Use of Ultrasonography in Fetal Development and Monitoring
Qatar Racing and Equestrian Club, Doha, Qatar
Ultrasonography offers safe and continuous viewing of fetal life in the mare, from completion of fetal organogenesis (day 40) to term. The combination of transrectal and transabdominal scanning techniques provides extensive investigation of fetal growth and development and monitoring of the fetal environment [1,2,3,4,5,6,7,8,9]. Doppler technology represents an additional tool to assess fetal viability, by characterizing blood flow through maternal, fetal, and placental circulations. Clinical applications of Doppler technology to the pregnant mare are currently limited by the necessity of specific equipment, the difficult visualization of some vascular structures in advanced gestation (i.e. the umbilical cord), and the lack of reference values for physiologic and pathologic conditions of equine gestation.
Applications of ultrasonography in early gestation (40–120 days) generally include identification and management of multiple fetuses [10,11] and fetal sexing [12,13,14]. In addition, elective feto-placental evaluations are carried out from 4 months of gestation to term in mares with histories of previous complicated pregnancies and potentially at risk of an unfavorable gestational outcome. Emergency feto-placental evaluations are performed when gestational complications or maternal disease are recognized and fetal distress or compromise may be anticipated [4,5]. Repeated examinations are usually required to assess fetal viability and response to treatment. Finally, routine scanning of term mares is carried out on some breeding farms to assess presentation and identify other factors that may complicate birth, suggesting the need for assistance to the mare at delivery or to the neonate shortly after birth.
Equipment and Technique
Linear, convex and sector ultrasound technologies can all be successfully used to image the equine pregnancy. Complete ultrasonographic feto-placental assessment is based on the acquisition of a series of biophysical parameters that requires both transrectal and transabdominal imaging. Transrectal ultrasound scanning provides access to the caudal aspects of the gravid uterus and should always be performed at any stage of gestation. Ultrasound scanning per rectum usually employs linear technology with frequencies ranging from 5–10 MHz. Convex and sector transducers better adapt to the curvilinear contour of the mare’s abdomen and are preferred for percutaneous ultrasonography of the gravid uterus. The equine fetus becomes visible by this route from day 100 of gestation on. A wide range of frequencies, spanning from 2–8 MHz, is needed by this route to reach scanning depths of up to 30 cm and visualize the growing fetus from mid-gestation to term. Optimal skin preparation greatly enhances image quality, as per standard percutaneous ultrasonography.
The sonographic profile of the feto-placental unit requires the establishment of a minimum database to ensure adequate fetal growth and development for the stage of gestation and to demonstrate appropriate levels of activity and responsiveness within an adequate environment.
Fetal Growth and Development
Several parameters can be measured to estimate fetal size in order to identify growth trends. Orbital diameters/eye volume [9,15,16] (Figure 19.1), aortic diameter (Figure 19.2), bi-parietal diameter (Figure 19.3), and, to a lesser extent, fetal chest and femur length  have all been reported as useful indicators of fetal growth. The aortic diameter correlates to fetal size more efficiently than any other anatomic structure and measurement should be taken in systole, on a longitudinal scan of the fetal left hemithorax, in close proximity to the spinal cord  (Figure 19.4). Ultrasonography provides excellent anatomic detail of the fetus in mid to late gestation (up to 9 months) (Figures 19.5, 19.6, 19.7, 19.8), when fetal sexing diagnosis can be easily accomplished (Figure 19.9, 19.10, 19.11, 19.12, 19.13) and congenital abnormalities identified. Developmental abnormalities commonly detected during late gestational scans include microphthalmus (Figure 19.14), hydrocephalus, small and large intestinal segmental atresia (Figure 19.15), and renal abnormalities (Figure 19.16). After 9 months of gestation, the quality of the image may decline due to fetal size and positioning within the mare’s abdomen. Ossified remnants of the vitelline sac can be visualized from early to late gestation with no need for concern (Figure 19.17).