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Ultrasound for Pregnancy Diagnosis in the Mare

By: Dr. Sylvia Bedford-Guaus

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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:

  • Irregular shape of an embryonic vesicle any time between 10 to 17 days, since at these stages the vesicle should be perfectly round in shape.

  • Embryonic vesicle that is small for its age as long as the ovulation date is known accurately.

  • The absence of a heartbeat in any exam performed after day 25

  • The presence of free fluid within the uterus, but outside the embryonic vesicle or fetal membranes. This is usually a sign of endometritis (uterine inflammation/infection).

  • Abnormal appearance of the membranes around the fetus, such as very bright white (hyperechoic) or being unattached from the surrounding uterine lining

  • Abnormal appearance of the fluid within the embryonic vesicle or fetal membranes, such as when small floating (white) particles are seen within the fluid.

    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.

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