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Ultrasound for Pregnancy Diagnosis in the Mare

By: Dr. Sylvia Bedford-Guaus

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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.

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