80813 P1130025

The embiyonic veside grows at an average ratę of 3 to 4 mm per day during the second week of gęstation until the beginning of the third week to reach a diameter of 20 to 25 mm by Day 16(Fig. 1.55). After this datę the growth curve flattens considerably. The diameter of the conceptus appears to increase only marginally and seems to rest on a plateau between the 17th and 25th days of pregnancy (Palmer and Drlanoourt 1980, Valon et al. 1982, Ginther 1983 b). Breed related differcnces in the size of embiyonic vesicles are negli-gjbie during the first three weeks of pregnancy. Even the growth curves of pony and large horse breed embiyos are virtually identical (Ginther 1986).

A remarkable phenomenon in the horse is its em-btyonic mobility which can be observed by ultrasono-graphy from the time the conceptus first becomes visi-ble until Day 17 of gestation (Ginther 1983 a). The conceptus moves from one hom to the other, and through the uterine body several times eveiy day and may be found in different positions within the uterus during successive examinations (Ginther 1984a). This movement of the embryo can sometimes even be ob-served if a marę is scanned continuously for a number of minutes. The mobility is particularly strong from Day 11 to 14 and comes to a halt on Days 15 to 17. It has been shown through frequent examinations in 5 minutę inter-vals that the embryo moved from one hom to the other or into the uterine body an average of 7 times within 2 hours during the period of maximal embiyonic mobility between Days 11 and 14 (Leith and Ginther 1984). From Days 9 to 11 the embryo was found in the uterine body in 60 % of the cases, from Day 12 to 14 in 30 % of cases and thereafter only rarely. The positional changes of the conceptus appear to be caused by contractions of the uterine wali and can be coritrolled by the conceptus itself (Ginther 1984 b, Leith and Ginther 1985). The careful sonographic examination of the corpus uteri during this stage of early pregnancy is of particular diagnostic importance. Due to its intense mobility the conceptus £requently lies in the uterine body. If only the uterine homs are scanned by ultrasound the further caudally positioned conceptus may remain undetected inside the uterine body. This is one of the most common reasons for not detecting or overlooking a conceptus or twin pregnancy. The intra-uterine positional changes of the equine conceptus can lead to, among other things,

| implantation not only in the hom ipsilateral to the ovu-lating ovaiy, but, in about half the cases, in the con-tralateral hom. Increascs in uterine tonę and size of the conceptus seem to be the cause for the ccssation of conceptus mobility. The preferred site of implantation lic$ just next to the bifurcation in one of the uterine homs.

In the horse the early conceptus has a strietły spheri-ca) shape until Day 15 of gestation, thereafter first be-coming ovoid until day 17 and, between Days 18 and 21 it is usualiy pear shaped, after which it assumes an irregular shape (Fig. 1.56 to 1.59).

From the time the conceptus is first detected until Day 18 the uterine wali is 10 to 15 mm thick. It becomes thinner over the next few weeks and ranges from 5 to 10 mm. Initially, the uterine wali is of uniform thickness around the entire conceptus (Fig. 1.52 and 1.54); from Day 18 to 20 on its thickness decreases, particularly ven-trally to the conceptus (Fig. 1.58).

There are numerous criteria which can be used to differentiate the early embryonic vesicle from patho-logical conditions such as endometrial cysts, the secre-tions in cases of endometritis or embiyonic death. At this stage the intact conceptus is a round spherical ves-icle clearly visible and contrasted against the echoic uterine wali. There are no reflections inside the vesicle so that its fluid contents appear black. I f the probe is swept across the vesicle the inereasing and decreasing diameter reveals the spherical shape of the conceptus. At the front and back sections of the conceptus, where the sound waves impact vertically onto the wali, a short hyperechoic linę can be seen (Chevauer and Palmer 1982). These bright lines are the result of the specular reflections of the ultrasound waves from the wali of the tense embiyonic vesicle and should not be mis-interpreted as the embiyonic disc (Fig. 1.53).

In veterinary practice there exists concem about the optimal time for early sonographic pregnancy detec-tion. For several reasons the time from Days 15 to 18 can be considered most appropriate. At this stage it is no longer difficult to demonstrate the early embiyonic vesicle. Not only are twin pregnancies easy to recognize at this time period but it is quite favorable for a timely inteivention where indicated. In case of detection of non-pregnancy the ultrasound examination can be used to assess follicular development, because the following estrus should be imminent.

Once an early pregnancy has been diagnosed in a marę during the first few weeks of gestation, it is useful to reexamine the marę at about Day 30 of pregnancy so that a possible early embryonic death can be detected.