Chapter 9



The placenta comprises the chorioallantois, the amnion, and the placental vasculature that constitutes the umbilical cord. All of these tissues are of fetal origin, and, although there are very significant effects upon the mare consequent upon failure to expel all or part of the placental membranes, examination of all the placental tissues is vital for the foal. Indeed, it is suggested that examination of the placenta provides as much information about the pregnancy and the foal itself as almost any other procedure. A considerable amount of information can also be gleaned about the status of the endometrium, which might have implications for subsequent pregnancies.

The equine placenta can be defined as diffuse, microcotyledenary and epitheliochorionic in character. The structure of the placenta and its relationship with the maternal endometrium are illustrated in Fig. 9.1.

This placental arrangement has a number of significant implications:

• Placental transfer of nutrients to the foal is relatively poor; only one foal can be fully supported.

• The entire endometrial surface is required to provide adequate nutrition and gas exchange for a single fetus.

• Areas of scarring on the maternal endometrium do not have the normal structure that allows placental attachment and transfers, and are avillous. The result of endometrial scarring or other avillous areas is the loss of effective transfer capacity. The extent of the deficits is reflected in deficiencies of growth or maturation in the foal. Minor placental inadequacy as a result of endometrial scars or cysts may have no detectable effect on the foal’s well-being, but more extensive ones will have an increasing influence. Thus, a foal born to an affected mare may be less developed than it should be.

• If twin pregnancies are present, the problem of avillous areas of endometrium is relatively minor. The problem here is that the two placentas will abut each other and so the placental surfaces will reflect avillous areas that do not correspond with endometrial damage (Fig. 9.2). This may perhaps explain the significant number of abortions or stillborn foals and the widely held belief that twins are usually disastrous. However, it is also important to realize that a very small percentage (<1%) of twins are born relatively normal and can also grow well and mature into normal adults. More commonly one foal is better than the other and the large majority of twin births result in poorly grown foals that do not adapt well to extrauterine life. A mare that is known to be carrying twins or has previously had twins is therefore classified as high-risk (see p. 252). Proper management of a twin pregnancy at an early stage can reduce the risks of twinning (see p. 242).

• Placental transfer of large protein molecules is not possible. There is no effective/significant acquisition of immunoglobulins before birth. Consequently, the newborn foal is best regarded as immunologically naive.

• There is little chance for transfer of infective organisms to the foal without concurrent placentitis, and abortion will commonly ensue in such cases. Even minor placental inflammation can result in rapid abortion or significant compromise in the development of the foal. Therefore the health of the pregnant mare is critical for the delivery of a healthy foal at full term. Serious infections carry a significant danger of early termination of the pregnancy.

• Endometrial bleeding, such as is seen in the human hemochorionic placentation, is not possible. Significant blood loss at parturition usually means maternal (cervical, uterine or vaginal) trauma or premature rupture of the umbilical cord. Fresh bleeding after delivery of the foal (and the placenta) is usually a result of uterine, cervical, vaginal or vulvar damage.

• Premature separation of the umbilical cord is a relatively frequent event in mares foaling under disturbed conditions. In this situation, the foal can be deprived of up to 1.5 liters of blood.1 This can result in considerable problems with metabolic acidosis and failure to adapt quickly to the free-living state.2 There is debate about the significance of the early separation of the cord and the importance of the blood remaining in the placenta.3 However, it seems reasonable that a foal will be slower to adapt and may be more susceptible to infection if it is deprived of the maximum amount of blood.

• Any factor that adversely affects the intimate relationship of the placenta with the endometrium has the potential to cause serious problems, such as abortion, fetal reabsorption, deformity, or neonatal infection or compromise.

Abnormalities of the placenta are extremely important and allow the attending clinician to make a rational and often life-saving assessment of the viability of the foal before any clinical abnormality becomes apparent.

The problems associated with placental retention in the mare are described on p. 313.


The stud manager/owner should place the unwashed placenta carefully into a plastic bag immediately it is passed, and store it in a cool place (not a freezer) until examination. The placenta should be examined within 12–24 hours; usually this will conveniently coincide with the primary (2–6-hour) examination or the second routine examination of the foal (at 24 hours).

The stud manager or groom should be asked to write down the findings as the clinician dictates them. It is unwise to rely on memory. An examination form can be useful in ensuring that nothing is omitted and that the results are recorded correctly. (An example of a suitable form is shown in Fig. 9.25 at the end of this chapter.) In an increasingly litigious society, a meticulous record of events should be kept.

Step 2: Prepare all equipment required

• Disposable gloves/apron/mask.

• Scissors.

• Scalpel handle and blade (preferably disposable).

• Sterile swabs with bacterial and viral transport medium.

• Sampling bottles:

• Disinfectant for washing up afterwards.

• Notepad and paper; record findings as they are identified (it is often useful to dictate to the stud groom as the procedure is performed or use a tape recorder). A suitable protocol is shown at the end of this chapter (pp. 340–342).

• A refrigerator and freezer are also useful, but potentially infected specimens must never be stored in a freezer or fridge that also contains other material such as colostrum or plasma.

• If there is a possibility that viral agents are involved, a pack of dry ice is often useful as many specimens deteriorate markedly unless cooled quickly.

Step 4: Examine the placenta

It is essential to examine the placenta thoroughly from both sides (chorionic and allantoic) and to examine the amnion and the cord. Failure to do this could result in failure to recognize vital clues that might impact on the foal and/or the mare.5

i. Place the placenta on a clean dry surface (e.g. concrete floor) well away from any area where other horses might make contact with it (it might carry infectious agents).

ii. Lay the placenta out in the shape of an ‘F’, the way in which it was delivered (see Fig. 9.3). The smooth gray, shiny allantoic surface of the allantochorion is exposed (this is the inside of the chorioallantois; it should not be confused with the thinner, tough, smaller amnion, which should be identifiable as a separate entity).

iii. The bottom leg of the ‘F’ is the cervical end; the cervical star and rupture line should be situated here. The vertical part of the ‘F’ corresponds to the uterine body. The larger upper (longer, wider and thicker) side-arm of the ‘F’ corresponds to the pregnant horn, and the lower, smaller and narrower side-arm corresponds to the nonpregnant horn.

iv. The whole visible allantoic surface should be examined carefully, then turned over to inspect the other side. Particular care must be taken to examine any area that may indicate uterine injury from the foal’s feet or legs. This may be the only outwardly detectable indicator of trauma to the uterine wall.

v. The integrity of the blood vessels that lie on the allantoic surface should be examined: missing portions of the placenta are characterized by blood vessels that do not correspond; tears in the membrane can be recognized when the vessels can be re-aligned accurately.

vi. The placental blood vessels are examined in detail for the presence of any inflammatory margin, which is characteristic of some bacterial infections (particularly those that are liable to cause septicemia in the foal such as Escherichia coli and Klebsiella spp.).

Examine the umbilical cord and the amnion

The umbilical cord should be examined closely for evidence of abnormal twists and bruising or other damage. The normal cord has a number of regular nonobstructive twists and shows no evidence of hemorrhage at any point (Figs 9.4, 9.5).

Jun 18, 2016 | Posted by in EQUINE MEDICINE | Comments Off on THE PLACENTA

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