Pregnancy and Parturition - Horse Anatomy

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Mares are monotocious, meaning they normally produce a single offspring. Twin pregnancies do occur in the mare, but are rarely carried to full term and veterinary intervention is often used to prevent twin pregnancies in this species. Gestation length in the mare is approximately 330-340 days, however it is usually consistent within the individual mare from one pregnancy to the next.

Maternal Recognition of Pregnancy

The presence of a conceptus prevents luteolysis. In the presence of a conceptus, endometrial production of PGF2α is significantly reduced. The conceptus must migrate within the uterus from one uterine horn to the other 12-14 times a day during days 12, 13 and 14 of gestation in order to inhibit PGF2α. This migration is necessary, as the equine conceptus does not elongate, so there is less contact between the conceptus and the endometrial surface. It must move to distribute pregnancy recognition factors to the endometrial cells. The conceptus does produce proteins to effect the recognition of pregnancy, but specific roles are unknown. The luteolysin in the non-pregnanct cycle is thought to be PGF2α.

The embryo takes 6 days to traverse the oviduct (in other species this is normally ~4 days). It stops at various spots in the uterus, spending 5-20 minutes in each. The conceptus remains spherical in shape.

  • Day 6-22: the trophectoderm secretes a glycocalyx, which hardens to form a capsule. This prevents attachment of the embryo to the uterine endometrium.
  • Day 7-17: Peristaltic contractions of the uterine myometrium move the embryo around the uterus. The conceptus begins to secrete oestrogens, but their role is unknown. If pregnant, upregulation of oxytocin receptors between day 10-16 is inhibited
  • Day 17: The myometrium clamps the embryo in position at the base of the uterine horns, preventing movement.


The placenta signifies the "second" or "embryonic" period of pregnancy (after the implantation period) and describes the establishment of a fully functional placenta. The placenta is an apposition of foetal and parental tissue for the purposes of physiological exchange. There is little mixing of maternal and foetal blood, and for most purposes the two can be considered as separate.The placenta provides an interface for the exchange of gases, food and waste. It also facilitates the de novo production of fuel substrates and hormones and filters potentially toxic substances.The placenta has two distinct seperate compartments; the foetal side consisting of the trophoblast and chorionic villi and the maternal side consisting of the decidua basalis.

The placenta consists of a foetal portion, formed by the chorion, and a maternal portion formed by the decidua basalis. The uteroplacental circulatory system begins to develop from approximately day 9 via the formation of vascular spaces called trophoblastic lacunae. Maternal sinusoids develop from capillaries of the maternal side which anastamose with these trophoblastic lacunae. The differential pressure between the arterial and venous channels that communicate with the lacunae establishes directional flow from the arteries into the veins resulting in a uteroplacental circulation.

Blood begins to circulate through the embryonic and cardiovascular system, and therefore into the placenta, at approximately 21 days. Seperation of the maternal and foetal blood is referred to as the "placental barrier". The placental barrier is made up of a number of layers;

  • Syncytiotrophoblast
  • Discontinuous inner cytotrophoblast layer
  • Basal lamina of the trophoblast
  • Connective (mesenchymal) tissue of the villus
  • Basal lamina of the endothelium
  • Endothelium of the foetal placental capillary in the teriary villus

Macroscopic Physiology

The physical contact surfaces used within the process of circulatory exchange are the foetal membranes and the endometrium and this exchange takes place via microscopic chorionic villi that invade the endometrium. These chorionic villi are covered by epithelium. Horses have many small contacts spread over the entire surface of the foetal membranes and this form of placenta is termed a diffuse placenta.

Microscopic Physiology

The horse has an epitheliochereal placenta. This type of placenta can be said to be the most complete form, where the interface between the chorion (chorionic epithelium)and uterus (endometrial epithelium) consists of intact layers of epithelial cells with a basal laminae on each side. Both sides of the placenta have supporting connective tissue and a high density of blood capillaries.

Placental Blood Supply

Maternal blood carrying oxygen and nutrient substrate to the placenta must be transferred to the foetal compartment and this rate of transfer is the rate limiting step in the process. Therefore the placenta has a significant blood to facilitate improved exchange. Foetal blood enters the placenta via a pair of umbilical arteries which have numerous branches resulting in foetal chorionic villi within the placenta, terminating at the chorionic plate. The foetal chorionic villi are then surrounded by maternal tissues. This physiology is referred to as invasive decidualisation as the foetal chorionic villi effectively invade the maternal tissues.

Oxygen and nutrient rich blood returns to the foetus via the umbilical vein. Maternal blood is supplied to the placenta via 80-100 spiral endometrial arteries which allow the blood to flow into intervillous spaces facilitating exchnage. The blood pressure within the spiral arteries is much higher than that found in the intervillous spaces resulting in more efficient nutrient exchange within the placenta.


The mare is not able to confer immunity via the placenta and instead relies on the passive transfer of antibodies via colostrum.

Histotrophic Exchange

This type of exhange facilitates nourishment of the embryo prior to implantation, i.e. where no placenta exists. In horses, this type of exchnage is very important as there is a long period prior to implantation (up to 35 days). Nutrition is supplied by uterine secretions/debris, often referred to as uterine milk. Uterine milk secretions are usually maintained by progesterone. Pinocytosis (cellular drinking) is the main exchange mechanism.

Haemotrophic Exchange

This type of exchnage utilises direct transfer of nutrients from the maternal to foetal blood via simple diffusion, facillitated diffusion, active transport and complex diffusion.

Placental Blood Supply and Drainage

Umbilical Arteries

The paired umbilical arteries arise from Iliac arteries along with vesicular arteries to the bladder. In the adult, the remnance of these vessels form the ventral ligament of the bladder. The umbilical arteries carry deoxygenated blood and waste products from the foetus to the placenta.

Umbilical Veins

A single umbilical veins runs from the foetus and joins the hepatic portal vein, effectively circumventing the liver which is not yet fully patent. The umbilical vein transports oxygen-rich and nutrient-rich blood from the placenta to the foetus.


There are a number of foetal circulatory shunts that are related to the umbilical arteries and veins. The three major shunts are covered in more detail here but are important to ensure that organs are always supplied with oxygen and nutrient rich blood, to prevent waste accumulation and protect organs that are not yet fully patent. The main foetal circulatory shunts are the Ductus venosus, Foramen ovale and the Ductus arteriosus.


The step in the reproductive process that immediately precedes lactation, uterine involution and return to cyclicity. It is initiated by the foetus and involves a complex cascade of endocrine events. Parturition is the process by which the conceptus (foetus, placenta and placental membranes) is expelled from the uterus; this requires cervical softening, coordinated myometrial contractions and contraction of abdominal muscles to occur.

Stages of Parturition

  • Stage 1: This is the preparatory stage, starting at the onset of regular uterine contractions followed by cervical dilatation and the foetus assuming the correct disposition for passage through the birth canal. This takes 1-4 hours.
  • Stage 2: The expulsive stage, characterised by the onset of abdominal contractions which together with uterine contractions lead to foetal expulsion. This takes 12-30 minutes.
  • Stage 3: Separation and expulsion of the foetal membranes. This takes 1 hour.

Stage 1

This stage begins with mammary hypertrophy, waxing of the teats and possible escape of milk from the glands. Patchy sweating may be noted behind the elbows and around the flanks. It commences ~4 hours before the birth of the foal and increases as the stage progresses. The mare usually yawns frequently and shows no indication of pain. Food is taken readily. Respirations are normal and the pulse is ~60. Body temperature may become slightly subnormal (36.5-37◦C). The mare becomes restless and wanders aimlessly, she may swish the tail or slap it against the anus. The tail is frequently raised or held to one side. Towards the end of this stage, the mare may kick the abdomen, crouch, straddle the hindlimbs, go down on the knees or sternum before rising again, glance at the flank. This stage terminates with rupture of the allantochorionic membrane and the escape of allantoic fluid from the vulva. There is no visible straining during this period.

Stage 2

This stage is abrupt in onset and lasts an average of 17 minutes. It begins with the appearance of the amnion or commencement of forceful straining. There is not much delay between contractions,they often coincide. Soon after straining begins, the mare goes down and lies on her side with the limbs extended. She generally remains in this position until the foal is born.A transperent blue/white 'water bag' (amnion) visible at the vulva, quickly followed by the appearance in it of a digit. Straining should be at regular intervals. Each comprises 3/4 powerful expulsive efforts followed by a period of rest. Expulsive efforts are generally 3 minutes long. One forelimb procedes the other by 7-8cm. One elbow passes through the pelvic inlet before the other preventing minimal obstruction. The head is usually in the oblique position, but may be transverse with the cheek lying on the limbs. The greates and longest effort is with birth of the head. After expulsion of the foal, the mare may remain on her side exhausted for up to 30 minutes. The umbillical cord is intact when the foal is born. It subsequently ruptures 5-8cm below the belly due to movement of the mare or foal. The foal is usually born within the amnion, which is ruptured by movements of the forelegs. Lower portions of the hindlimbs often remain within the vagina for minutes after the rest of the foal is born.

Stage 3

Membranes are expelled quickly after the birth of the foal, usually within 3 hours. The duration of this stage is ~30 minutes. The afterbirth is expelled by coordinated myometrial contractions with no straining.

Placental Changes

During the last 5 days of gestation, there are changes in the placenta.

  • Collaginisation of the placentome.
  • Flattening of maternal crypt epithelium.
  • Leucocyte migration and increased activity.
  • Reduction of binucleate cells in the trophectoderm.


Contractions open endometrial crypts. Foetal villi have shrunk due to the escape of blood from the foetal side of the placenta when the umbillical cord ruptures.Myometrial contractions aid exsanguination of the placenta and separation of foetal membranes. The apex of the allantochorionic sac becomes inverted. As the sac is 'rolled' down the uterine horns, foetal villi are drawn out of the crypts. When a large portion becomes detached and inverted, it forms a mass in the maternal pelvis. This stimulates reflex contractions of abdominal muscles and completes expulsion of the allantochorionic sac. The final stage of allantochorionic expulsion lasts 1 hour in the mare.

Placental Expulsion

Expulsion of foetal membranes quickly follows expulsion of the foetus. After the birth of the young, regular abdominal contractions largely cease, but myometrial contractions persist. They are of decreased amplitude, but become more frequent and less regular. This is important for dehiscence and expulsion of foetal membranes. Waves of contractions from uterus to the cervix persist.

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