Retained fetal membranes (RFM), also referred to as retained placenta, is a condition that affects a small percentage of broodmares. RFM can lead to serious medical complications in affected mares and requires prompt treatment. [1]

Potential complications of RFM include laminitis, the accumulation of toxins in the bloodstream (toxemia), and an enlarged uterus (metritis). RFM can also lead to sepsis, a potentially life-threatening condition that results when the body’s response to an infection damages its own tissues. [1]

Proposed causes of retained fetal membranes are associated with hormonal and nutrient imbalances in the mare, abnormal adhesions between the placenta and the tissue that lines the uterus, foaling complications, abortion, and infections. [1]

Mares that are draft breeds and Friesians may have an increased risk for RFM, possibly due to genetic causes. [1][2] The risk of RFM is believed to increase if it has occurred previously. [1]

Common treatment options for retained placenta in broodmares include administering oxytocin, antibiotics, and uterine lavage. [1] Less common treatments include manual removal of the placenta and applying traction. [1]

In most cases of RFM, the prognosis for affected mares is very good if the condition is treated promptly.

Fetal Membranes and the Placenta

A temporary organ, the placenta is an important structure that provides a physical connection between a developing foal and its mare. It begins developing in the uterus from a cluster of cells produced by a fertilized egg.

Fetal membranes, including the amnion, chorion, allantois, and yolk sac, protect the embryo and fetus before eventually becoming a part of the placenta, which develops as gestation progresses.

Function

The placenta provides nourishment for the developing fetus until birth. An umbilical cord connects the developing embryo/fetus and the placenta that is attached to the maternal uterine wall.

The placenta enables the exchange of nutrients, waste, and gases between the fetus and mare throughout pregnancy. This exchange is facilitated by microvilli, a network of structures that are filled with blood vessels and connect the placenta and uterus.

During pregnancy, the placenta influences both maternal and fetal physiology. After birth, the placenta is no longer needed.

After Birth

When the umbilical cord breaks postpartum, the blood vessels within the placenta collapse and cause the microvilli to retreat from the uterus wall. [1]

A normal occurrence following the birthing process, the placenta is typically expelled from the mare’s uterus once the foal has been delivered. Rhythmical uterine contractions promote the placenta to be expelled from the uterus, exiting through the vagina. [1]

The entire placenta may be retained or only a part of it expelled. Typically, the tips of one or both placental horns may remain. [1]

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Retained Fetal Membranes in Horses

When parts of the placenta or the complete structure remain in a mare’s body after she delivers her foal, the condition is referred to as retained fetal membranes (RFM). [1]

Fetal membranes are retained if they are not expelled within one to six hours post-partum. However, the time frame of when fetal membranes are classified as retained is not clearly defined by veterinary professionals. [1]

One study determined that fetal membranes are expelled within one hour following birth in approximately 66% of mares. [5]

Only 2% to 10% of broodmares are affected by RFM. [3][4]

Causes

The exact cause of why RFM affects some mares is unknown. According to research, potential causes of retained placenta include: [1]

Abnormal Adhesions

Research suggests mares that are draft breeds are more likely to have RFM because their fetal membranes are strongly connected to the tissue that lines the uterus (endometrium). [6]

A study of 90 draft-breed mares found that retained fetal membranes were associated with an abnormal connection (adhesion) of the allantochorion component of the placenta to the endometrium in 88% of the mares. [6]

Breed and Genetics

Research in draft horses notes that these breeds may be genetically predisposed to RFM. [7]

A study of 495 births by Friesian mares found that 54% had retained fetal membranes. [8][9] This study also noted that mares that have had RFM are 2.9 times more likely to develop the condition again in future pregnancies. [8]

Inbreeding is believed to promote an increased risk of RFM in Friesian broodmares. [10]

Foaling Complications

Although RFM can occur in horses that foal in an apparently normal manner, the risk for the condition is believed to increase after a difficult birthing process (dystocia). [11][12]

Dystocia may occur if the fetus is large, malformed, or awkwardly positioned.

The risk for RFM may also increase if the uterus fails to function normally. [1] This can occur because of uterine inertia or exhaustion leading to failed expulsion of the membranes.

Additional Risk Factors

Additional factors that increase the risk of retained placenta include: [1][9][11]

  • Abortion
  • Hormonal imbalances
  • Stillbirth
  • Uterine infection
  • Uterine inertia (occurs when the uterus fails to contract or relax properly)
  • Calcium and phosphate imbalance
  • Placental infection
  • Abnormal regulation of macromolecules and minerals present outside of cells (referred to as the extracellular matrix)
  • Damage to or stretching of the smooth muscle tissue of the uterus (myometrium)
  • A history of RFM occurrence in previous deliveries
  • Advanced age (mares that are older than 15 years)
  • Poor body condition of the mare
  • Prolonged gestation
  • Interference with a natural birthing process (i.e. cesarean sections)
  • Inhibited natural inflammatory processes that promote expulsion of fetal membranes

Complications

Retained fetal membranes can result in multiple health complications in affected mares, including:

Systemic Illness

Postpartum, the placenta is no longer a necessary organ. As it degenerates in the uterus, it results in toxin production and bacterial overgrowth. Complications of RFM may occur as soon as 12 hours after foaling. [1]

Sepsis

When bacteria enter the bloodstream from the postpartum uterus, which is rich in blood vessels, a serious systemic infection (sepsis) can result. [1]

Endotoxemia

As toxins increase in the bloodstream, it can result in endotoxemia, an inflammatory response that can lead to the failure of the circulatory system and vital organs. [1]

Metritis

Metritis refers to