to avoid inserting tail hairs into the rectum. The arm fitted with a lubricated disposable plastic sleeve is placed inside the rectum for careful removal of feces, before scanning the uterus. If performed carefully, this is not a dangerous procedure and cautious palpation or ultrasound waves should not harm the early embryo or fetus.

In the mare, pregnancy diagnosis can be done as early as day 11 after ovulation, when a small vesicle (4 to 6 mm or a 1/4 inch in diameter) can be found anywhere within the uterine horns or body. However, diagnosing pregnancy at such an early stage requires good quality ultrasound equipment and extensive experience in the field. Therefore, most practitioners will check your mare at 12 to 14 days after ovulation when the vesicle is already 12 to 16 mm (around 3/4 inch) in diameter. At this stage, the vesicle is readily found in pregnant mares, she can still be bred back if she happens not to be pregnant, and it is still within the window of time that allows successful twin reduction if two embryos were to be found.

At the same time that pregnancy diagnosis is done, the veterinarian should also carefully palpate the uterus and cervix and check for tonicity (tightness). The early pregnant uterus feels very tonic on palpation and the cervix should always be closed. Evaluation of the ovaries with ultrasound should reveal the presence of a corpus luteum (or two in case of a double ovulation). The corpus luteum is the structure that develops in place of a follicle after ovulation, and it is responsible for progesterone release, the hormone that will allow pregnancy to be maintained and to progress normally.

Most pregnancy losses in mares occur within the first 45 days of pregnancy. Hence it is important to monitor the mare up to this stage in case rebreeding was to be performed within the same season. After initial pregnancy determination, recheck the mare at least around 21 to 25 days, and then again at 30 and 60 days after ovulation. Abnormal pregnancies, twin reductions and mares that have previously lost a pregnancy may warrant more frequent examinations.

Around 21 to 22 days, one can visualized the embryo proper for the first time. At 25 days the heartbeat can be seen for the first time as a flickering white spot in the middle of the embryo; this requires close observation. Between 25 and 30 days, the developing fetal membranes (placenta) can be visualized for the first time. The membranes and umbilical cord are completely developed by day 50. After this stage the fetus and fluid around it become larger as pregnancy advances. Days 60 to 70 are the best time to determine the sex of the fetus. Ultrasound examinations after day 70 are not usually required, unless a new problem is detected or the mare has lost a pregnancy at this stage prior. Your veterinarian may still want to check the uterus and fetus by palpation per rectum a couple of times during the rest of the pregnancy, so that your mare can be managed accordingly.

Detection of Problems During Pregnancy

Checking a pregnancy by repeated ultrasound examinations provides useful information about the health of the embryo or fetus. During the early stages (10 to 50 days) the size and/or appearance of the embryonic vesicle or of the embryo proper and associated membranes are useful indicators of embryonic health. Early growth of the equine embryo is consistent in the mare and one can easily determine whether an embryo is in trouble just by its size and general appearance. Additionally, after day 25, the presence of a heartbeat should be closely monitored.

Several signs are consistent with imminent embryonic or fetal loss. These include:

Uterine cysts are dilated lymphatics that are sometimes seen within the wall of the uterus or protruding over the lumen of the uterus as a fluid-filled pouch, most commonly present in older mares. These appear as fluid filled (black) vesicles on ultrasound examination, sometimes identical to an embryonic vesicle. When one or more of these cysts is detected on routine pre-breeding examination of the mare, one should carefully record the size and location, so they are not confused with an embryo when performing pregnancy diagnosis after breeding. Additionally, a cyst will not move, change in shape or grow significantly within a short period of time like the early embryo will.

Do cysts interfere with pregnancy? This is a controversial topic. Many mares successfully become pregnant and carry a foal to term despite the presence of one or several uterine cysts. Occasionally, however, it is thought that numerous and/or large cysts may interfere with early embryo migration (which is important for maintenance of pregnancy) and/or with early placentation. Therefore, if a mare has numerous cysts and she does not successfully maintain a pregnancy, removal of the cysts may be warranted.

Early twin detection and elimination of one of the embryos (twin reduction) is a very important management tool. Most mares that manage to carry twin foals to an advanced pregnancy stage (over 2 months) will probably abort both fetuses close to term (8 to 10 months) or will give birth to small weak foals that require intensive care to survive. Most twin pregnancies in the horse result from the fertilization of two different eggs (double ovulations). If you suspect that your mare could be carrying twins, either because she had a double ovulation in that breeding cycle or because she carried twins sometime in the past, you should have her checked no later than 14 days post-ovulation for appropriate management.

After day 16, the vesicles may become lodged together or in separate uterine horns. It has been found that about 70 percent of twin embryos become lodged in the base of the same uterine horn, and about 80 percent of those may spontaneously reduce to a single pregnancy. But what about the remaining 20 percent? Trying to reduce an embryo manually in these circumstances may result in pregnancy loss. On the other hand, twins lodged in separate horns after day 16, may be manually reduced to a single pregnancy, but the success rate for survival of one of the embryos diminishes as the pregnancy advances.

In any case, early detection and manual reduction before day 16 is the key to success. At this stage, both vesicles are still traveling around the uterus and one can ultrasound the mare at frequent intervals until each embryo is visualized at almost the tip of a different uterine horn. At that point, one embryo (usually the smallest) is moved towards the tip of its uterine horn and then manually crushed by holding and pressing over the uterus through the rectal wall. Several attempts or sessions may be necessary for successful reduction.

The procedure causes minimal discomfort to the mare, although she may be sedated if deemed necessary. Adjunctive progestagen and/or anti-inflammatory therapy is usually recommended to optimize survival of the singleton. Additionally, the mare should be checked several times in the following weeks to ensure that one of the twins was successfully reduced, and that the remaining embryo is maintained and growing as expected for its age.

Fetal Sexing

Ultrasound examination of the mare after day 58 of pregnancy will allow determination of the sex of the fetus. The best window of time for fetal sexing is between 60 to 70 days of pregnancy. After day 70, the fluid depth of the pregnant uterus does not allow proper examination of the fetus due to inadequate penetration of ultrasound waves when using portable ultrasound scanners fitted with a 5Mhz transducer. Determination of fetal sex requires extensive practice and experience. Guessing the sex does not yield any better results than assuming a 50:50 sex ratio.

The key to successful fetal sexing is the visualization of a structure termed the genital tubercle, which will become the prepuce and penis in the male, and the vulva in the female. Between 60 and 70 days of pregnancy, the genital tubercle appears like a bright white (hyperechoic) structure on the ultrasound monitor, just like bone. In the male fetus, the tubercle is observed just behind the opening of the umbilical cord on a sagital (longitudinal) section of the fetus. In a female fetus, there is nothing remarkable around the umbilical cord; on a cross section, however, the tubercle is found at the most posterior end of the fetus, in a centered point just in between the tail, hip bones and rear limbs. Needless to say that cooperation of the mare and fetus is needed to obtain all the images required for accurate sexing. For 100 percent accuracy, one must visualize an image consistent with one of the sexes while ruling out the opposite sex.

Ultrasound in Late Pregnancy

Mares are seldom checked by transrectal ultrasound examination late in pregnancy (after 3 to 4 months) unless a problem is suspected. If your mare develops a discharge through the vulva or early udder development and lactation at any stage during pregnancy you should seek immediate veterinary assistance, since your mare may have developed a placentitis (infection of the placenta). Early udder development is also consistent with the presence of twins and/or imminent abortion. Transrectal ultrasound in later stages of pregnancy allows evaluation of the fetal membranes (placenta) and associated fluids. If the chest of the fetus and thus the heartbeat is visualized, this is always the best indicator of fetal health and viability.

Signs on transrectal ultrasound examination consistent with placentitis and/or fetal death include:

Additionally, if a mare develops colic at any stage during pregnancy, palpation and ultrasound through the rectum will allow not only to check for compromise of the fetus, but also to rule out problems related to the pregnancy such as uterine torsion (twist) or broad ligament hematomas (bleeding through the uterine arteries), both of which can be life-threatening conditions for the mare. Transabdominal ultrasound (ultrasound done through the abdominal wall) at a referral practice may be required to confirm diagnosis of any of the above-mentioned problems. Furthermore, a mare with any of these problems may require intensive monitoring and care at a referral hospital.