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Ultrasound for Pregnancy Diagnosis in the Mare

By: Dr. Sylvia Bedford-Guaus

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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:

  • Abnormal thickening of the fetal membranes at or around the cervix.

  • Separation of the fetal membranes from the uterine lining with or without pockets of fluid in between membranes and uterus.

  • Presence of copious amounts of fluid in the vagina, just behind the cervix.

  • Numerous floating particles within the fluids surrounding the fetus, which may indicate the presence of pus or hemorrhage.

  • Absence of a heartbeat if the chest of the fetus is visualized.

  • Visualization of two different chests/hearts or two adjacent placentas is consistent with the presence of twins, but usually not successful when scanning the mare per rectum.

    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.

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