University of wisconsin-madison

Growth of the Fetus And Rectal Palpation

At day 18 to 20 the embryonic vessicle becomes large enough that it can now be detected by rectal palapation of the uterine horns and body. Prior to ultrasoundography, this was the main method to diagnose pregnancy in the mare. The following tables first describe major distinquishing structures used to determine pregnancy at increasing days in the mare and then a table of images and diagrams of embryos through day 50 of gestation.

Rectal palpation

Time Diagnosis
15-18 days Embryonic vesicle is 15-20 mm, palpate the uterine tone and not the embryo itself
20 days Embryonic vesicle is 30-40 mm, uterine bulge between the uterine body and horn
30 days Can feel fetal bulge 40-50 mm, can detect twins after 35 days
40 days Can feel fetal bulge 65 mm
50 days Can feel fetal bulge 80 mm, bulge extends into the uterine body
60 days Can feel fetal bulge 100-130 mm, uterus pulled into the abdomen
3-4 months Can palpate fetus

Images of equine embryo development (day 18 - 50).

day 19 vesicle

Day 18

By day 18 the cervix is elongated and firmly closed. The uterine horns are palpably turgid, making the bifurcation between the two uterine horns prominent on palpation per rectum. The vesicle becomes more triangular when viewed with ultrasound.

day 18 vesicle and uterus
Day 18 uterus and vesicle
drawing of 18 day uterus

diagram of embryonic vesicles of day 21 and day 25

Day 20-24

The fetus can be detected at day 20 to 23 and the heart beat is seen by day 24. By day 20-30, the fetus moves to the top of the vesicle. The allantois can also be seen by day 24. As the allantois grows, the yolk sac gets smaller.



day 24 fetus
Day 24 vesicle and fetus

ultrasound image of 22 day vesicle
Day 22 ultrasound

day 30 and day 40 labelled diagrams


day 30 fetus and vesicle
day 30 vesicle and embryo

day 30 pregnant tract

Day 30 pregnancy

Day 35-40

By day 35-40, the pronounced uterine tone and tubularity typical of early pregnancy remains, along with the palpable elongated cervix. A resilient sperical or ovoid bulge approximately the size of a tennis ball or baseball is present at the base on one uterine horn.

By ultrasound, the yolk sac has degenerated, the umbilical cord is elongated and the fetus moves toward the ventral floor of the uterus. The umbilical chord can be seen by day 46.

drawing of the 35-40 day pregnant tract

40 day fetus
Day 40 ultrasound

photo of day 50 pregnant uterus
Day 50 pregnancy


drawing of uterus of 45-50 day pregnancy


Day 45-50

By day 45-50, the pronounced uterine tone and elongated cervix remain. A resilient sperical or ovoid bulge approximately the size of a softball or grapefruit is present at the base on one uterine horn.

day 50 fetus

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