Fig. 15. Transrectal scan of the caudal third of a Grade 237 cervix at 6 months gestation.
Fig. 17. Color and spectral Doppler flow evaluation of the left uterine artery at 8 months gestation.
Cervical Parameters
Recent data on cervical size and echotexture in the pregnant mare suggest a high degree of cervical tone maintained up to 9 months gestation, followed by progressive cervical relaxation until delivery.37 A high degree of correlation between cervical size and sonographic appearance was also demonstrated (Figs. 15 and 16).
Doppler Ultrasonography
Doppler ultrasonography represents an additional diagnostic instrument to characterize blood flow in the pregnant mare and provides an insight on fetal (umbilical and carotid arteries), maternal (uterine arteries) (Fig. 17), and placental circulations (intra- placental vessels). In addition, two distinctive color Doppler signal patterns differentiate male
from female fetal gonads, offering an auxiliary tool in the diagnosis of fetal gender.38 Doppler ultrasonography has become an impor-
tant clinical instrument for the assessment of pla- cental performance in healthy and high risk human pregnancies, but applications to the equine pregnan- cies are still limited due to the lack of reference values. In normal pregnancies, haemodynamic changes in the uterine arteries progress from a high resistance/low flow pattern during the first half of gestation to a low resistance/high flow system in the second half.39 The transition correlates closely with the onset of placental angiogenesis in response to fetal growth and the development of the placental microcirculation. Ousey et al (2012) reported a threefold total blood flow volume increment during late gestation (210 days to term) when fetal body mass seemingly increases three to fourfold.39 Doppler velocimetry indices of the umbilical vas- culature and carotid artery are currently being in- vestigated in order to establish fetal hemodynamic patterns throughout gestation. Signs of circulatory derangement indicating fetal hypoxia and intra- uterine-growth-restriction could then be identified as routinely done in the US evaluation of the human pregnancy.
Acknowledgments
Conflict of Interest The Author declares no conflicts of interest.
References 1. Curran S, Ginther OJ. Ultrasonic fetal gender diagnosis during months 5 to 11 in mares. Theriogenology 1993;40: 1127–1135.
Fig. 16. Transrectal scan of the cranial third of a Grade 437 cervix at 10 months gestation.
2. Holder RD. Fetal sex determination in the mare between 55 and 150 days gestation, in Proceedings. Am Assoc Equine Pract 2001;47:321–324.
3. Bucca S. Fetal gender determination from mid to advanced gestation by ultrasound. Theriogenology 2005;64:558–563.
