Early detection of pregnancy in mares is strongly demanded by owners in order to maximize breeding efficiency and economic investment. The most reliable and earliest available tool for conceptus detection is ultrasonography. Other diagnostics include palpation per rectum and hormonal blood tests (i.e. estrone sulfate in serum or urine; equine chorionic gonadotropin in serum) as these methods are useful when examination by ultrasound (per rectum or transabdominally) is not possible or feasible.
Transrectal ultrasonography is an essential tool for reproductive veterinarians. However, this tool cannot replace palpation per rectum and should be used concurrently. This technique has its disadvantages such as the equipment can be expensive. Also, it can be difficult to scan transrectally between 3 and 5 to 6 months of pregnancy because the gravid/pregnant uterus extends into the abdomen, possibly out of reach of the examiner. Despite these constraints, this tool is invaluable especially for early pregnancy detection, twin management, and assessment of fetal viability.
Early Pregnancy Detection and Twin Management
First, the earliest an embryonic vesicle can be detected by ultrasound is 9-10 days post ovulation. By Day 14-15, the accuracy of vesicle detection increases to 99%. One of the most important roles of ultrasonography in equine pregnancy diagnosis is for twin detection and reduction. For initial pregnancy detection, it is beneficial to examine mares by ultrasound between Day 13-16. If a mare is noted to have double ovulated synchronously or asynchronously, this time period is ideal for imaging the potential twin vesicles and planning a manual reduction of one of the conceptuses. Between 16- and 17-days post ovulation, the increase in uterine tone, lack of uterine contractility, and cessation of embryonic mobility lead to the fixation of the conceptus at the base of one of the uterine horns. Later detection after Day 17-20 may increase the difficulty of manual reduction if both twins fix at the base of one uterine horn, termed unilateral fixation. If each conceptus occupies one uterine horn in bilateral fixation, it is easier to reduce a twin post fixation.
Between Day 21 and 25, detection of the embryo proper, which eventually develops into the fetus, is possible. If twins are still present, intervention via reduction or transvaginal aspiration can still be performed, but survival of the remaining twin tends to be lower. If twins were reduced before this period, this ultrasound can confirm one pregnancy is still present. Furthermore, the heartbeat in the embryo proper can be detected as early as Day 22 with some ultrasounds and consistently at Day 25 or after in most ultrasounds. If no embryo proper develops or the pregnancy is lost, the mare can be rebred before endometrial cup formation occurs. Twin management is reviewed in other articles. It is important to note that two ultrasounds should be performed before Day 30 of pregnancy to increase the chance of twin detection.
For monitoring embryonic and fetal growth, charts are available that show images of how conceptuses should appear on the ultrasound at particular time points. Being familiar with these images, a skilled reproductive veterinarian can approximate embryonic/fetal age or detect early embryonic loss. Using transrectal ultrasonography, practitioners can assess embryonic and fetal viability, evaluate placental health by measuring combined thickness of uterus and placenta (CTUP), and determine fetal sex during particular time periods. Prompt detection of pregnancy loss or placentitis allows for more efficient treatment and monitoring plan for the mare.
Furthermore, transrectal imaging of the genital tubercle between Days 55-75 may be used for fetal sexing. Between Day 90 and 150, transrectal ultrasound may be used to determine fetal sex from the gonads and/or external genitalia. The best time in this later window depends on the location of the gravid uterus.
Though not discussed at length in this article, transabdominal ultrasonographic examination can also be used after Day 80 until term for monitoring fetoplacental health including heart rate, movement, and development, for fetal sexing, and for late twin management.
Overall, it is beneficial to have multiple scans at particular time points to ensure proper embryonic/fetal growth and viability. If the mare has had issues with early embryonic loss, twins, fetal loss, or placentitis, additional ultrasonographic examinations may be performed according to your veterinarian’s advice.
These examinations allow owners to not only know that their mare is pregnant but also to prepare for an improved breeding plan for the following season, if the mare is open.
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