Transrectal Ultrasonography of Early Equine Gestation – the First 60 Days

Transrectal Ultrasonography of Early Equine Gestation – the First 60 Days

Chelsie A. Burden

Goulburn Valley Equine Hospital, Victoria, Australia


Early examination of the mare’s reproductive tract for the purpose of pregnancy detection, evaluation of the uterine environment, and development of the pregnancy during the first 60 days of gestation is best accomplished via transrectal palpation and ultrasonography. Effective and efficient breeding management of the mare is facilitated by an accurate detection of pregnancy. Furthermore, prompt detection of mares who fail to become pregnant on a given breeding cycle, or whose pregnancies fail to develop properly during early gestation, present the opportunity for follow-up diagnostics and rebreeding attempts within the confines of a limited breeding season. Early detection of multiple or twin pregnancies provides the opportunity for effective reduction prior to endometrial cup formation. For the purposes of this discussion, gestational age will be measured as days from ovulation, where the date of ovulation is identified as day 0 (zero).

Transrectal Technique

Transrectal palpation and/or ultrasonography of the reproductive tract of the mare is a standard part of any reproductive practitioner’s examination. The practitioner is essential to successful transrectal examination on multiple levels. Palpation skills are necessary to locate structures of interest such as the uterus and ovaries and differentiate these organs from the intestinal tract [1]. Appropriate restraint of the mare is indicated for the safety of both the mare and clinician. The risk of damage to the rectal mucosa and the possibility of creating a fatal rectal tear during the course of any transrectal examination should be foremost in the examiner’s mind; movement and manipulation within the rectum should be gentle and vigilant to reduce such risk. Methods of restraint may include physical or chemical restraint. A mare might be placed in stocks or be twitched to reduce movement during examination, and foals should be positioned or confined to the front or side of the mare to reduce the anxiety of the mare [2]. A maiden mare or fractious mare may require sedation and/or muscle relaxants to reduce the risk of injury during transrectal examination. An alpha-2-adrenergic agonist, e.g., detomidine (0.01–0.08 mg/kg IV) and butorphanol (0.03–0.08 mg/kg IV) is a useful combination along with a smooth muscle relaxant such as N-butylscopolammonium bromide (0.3 mg/kg IV; Buscopan®) or propantheline bromide (0.014–0.07 mg/kg IV; Propan-B, 2.5–3 ml/500 kg mare) [3]. The ample use of lubricant facilitates the safe removal of fecal material from the mare’s rectum and the safe movement of the examiners hand and arm against the rectal mucosa, as well as providing effective contact between the ultrasound probe and rectal mucosa. A lubricant should be of the proper consistency and free of air bubbles to establish adequate contact and achieve the best image during ultrasonography. Furthermore, during pregnancy diagnosis and evaluation, it is important that practitioners be gentle but systematic in their manipulation in order to thoroughly ultrasound the mare’s reproductive tract without damage to the developing embryonic vesicle [2].

Once all fecal material has been safely evacuated, the practitioner first performs a gentle palpation of the mare’s internal reproductive tract to include ovaries, uterus, and cervix. The transrectal palpation allows orientation of the reproductive organs prior to ultrasonography along with evaluation of uterine and cervical tone. Visualizing an accurate mental picture of the reproductive anatomy will help accurately facilitate the interpretation of what is imaged with ultrasonography.

The cervix, in early pregnancy (10–30 days), can be palpated on the floor of the pelvis as a longitudinal, rigid, toned structure such as in diestrus. The ovaries should be palpated for completeness of examination; however, the corpus luteum is not palpable in the mare and adds little knowledge to pregnancy diagnosis. Uterine tone in the normal pregnant mare will be consistent with diestrus with a prominent tubular anatomy. The swelling of the uterus, indicating presence of a conceptus, is not palpable until 28 to 30 days gestation (slightly earlier in the maiden mare) at the base of the fixated horn. The swelling at this stage of gestation is a 3-4 cm in diameter ventral bulge [4]. With the development of pronounced uterine tone in the pregnant mare, the uterine horns may become kinked, especially at the base, sometimes turning back along the uterine body. Palpation can help identify this when it is present, and allows for gentle manual repositioning to facilitate a complete examination. By 60 days gestation, the conceptus swelling fills the pregnant horn and measures roughly 12 cm in diameter [4]. In the author’s experience, mares palpated under chemical restraint will often have less palpable tone to the reproductive tract due to smooth muscle relaxant effects of the sedative. Once palpation is completed, the ultrasound probe can be introduced into the mare’s rectum.

Imaging Technique

In most instances, it is standard practice to use a 5–10 MHz linear probe to perform transrectal reproductive ultrasonography in real time (B mode) [1,2]. All figures provided in this chapter were captured using a SonoSite M-Turbo® ultrasound system. Care should be taken to choose a probe that has smooth sides and rounded edges to protect the rectal wall during the course of the exam. The ample use of lubricant will help produce good contact between the probe surface and rectal wall with minimal pressure. The goal is a clear image without distorting the shape of, or possible damage to, the structures being examined (Figure 16.1). Scanning technique, instrument adjustment, and an understanding of the anatomic structures examined will provide the best image for interpretation [1]. It is helpful to position the ultrasound scanner close to the operator and at eye level to reduce body strain and facilitate viewing and adjustment of the image quality. Again, the mare must be properly restrained for safety and interpretation; excessive movement will interfere with image quality and could lead to misinterpretation of an image or inability to image the entire reproductive tract [1,2].

Figure 16.1 Probe cupped within the examiner’s hand allowing palpation of the internal reproduction tract while controlling the probe at all times. The tips of the practitioner’s fingers are free to gauge the rectal wall and “hook” the cranial aspect of the uterus facilitating complete imaging of the entire uterine lumen.

While well described in previous chapters, liquids (follicular fluid, yolk-sac fluid) do not reflect sound waves and produce a non-echogenic or anechoic, black image. In contrast, dense tissues (fetal bone) or an air interface reflect most of the sound beam producing a hyperechoic, white image. In addition to tissue density, the organization of the tissue results in characteristic patterns allowing identification of internal organs including the uterus and cervix. These patterns will vary depending on the reproductive status (estrus, diestrus, pregnancy) [4]. The early spherical equine embryo is mobile within the mare’s uterus for the first 16 days of gestation allowing maternal recognition of pregnancy to occur. Fixation occurs at the base of a uterine horn around gestational day 17 [4]. Detection of an equine pregnancy prior to fixation requires the practitioner to thoroughly scan the entire uterine lumen for presence of embryonic vesicle(s).

The uterine body is imaged from cervix to apex in a longitudinal plane and each horn in a cross-section [4] (Figure 16.2). As the tip of each horn is visualized to its conclusion, each ovary should then be imaged noting the presence or absence and number of corpora lutea (CLs) present (Figure 16.3). The practitioner should develop a routine whereby they systematically and completely scan the uterus (Figure 16.4a). When imaging the cross-section of the horns it is important to center the round, cross-sectional image on the ultrasound screen noting the direction of orientation of the probe and the image. This helps the practitioner stay on the uterus and avoid skipping over a section of the uterine horn (Figure 16.4b). This is especially true at the bifurcation as the dependent weight of pregnancy can position the uterus ventrally at the base of the gravid horn making visualization difficult in early gestation. When scanning longitudinally along the uterine body, gently rotating the probe along the long-axis so that the image of the uterine body appears as thick as possible dorsal to ventral, helps image the lumen of the body completely. Lastly, carefully scanning the cervix and vagina and noting the bladder as the probe is withdrawn from the rectum completes the examination and will highlight the presence of urovagina or pneumovagina.

Figure 16.2 (A) Longitudinal view of the uterine body sitting dorsal to the bladder. (B) Cross-section of the uterine horn sitting dorsal to the distended bladder.

Figure 16.3 (A) A single corpus luteum (arrow) appears as a homogenous structure next to an anechoic, fluid-filled follicle on the ovary. (B) An early pregnancy ultrasonography examination with two distinct corpa lutea on one ovary alerting the practitioner to the possibility of twin pregnancy as he/she completes the examination of the uterine horns and body.

Figure 16.4 (A) Cross-sectional ultrasonography of cranial aspect of the uterine horn. A 15-mm vesicle is present at the tip of the uterine horn. A technique for completely evaluating the uterus is important as the early pregnancy is motile and can be imaged throughout the entire uterine lumen from days 10 to 16 post-ovulation. (B) A technique for completely evaluating the entire uterus is described. As the bifurcation of the uterus is an easily palpable point of orientation: 1) ultrasonography begins at the base of one horned out to its tip, back down to the horn base; 2) thoroughly through the bifurcation from the base of one horn to the other; 3) from the base of the opposing horn out to the horn tip and back down again to the base of the horn and bifurcation; and 4) from the bifurcation and apex of the uterine body moving caudally through the uterine body back to the cervix. Longitudinal ultrasonography of the uterine body should be done slowly and methodically to visualize the entire lumen of the uterine body.

Early Embryonic Imaging

The normal equine pregnancy develops in a predictable sequence in regards to size and development of visible structures through the embryonic stage (< 40 days) and throughout the fetal period [4]. There is no significant difference in the size of the conceptus between pony mares and horses in the first 40 days of pregnancy development; however, fixation occurs earlier at day 15 in the pony mare than the horse at days 16 to 17 [5,6]. While being gentle and non-disruptive to the developing pregnancy, it is good technique to carefully scan through the entire embryonic vesicle and piece together a three-dimensional image in the mind’s eye of what is presented on the screen (Figure 16.5). Ideally, the practitioner will already have performed transrectal ultrasonography on the mare and made note of any abnormalities such as uterine cysts that are of similar size to a developing embryonic vesicle (Figure 16.6).

Figure 16.5 (A) Ultrasonography of the uterine horn containing the developing embryonic vesicle. The yolk sac is anechoic surrounded by the gray soft tissue density of the uterine wall. The spherical vesicle is positioned centrally within the uterine lumen; the toned uterine lumen “expands” immediately around the embryo accommodating its size while remaining in visual apposition immediately in front of and behind the embryo(s). While the early developing embryo is visualized as a simple growing anechoic sphere on ultrasonography, cellular differentiation is rapidly occurring. (B) During embryonic development (day 12) the embryonic vesicle is composed of a spherical layer of trophoblast cells (ectoderm) and endoderm. The capsule protects the embryo and helps the developing vesicle maintain a spherical shape around its yolk sac. By day 14, as seen in Figure 16.5A, a mesodermal layer is added with a specialized embryonic disc region expanded against the glycoprotein capsule [5].

Figure 16.6(A) A large cluster of uterine cysts visible at the uterine bifurcation. (B) A 23-mm embryonic vesicle is located adjacent to the cluster of cysts. The embryonic vesicle imaged is spherical and of appropriate size for the stage of gestation. If the practitioner is familiar with the mare or has previously mapped the cysts, this is enough to confirm the mare is pregnant. If in doubt, a repeat ultrasonography examination in 1 to 3 days to confirm growth or change in location of the developing vesicle is warranted.

Days 9 to 16

Although the developing embryo enters into the uterus between days 5 and 6 post-ovulation [4], it is not possible to identify a developing embryonic vesicle via ultrasonography until days 9 or 10 post-ovulation when sufficient yolk sac fluid has developed. By day 12, the vesicle reaches ~ 8 mm in size, growing 1.5 mm/day [4] (Figure 16.7). The stage of early diagnosis is dependent on practitioner skill and the quality of image and machinery available. Some practitioners may choose to delay early pregnancy detection until day 14 when the conceptus is 14 to 18 mm in diameter and growing at a faster rate (3.5 mm/day) (Figure 16.8a

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Nov 6, 2022 | Posted by in EQUINE MEDICINE | Comments Off on Transrectal Ultrasonography of Early Equine Gestation – the First 60 Days
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