A Pregnancy Story



With breeding season upon us, My Stable Life has dove into the world of equine reproduction and taken an inside look at some of the specifics of ultrasounding mares. If you missed the first two blogs you can catch them here: Ultrasounding 101 and Understanding Estrus on Ultrasound. In this final ultrasounding post, we will take a look at a typical equine pregnancy and follow a follicle along in normal breeding development.

Age of the embryo: Day 0.
What is happening: Day of ovulation.
Image on Ultrasound: A CL is seen on the ovary. Uterine edema is resolved. This can
best be determined by following follicular development through the estrus period.



Age of the embryo: Day 14 of a pregnancy.
What is happening: Early detection of an embryo can be determined at 11 to 15 days.
Most vets prefer to check at Day 14, since an early detection of twins is extremely
important for the health of the mare. If twins are present, both vesicles are visible at Day
14. At this time they are still highly mobile within the uterus, allowing manual reduction
of one twin to be possible.
Image on Ultrasound: The embryonic vesicle is seen as a spherical black structure
approximately 14-15 mm in diameter. The yolk sac is highly visible and the embryo is
highly mobile in the uterine lumen and can found anywhere in one of the uterine horns
or the body. The mareʼs uterus is tightly toned.



Age of the embryo: Day 16 of a pregnancy.
What is happening: Fixation of the embryo at the base of the uterine horn.
Image on Ultrasound: The black spherical shape has grown in size and has implanted
itself at the base of the uterine horn. A healthy pregnancy would not indicate the
presence of any fluid, edema or cysts in the uterus.



Age of the embryo: Day 28 of a pregnancy.
What is happening: A healthy embryo is developing. Detection of a heartbeart can be
done as early as Day 22, as a fluttering movement within the echogenic mass of the
embryo. The developing allantois can also be determined at Day 24.
Image on Ultrasound: At Day 28, the allantoic sac occupies 50 per cent of the vesicle
and the embryo is located in the middle of the embryonic vesicle. A visible membrane
separates the yolk sac (top) and the allantoic sac (bottom).



Age of the embryo: Day 42 of a pregnancy.
What is happening: From Day 42 to 48, the fetus descends.
Image on Ultrasound: The fetus begins to descend back to the ventral (underside) part
of the vesicle, hanging from the umbilical pole attached at the dorsal (upper side) aspect
of the vesicle. The yolk sac is now enclosed in the umbilical cord and can sometimes be
seen as a black structure. On Day 48, the fetus is on the floor of the vesicle and the
umbilical cord can be seen hanging from the top.



According to the University of California, Davis, School of Veterinary Medicine and the
Department of Reproduction, only nine per cent of mares with twin embryos will carry
both foals to term. Of the rest, 60 per cent will deliver one live foal and 31 per cent will
lose both pregnancies. This is simply due to the fact that the mareʼs placenta is not
designed to support twin pregnancies and combined with the birth weight of the twins, if
carried to term, twins rarely exceed the normal birth weight of a single foal.

This is why, an ultrasound at Day 14 to determine twinning is very important, prior to
implantation of the embryo(s) at the base of the uterine horn. If your vet determines that
twins are present, he or she may choose to leave the mare for a few days (only up to
Day 19 at the latest), to see if one vesicle regresses on its own. This is something that
may require daily ultrasounds to monitor.

If the vesicle does not resolve naturally, your vet will likely rupture one of the vesicles
transrectally between their finger and thumb, or with the use of a transducer by trapping
it at the top of the horn. Manual rupture of one vesicle is highly effective if the twins are
fixed bilaterally. Unilateral fixation is much more difficult.

After Day 25, correction of twins is increasingly difficult and abortion of both twins may
be necessary to ensure the health of the mare.

Ultrasonography has gained wide acceptance and is a very beneficial tool in equine
reproduction. While the facts in this article have mostly detailed an ideal pregnancy,
ultrasound imaging can also be very beneficial for monitoring ovarian and uterine
abnormalities and pathology. Some machines can additionally be used for fetal sexing
at Day 60 of a pregnancy.

Thank-you to Dr. Tammi Roalstad of Scottsdale, AZ, for providing the ultrasound images and information used in this article.


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