Transrectal Ultrasonography of Early Equine Gestation – the First 60 Days
Early Winter Equine PLLC, Lansing, NY, USA
Direct examination of the mare’s reproductive tract for the purposes of pregnancy detection and evaluation 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 early and accurate detection of pregnancy . Prompt detection of mares who fail to become pregnant on a given bred cycle, or whose pregnancies fail to thrive prior to 35 days of gestation, presents the opportunity for follow-up diagnostics and rebreeding attempts within the confines of a limited breeding season . Early detection of multiple embryos (see Chapter 17) provides the opportunity for effective reduction management 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.
Transrectal examination of the reproductive tract of the mare is a standard part of any reproductive practitioner’s repertoire of technical skills. Appropriate restraint of the mare is indicated for both the mare’s and the clinician’s safety . The risk of damage to the rectal mucosa and the possibility of producing a fatal rectal tear during the course of any transrectal examination should be foremost in the examiner’s mind and govern the movement and manipulation within the rectum so that it is gentle and careful. The ample use of rectal lubricant facilitates the safe removal of fecal material from the mare’s rectum and the safe movement of the examiner’s hand and arm against the rectal mucosa, as well as providing effective contact between the ultrasound probe and rectal wall [3,4]. In the case of pregnancy diagnosis and evaluation it is important that practitioners be gentle in their manipulation in order to thoroughly ultrasound the mare’s tract without damaging a developing embryo.
Once all fecal material has been safely evacuated, the practitioner first performs a gentle palpation of the mare’s uterus, cervix, and ovaries. This helps orient the position of the structures in the “mind’s eye” and provides for an assessment of uterine and cervical tone. Visualizing an accurate mental picture of the reproductive anatomy of the mare’s ovaries, uterus, and cervix during the course of both the palpation and ultrasound examinations of the tract will help accurately guide the examiner’s hand and facilitate interpretation of what is being felt and visualized.
Uterine tone in the normal pregnant mare will be consistent with diestrus and becomes more prominent, with a distinct “two-humped” feeling to the base of each uterine horn (uterine bifurcation between them), beginning around 1–15 days of gestation. Uterine tone goes on to become especially pronounced and tubular between 18 and 30 days. 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 facilitating a complete examination. Starting at around 30 days of gestation (especially in maiden mares) the development of a palpable ventral bulge in the base of one uterine horn will aid in the identification of pregnancy. The cervix in the normal pregnant mare will be tubular and firm. Once palpation is completed the ultrasound probe can be introduced into the mare’s rectum.
In most instances it is standard practice from 0–60 days of gestation to use a 5–10 MHz linear probe to perform the ultrasonographic pregnancy examination in real time . 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 possibly even damaging, the structures being examined (Figure 16.1). The early spherical equine embryo is mobile within the mare’s uterus for approximately the first 16 days of gestation until it becomes “fixed” within the uterine lumen. Therefore, detection of an equine pregnancy prior to fixation requires the practitioner to be able to scan the entire uterine lumen during the course of the ultrasound examination before the presence of an embryo or embryos can accurately be determined.
The uterine body is imaged from cervix to apex in a longitudinal plane and the horns are imaged from base to tip in cross-section  (Figure 16.2). As the tip of each horn is visualized to its conclusion, each ovary should then be imaged with careful note being made of the presence or absence and number of corpora lutea (CLs) present. The practitioner should develop a routine whereby they systematically and completely scan the uterus (Figure 16.3a). 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 orientation of the probe and the image. This helps the practitioner to stay on the uterus and notice if the probe skips over a section of the uterine horns (especially at the bifurcation). Likewise, 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 (Figure 16.3b). Lastly, carefully scanning the cervix and vagina and noting the bladder and urethra as the probe is withdrawn from the rectum completes the exam and helps check for the presence of uro- or pneumovagina.
Image Milestones in Embryonic, Placental, and Fetal Development
The normal equine pregnancy develops in a predictable sequence in regards to size and development of visible structures through the embryonic stage (days 0–39) and through the fetal period covered in this chapter (days 40–60) [5,6]. While being careful to be gentle and non-disruptive to the developing pregnancy it is good technique to carefully scan through the entire pregnancy vesicle and piece together a three-dimensional image in the mind’s eye of what is being presented on the screen.
Prior to day 9 post-ovulation, it is not possible via ultrasound to identify the developing blastocyst after it arrives within the uterine lumen from the oviducts on or about day 6 post-ovulation [5,6,7]. The image of the pregnant mare’s uterus at this stage will be consistent with a normal, non-pregnant diestrus mare. Uterine and cervical tone should be palpably increased relative to that of an estrus mare, and there should be no endometrial edema or free uterine fluid visible. One or more ovulatory CLs should be readily identifiable on one or both ovaries, and follicle sizes will depend on the point of follicular wave development at the time of the examination. In some cases (pristine uterus, excellent image quality, and sharp-eyed examiner) the developing embryo can be identified via ultrasound as early as day 9–11 when it is only <3–5 mm in diameter [5,6], but this is unreliable in most mares under field conditions and will necessitate re-examination regardless.
In the author’s experience the normally developing embryo (Figure 16.4) can typically be first reliably identified starting at day 12 post-ovulation when it appears as an approximately less than or equal to 1-cm diameter, anechoic (fluid-filled), circular (spherical) structure located within the uterine lumen. After day 12, ultrasound imaging detects the anechoic, fluid-filled yolk sac [5,6] within the uterine lumen in contrast to surrounding uterine soft tissue echogenicity. There is often a characteristic, definable specular reflection, visible as lines of bright echogenicity dorsally and ventrally, on the tangent of the circumference of the embryo that is parallel to the probe (Figure 16.5) [5,6]. The embryo maintains its general anatomy, but increases in size in a linear fashion through day 16 (Figure 16.6). The embryo remains mobile within the uterine lumen until its increasing size and increasing uterine tone at approximately day 16 causes it to become “fixed”, typically at the base of a uterine horn [5,6]. This embryonic migration is most pronounced around 14 days gestation when successful maternal recognition of pregnancy in the mare requires that the embryo make contact with the entire endometrial surface in order to block prostaglandin release by the uterus [5,6,8]. During the mobile period it is possible to visualize the contractions of the myometrium moving the embryo back and forth in real time if the examiner carefully stills the probe over the embryo and watches for them [5,6, and personal observation].