Embryology – Different Stages of the Early Equine Pregnancy
– Equine embryology – early pregnancy images. A pictorial showing embryonic developmental images through to the fetal stage of 50 days.
All day numbers given here are the number of days post-ovulation on the conceiving cycle.
This image shows an expanded blastocyst. Note the regularity of the circular shape. The blastocyst stage of development is considered to be reached when the blastocele cavity, or yolk sac, is seen to be developed within the embryo. It is at this stage of development that embryo transfer is most commonly practised.
By 17 days the regularity of shape is starting to deteriorate – note the “ragged” edges now seen on the 17-day pregnancy compared to the 16-day. This is perfectly normal and not generally an indication of a problem – do not give prostaglandin with the intention of starting over, or Regumate (or any progestin) to “maintain the pregnancy”!
By 21-23 days (and earlier with a good ultrasound and technical ability) the embryo proper can be seen with ultrasound – here it is visible between the red calliper marks on the image. The heartbeat can also start to be detected at this stage. Note here how the regular rounding has completely deteriorated, and the conceptus is appearing oval, and without “smooth” edges.
At 33 days, the developing allantois (the outgrowth of the embryo’s hindgut which forms the bladder, carries blood vessels in the umbilical cord, and later combines with the chorion to form the placenta) is visible, and the yolk sac (visible here as the upper portion of the dark section) is regressing. The apparent “division” in the conceptus at this stage is not to be mistaken for twin embryos – of which clearly only one is visible.
A different view of a 35 day pregnancy shows that there can be differences of embryo position and yolk/allantoic sac size. The rotation of the uterus or horn can affect the apparent embryonic position.
Around day 40 the embryo officially becomes a fetus with the commencement of the placental formation. As can be seen in this image of a 50-day fetus, the legs, head, tail and eye are all clearly defined externally. Internally organs and skeletal structures are in place. The overall size is a little deceptive with this image, and it is in fact about one inch in length. Within 10 days or so, the sex of the fetus can be determined using ultrasound, by establishing the direction of migration of the pedicle that will become either the penis or the clitoris. Upward migration towards the anus of course indicates a female.
So, are they or not?
Are these twins? The ultrasound at left was done at 21 days, at right 27 days. What do you think?
Click Here To See The Answer and the reasons for it, as well as the final answer as to whether it was twins or not!
© 1991, 2020 Equine-Reproduction.com, LLC
Use of article permitted only upon receipt of required permission and with necessary accreditation.
Please contact us for further details of article use requirements.
Other conditions may apply.