Page 459 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

434 CHAPTER 2

VetBooks.ir • Days 23–55. At day 24 the allantois can be seen. generally appear vertical, compared with the
As the allantois expands, the yolk sac contracts
usually horizontal membranes of allantois and
and so the embryo is lifted dorsally in the vesicle
yolk sac of a 30-day-old vesicle.
(Fig. 2.36). After day 40, the yolk sac reduces • After day 60 it can be difficult to obtain a
dramatically and the umbilical cord is formed, complete view of the embryo (Figs. 2.37, 2.38).
which allows the embryo to drop down to the This is due to the uterus moving over the pelvic
ventral portion of the vesicle by day 50. With brim and into the abdomen. The presence of
knowledge of the approximate age of gestation twins at this stage can be very difficult to rule
and the growth characteristics given above, it out. The age of the fetus can be calculated by
should be possible to determine twins without measuring parts of the fetus and comparing
too much difficulty when using a 5-7 MHz these with reference measurements. The sex of
ultrasound transducer. For example, twin a foal can be determined at around 60–75 days.
vesicle walls, when in contact with one another, The primitive equine genitalia, the genital

2.35 2.36 2.37

2.38

Figs. 2.35–2.38 (2.35) 21-day pregnancy with the embryo
seen at the 7 o’clock position. (2.36) 30-day pregnancy with
the embryo seen in the centre separating the allantoic and
yolk cavities (both have equal volume at 30 days). (2.37) 65-day
pregnancy scan showing a colt foal. The hyperechoic genital
tubercle can be seen in front of the hindlimbs. (2.38) 65-day
pregnancy scan showing a filly foal. The hyperechoic area
lying between the tail (lower left) and two hindlimbs is the
genital tubercle of the filly.