Ultrasonographic Cervical Parameters Throughout Gestation in the Mare

The equine pregnant cervix can be safely and effectively evaluated by ultrasonography to assess stage of relaxation. In normal pregnancies, minimal changes can be observed in cervical size, tone, and echo-texture up to 9 months of gestation. In contrast, cervical relaxation is a consistent feature during the last 2 months of gestation. Authors’ addresses: Somerton Equine Hospital, Friarstown Kildare Co, Kildare, Ireland (Bucca); Irish Equine Centre, Johnstown Naas Co, Kildare, Ireland (Fogarty); e-mail: [email protected].

1. Introduction

Placental pathology is one of the leading causes of fetal and neonatal loss in the horse, worldwide.^1,2  Placentitis in mares is reported to be most commonly caused by bacteria ascending through the vagina³  and breaching the cervical barrier. A key role of the cervix in the pathogenesis of ascending placentitis is implicit. A high degree of tone is present in the cervical canal to maintain its vital role as a physical barrier throughout gestation. Transient cervical relaxation has been anecdotally reported during the last month of pregnancy, based on rectal palpation and vaginoscopy findings. Objective evidence of cervical physiologic changes during pregnancy is currently lacking in the mare. Work by Day et al.⁴  demonstrated a high correlation between cervical palpation findings and ultrasound appearance in the nonpregnant mare, concluding that the sonographic characteristics of the cervix can be expected to be consistent with a given stage of the reproductive cycle. The present study aims to identify ultrasound parameters to characterize size and echo-texture of the equine cervix throughout gestation in normal pregnancies. 

2. Materials and Methods

For the purpose of this study, the cervix of 32 light breed horse mares was examined by ultrasound, at 2- to 3-week intervals, throughout gestation, from day 60 to parturition. Gestation was divided by month, as follows: G3, from day 60 to 90; G4, from 91 to 120; G5, from 121 to 150; G6, from 151 to 180; G7, from 181 to 210; G8, from 211 to 240; G9, from 241 to 270; G10, from 271 to 300; G11, from 301 to 330; and G12, greater than day 331. Each month was additionally divided into two, A and B, A being first and B second half of the month. Breeds included Thoroughbred, Standardbred, Arabian, and Quarter Horse mares between 3 and 21 years of age and variable parity. Ultrasound parameters recorded on each examination included cervical measurements at three designated areas (cranial, mid, and caudal diameter), quality grading, and combined thickness of the utero-placental unit at the cervical pole. Cervical length and tone were estimated by palpation during the course of each examination, and findings were also recorded. Maternal and neonatal criteria for inclusion in the study are summarized in Table 1.
 

Cervical Parameters

Cervical Size

Linear, real-time ultrasound technology (GE LogiqE; Sonosyte MicroMaxx) was used for the purpose of this study (mounted with an 8- to 10-MHz multifrequency transducer). All determinations were carried out by two experienced ultrasonographers. Mares were restrained in stocks, their rectum emptied, and the caudal reproductive tract assessed by palpation, including an estimate of cervical tone and length. The ultrasound transducer was then inserted into the rectal ampulla, and the combined thickness of the utero-placental unit was measured at the cervical pole, when gestation was sufficiently advanced for the caudal end of the fetal membranes to adjoin the cranial cervical os (around 60 days of gestation). The ultrasound probe was then placed along the longitudinal axis of the cervix, and the serosal surface was identified as a distinct hyper-echoic peripheral border. Three different sites (cranial, mid, and caudal) were located along the longitudinal axis of the cervix, and the distance between dorsal and ventral serosa was recorded. All measurements were expressed in millimeters. Cross-sectional assessments were not always possible, particularly in late gestation, when fetal parts pressing against the caudal body of the uterus interfered with transducer positioning. During the course of each examination, the following parameters were recorded:

(A)  Cranial cervical vertical diameter: Adjacent to the cervical pole of the placenta.

(B)  Caudal cervical vertical diameter: Just cranial to the vaginal os of the cervix.

(C) Mid cervical vertical diameter: A point taken between the first two measurement points.

(D)  Cervical echo-texture.

(E)  Combined thickness of the utero-placental unit (CTUP) at the cervical pole.

(F) Evaluation of cervical length by palpation.

(G) Evaluation of cervical tone by palpation.
 

Cervical Echo-Texture

Cervical echo-texture was graded on a scale of 1 to 5, according to the sonographic characteristics observed, as follows:

Grade 1 (Fig. 1): Uniform echo-texture, where lumen and muscular layers of the cervix are not easily distinguishable; a faint central linear pattern may be present.

Grade 2 (Fig. 2): Mild loss of echo-density between cervical lumen and muscular layer and evidence of a central linear pattern.

Grade 3 (Figs. 3 and 4): Marked central linear striation with hypo-echoic borders.

Grade 4 (Fig. 5): Very loose central linear striation, within a wide band of echolucent tissue.

Grade 5: Anechoic cervical lumen in longitudinal and cross section views, with no evidence of central striation. Grade 5 was associated with “abnormal” gestations and was never recorded in any of the mares entered in this study.

CTUP at the Cervical Pole

Measurements of the CTUP were taken from the ventral aspect of the caudal uterine body, cranial to the uterine cervical os and using the ventral uterine vasculature as a landmark, as previously described.^5–7

Cervical Length and Tone

Cervical length could not be objectively measured by ultrasound during the course of this study, but it was evaluated by rectal palpation and estimated on a scale of 1 to 3, as follows:

(1) Short cervix, which could be easily palpated in its entirety and was completely visualized within the length of the ultrasound transducer (size of transducer footprint: 6.5 cm). [...]