Normal equine pregnancy



Production of a viable ovum and insemination and transport of viable spermatozoa is a highly regulated process.

At mating, spermatozoa are ejaculated into the uterus directly (en-utero insemination). From there they move to the uterine tube where the ovum will belocated - or will soon arrive.

Embryonic period

The conceptus remains in the uterine tube for 6 days. A signal, PGE2, from the viable conceptus allows its passage into the uterus. Dead embryos do not move into the uterus.

Once in the uterus, the conceptus receives nutrients from 2 sourses - the secretions of the endometrial glands, and directly from the endometrial blood supply by diffusion. As the embryo ages, the placenta develops. The choriovitelline membrane continues to develop.

The embryo moves about in the uterus until about day 16-17. At that time it attaches to the endometrium at the bifurcation of the uterine horns. The site of attachment is where the umbilical cord is located. The embryo can develop in either horn.

Formation of the placental interdigitations occurs at about day 40. Before attachment and after development of the placenta, the conceptus takes up fluid from the endometrial glands (histotroph). Specialised areas of the chorion take up this fluid. These are called 'areolae' and are microscopic.

The size of the embryo changes rapidly over time:

Day 9 - 2 mm - blastocyst

Day 12 - 4.5 mm yolk sac visible

Day 14 - embryonic disc visible

Day 16 - 5.2 cm - blood vessels visible Embryo is cylindrical.

Day 18 - 6 cm - embryo is C shaped. neural folds begin to close.

Day 23 - Embryo has a definite form. Allantois covers the amnion.

Day 25 - 6.8 cm. Allanois fuses with chorion.

Day 30 - 7.5 cm. Yolk sac regresses.

Day 36 - 9 cm.

Day 38 - 11 cm Embryo well formed.

Day 45 - villi form on Chorion. Interdigitate with endometrium



van Niekerk CH, Allen WR. Early embryonic development in the horse. J Reprod Fertil Suppl. 1975; 23: 495-498. PMID: 1060830.


The normal equine placenta is diffuse and occupies both horns and the body of the uterus

Absorption of nutrients is said to be haemotrophic through the microcotyledons and histotrophic through the regions between the microcotyledons where the secretions of the endometrial glands are available for absorption. The trophoblasts in this region are pseudostratified.

Wilsher S. The uterus and early pregnancy failure in the mare. Eq Vet Edu 2019; 31:214-224

Endometrial cups

Horses are unique in that their placenta produces hormones - equine chorionic gonadotrophin. This production begins with the invasion of special trophoblasts into the endometrium to form the endometrial cups - beginning on day 36-38. It likely stimulates the release of FSH to induce secondary corpus lutea.

Progesterone increases in concentration with gestation. The fetoplacental unit producing progestins from day 60 to 180. It then becomes undetectable until 10 days prior to parturition when ACTH stimulation of the fetal adrenal occurs.

Placental and fetal size





Placental wt (95%) kg

Cord length

Foal weight

cord twists

Whitwell and Jeffcott (1975)





50.9 (41-60)
27 (16-34)


Williams and Gibbons (1929)







Caslick (1932)







Walton and Hammond







Jennings (1941a)

TB and half






Jennings (1941b)







Rossdale (1966)







Prickett (1970)






Mariella (2018) STB 70 2.8-6.3 37-95 31-64 5
Mariella (2018) TB 38 4-8 34-81 48-85 5
Mariella (2018) WB 16 4.5-5 34-100 27-59 6
Whitehead et al (2009) STB 92 4.39 +- .867   53 +- 5.57  


Mariella J, Iacono E, Lanci A, Merlo B, Palermo C, Morris L, Castagnetti C. Macroscopic characteristics of the umbilical cord in Standardbred, Thoroughbred and Warmblood horses. Theriogenology. 2018; 113: 166-170.

Whitwell KE, Jeffcott LB. Morphological studies on the fetal membranes of the normal singleton foal at term. Res Vet Sci. 1975; 19: 44-55.

Pregnancy side and attachment

Whitehead et al (2009) found that placental implantation side and side of pregnancy varied in 22 of 92 mares.

Whitehead AE, Foster RA, Chenier T. Placental characteristics of standardbred mares. Society of Theriogenology meeting, 2003

Whitwell KE. Morphology and pathology of the equine umbilical cord. J Reprod Fertil Suppl. 1975; 23: 599-603.

Cardiovascular parameters

Fetal heart rate at day 150 is about 120 BPM. It declines to about 80 gradually between day 150 and 300. Peripheral resistance decreased toward birth.

Mean fetal arterial blood pressure is about 30-35 mm Hg at day 150 and increases to 80-90 mm Hg at term.


Fowden AL, Giussani DA, Forhead AJ. Physiological development of the equine fetus during late gestation. Equine Vet J. 2020; 52: 165-173.

Giussani DA, Forhead AJ, Fowden AL. Development of cardiovascular function in the horse fetus. J Physiol. 2005 Jun 15;565(Pt 3):1019-30. doi: 10.1113/jphysiol.2004.078469. Epub 2005 Mar 24. PMID: 15790668; PMCID: PMC1464542.

Endometrial vascularity

The equine conceptus modulates changes in the endometrial tissue and the development of vasculature before implantation. This begins in the dorsal portion of the endometrium that overlies the yolk sac. The number of endometrial glands and the amount of stroma is more prominent. VEGFR2, but not VEGF or VEGFR1, can be localized about day 14 to 21 of pregnancy. This establishes haemotrophic nutrition

Silva LA, Klein C, Ealy AD, Sharp DC. Conceptus-mediated endometrial vascular changes during early pregnancy in mares: an anatomic, histomorphometric, and vascular endothelial growth factor receptor system immunolocalization and gene expression study. Reproduction 2011; 142: 593–603


Toll like receptors 2, 4, and 6 were found in the chorioallantois.The trophoblasts and allantoic epithelium was positive by immunohistochemistry.

Schöniger S, Gräfe H, Wipplinger M, Schoon H-A. Expression of Toll-like receptors 2, 4 and 6 in the equine chorioallantois. Vet Immunol Immunopathol 2017 206-49-53.

Placental nutrients

The equine endometrium provides hemotrophic and histotrophic nutrition to the fetus. Trophoblasts at the base of the villi take in the secretion of endometrial glands (uterine milk) for histotrophic transfer.

Pregnancy steroids

Pregnancy in horses is maintained by placental hormones.

Placental estrogens require androgens as a substrate. In horses, the fetal gonads are the site of production of pregnenolone, an androgen, that is in high concentration in fetal blood. This hormone is converted to 5α-reduced pregnanes including 5α-dihydroprogesterone (DHP) in the placenta. Estrogens are responsible for uterine blood flow and myometrial contractility. Estrogens reach a maximum at 210 days and reduces dramatically at 280 days.


Legacki EL, Ball BA, Corbin CJ, Loux SC, Scoggin KE, Stanley SD, Conley AJ. Equine fetal adrenal, gonadal and placental steroidogenesis.Reproduction. 2017; 154: 445-454.

Umbilical cord

The normal umbilical cord is composed of two arteries, one vein and the urachus.

The length of the umbilical cord varies with the size of the breed, so it is shorter in a pony and longer in draft breeds (see the table above).

Blood flow is high volume low pressure environment.


Whitehead et al (2009) found that implantation side was not significantly different left to right. Pregnancy was more on the right. 22 of 92 had different implantation side to pregnancy side. Those with differing sides had a similar vasculature., and these had a longer umbilical cord. Multiparous mares had longer umbilical cords.


Jeawon SS, Katz LM, Galvin NP, Fogarty UM, Duggan VE. Determination of reference intervals for umbilical cord arterial and venous blood gas analysis of healthy Thoroughbred foals. Theriogenology. 2018; 118: 1-6.

Whitehead AE, Foster RA, Chenier T. Placental characteristics of standardbred mares. Society of Theriogenology meeting, 2003

Umbilical cord/remnants

The umbilical remnants in the equine fetus, as with other placental mammals, includes the umbilical vein, to umbilical arteries and the urachis.
The umbilical vein becomes the falciform ligament of the liver. The umbilical arteries become around ligaments of the bladder, and the rakers becomes the median ligament of the bladder.
These structures were followed by ultrasonography and they reduce in size over time. By three weeks of age, all structures are about one half of their original diameter. By six weeks of age the structures are about 70% of the original size.

McCoy AM, Lopp CT, Kooy S, Migliorisi AC, Austin SM, Wilkins PA. Normal regression of the internal umbilical remnant structures in Standardbred foals. Equine Vet J. 2020; 52: 876-883.


Nonlesion Lesions (normal structures)

Yolk sac remnant

The horse is the only domestic species that may retain part of the yolk sac.

These are located in the allantoic portion of the umbilical cord. Oftentimes they are within the cord and are a blind ended cord. At the most dramatic, they are about 30cm in diameter. They have a smooth outer surface, an ossified wall and a fluid center. They can strangulate the cord and cause fetal death, but usually they are incidental. They are not a twin.

Harbo JM, Mausling RD, Schlafer DH, Vanderwall DK. Theriogenology question of the month. Large ossified remnant of the yolk sac. J Am Vet Med Assoc. 2006; 229: 215-217