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 inflammation of the uterus that can develop following the birthing process if the fecal membranes are retained. [1]

Laminitis and Founder

Laminitis and founder may occur due to sepsis and endotoxemia associated with RFM. [1]

Laminitis describes a condition in which the sensitive layers of tissue (laminae) inside the hoof become inflamed. Founder occurs following laminitis and results in the coffin bone sinking and rotating within the hoof capsule, potentially penetrating the sole of the hoof.

Fever and Lethargy

Mares with sepsis and endotoxemia may develop additional signs of illness, including fever and lethargy. [1]

Diagnosis

A veterinarian can make an accurate diagnosis of retained placenta. Diagnostic strategies include:

Assessing the Placenta

Because partial retention of the fecal membranes can occur following birth, it is important to examine the membranes after they are expelled to ensure they are complete.

Uterine Examination

If the placenta cannot be located or it is not certain if it was expelled in its entirety, your veterinarian may perform a manual examination of your mare’s uterus to check for placental remnants.

In some cases of retained placenta, fragments cannot be reached, and further treatment is typically started before a definitive diagnosis.

Visual Observation

A partially or completely retained fetal membrane can be diagnosed by observing any part of it protruding from the vagina. [1]

Vaginal Palpation

Palpation of the vagina may be conducted to determine the presence of retained placenta in mares after they have given birth. [1]

Clinical Signs

Indications of colic may be a sign of retained fetal membranes following giving birth. [1]

Diagnostic Testing

Blood tests, including a complete blood count (CBC), biochemistry, and fibrinogen, may be used to assess the mare’s health to determine if any of the systemic complications associated with RFM have occurred. [1]

Treatment

If RFM has been diagnosed, it is critical to provide treatment promptly to prevent serious health complications and the potential death of affected mares.

Treatments for RFM aim to remove the retained placenta and prevent toxic and inflammatory products in the uterus from entering the bloodstream. Preventing the accumulation of toxins and bacteria in the bloodstream may help to avoid additional complications, including laminitis.

Consult with your veterinarian to determine the appropriate treatment for your broodmare. Multiple treatment options may be used to manage mares with retained placenta.

Oxytocin

Administering low doses of the hormone oxytocin is considered an important initial treatment for RFM. Oxytocin stimulates uterine contractions, which promote the expulsion of fetal membranes. [1][4]

Oxytocin is not typically administered for longer than 24 hours. Methods used to administer oxytocin include intravenous drip, large dose (bolus) intravenous injection, and intramuscular injection. A uterine pessary (a removable device) may deliver oxytocin directly into the uterus.[13]

The recommended dosage of oxytocin used to treat RFM ranges from 2 to 120 IU. [1] Low doses between 2 and 10 IU may be the most effective. [1] Doses of over 20 IU administered in a bolus are not advised due to the risk of severe abdominal pain and uterine spasms. [1]

Calcium may be administered in conjunction with oxytocin as it is believed to improve the response to the hormone. [1] This mineral is also known to improve uterine contractions.

Oxytocin must be used cautiously due to the risk of side effects.

Antibiotics

Mares with RFM are commonly administered broad-spectrum antibiotics to prevent bacterial growth in their uterus. [1] Types of antibiotics used for treating RFM include trimethoprim-sulfamethoxazole, penicillin, and gentamicin, among others.

Antibiotics may be administered systemically or delivered directly into the uterus.

Administration of antibiotics to mares with retained fetal membranes may continue for one to two weeks after the fetal membranes have been expelled from the uterus. [1]

Uterine Lavage

Uterine lavage involves flushing the uterus with fluid to remove small pieces of retained membranes, bacteria, and cellular debris from it. [1] This procedure may also help to promote uterine contractions and aid in uterus recovery following delivery.

Uterine lavage can be used before or after fetal membranes are expelled postpartum.

Fluids typically used to perform lavage include tap water, isotonic saline solution, and a solution of several salts dissolved in water called Ringer’s solution. [1] Antiseptic solutions may be used but are known to cause irritation to the uterus. [1]

Lavage is usually repeated until the fluid that is expelled from the uterus is clear.

Manual Removal of Placenta

Manual removal of the fetal membranes may be used to quickly address RFM. However, manual removal is controversial as this procedure poses various risks such as: [1]

  • Promoting the absorption of toxins in the uterus
  • Delaying recovery of the uterus to its pre-pregnancy condition
  • Bleeding
  • Damaging the lining of the uterus
  • Causing blood clots
  • Causing part of the uterus to turn inside out (uterine horn invagination)

Applying traction to the placenta may also be used to treat retained fetal membranes, but this strategy also poses potential risks. [1] There is limited research regarding the use of traction to treat RFM.

If the placenta is protruding from the vagina, a common approach is to tie it into a knot and leave it in place for its own weight to apply traction. The placenta weighs approximately 11% of the body weight of the foal. [18]

Umbilical Artery Infusion

Umbilical artery infusion with bacterial collagenase via injection into the placenta is a method used to treat retained fetal membranes in mares safely. [14]

In a study of 14 mares, umbilical artery infusion using bacterial collagenase resulted in retained placentas being expelled in less than six hours. [14]

Umbilical Vessel Water Infusion

A more recently developed technique, umbilical vessel water infusion involves injecting water directly into the umbilical cord vessels. Once these vessels are infused with water, they expand and promote the detachment of the placenta from the tissue of the uterine wall. [15]

Umbilical vessel water infusion provides a safe, gentle, and rapid means to encourage fetal membranes to be expelled. [15]

Additional Treatments

Your veterinarian may recommend using one or more of the following additional treatments: [1]

  • Non-steroidal anti-inflammatories (NSAIDs) such as flunixin may be administered to control pain and inflammation and help prevent endotoxemia.
  • Tetanus toxoids or anti-toxins may be administered to prevent tetanus and other bacterial diseases.
  • IV fluid therapy provides a means of rapidly delivering nutrients and medication.
  • Laminitis prevention strategies, including ice boots and medications, may be used to reduce laminar inflammation and improve blood flow to the hooves.

Prevention & Prognosis

There is no specific way to prevent RFM in mares.

Providing balanced nutrition for your mare and ensuring she is in optimal health status before and during pregnancy may help to support a healthy delivery. In particular, adequate selenium status is critical for reducing placental retention time and improving reproductive performance. [16][17]

Ensure your broodmare is fed a diet that meets her needs and the needs of the growing foal during pregnancy and lactation.

Learn more in our guide to caring for a broodmare in preparation for pregnancy and foaling.

The overall prognosis for and future fertility of mares with RFM is typically good if treatment is provided as soon as possible. [1]

Mares that develop complications from RFM have a guarded to poor prognosis.

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References

  1. Warnakulasooriya, DN. Et al. Treatment of Retained Fetal Membranes in the Mare-A Practitioner Survey. Front Vet Sci. 2018. View Summary
  2. Jaworska, J. et al. Is similarity in Major Histocompatibility Complex (MHC) associated with the incidence of retained fetal membranes in draft mares? A cross-sectional study. PLoS One. 2020. View Summary
  3. Canisso, I. A Clinical Approach to the Diagnosis and Treatment of Retained Fetal Membranes with an Emphasis Placed on the Critically Ill Mare. J Equine Vet Sci. 2013.
  4. Samper, J. et al. How to Deal With Dystocia and Retained Placenta in the Field. AAEP. 2023.
  5. Rosales, C. et al. Periparturient characteristics of mares and their foals on a New Zealand Thoroughbred stud farm. N Z Vet J. 2017. View Summary
  6. Rapacz, A. et al. Retained Fetal Membranes in Heavy Draft Mares Associated with Histological Abnormalities. Journal of Equine Veterinary Science. 2012.
  7. Jaworska, J. et al. Transcriptome Profiling of the Retained Fetal Membranes-An Insight in the Possible Pathogenesis of the Disease. Animals (Basel). 2021.
  8. Sevinga, M. et al. Retained placenta in Friesian mares: incidence, risk factors, therapy, and consequences. Pferdeheilkunde. 2001.
  9. Sevinga, M. et al. Retained placenta in Friesian mares: incidence, and potential risk factors with special emphasis on gestational length. Theriogenology. 2004. View Summary
  10. Sevinga, M. et al. Effect of inbreeding on the incidence of retained placenta in Friesian horses. J Anim Sci. 2004. View Summary
  11. Hospes, R. and Huchzermeyer, S. Treatment of retained placenta in broodmares in a 4-step-routine – Review of 36 cases. Pferdeheilkunde. 2004.
  12. Provencher, R. et al. Retained fetal membranes in the mare: A retrospective study. Can Vet J. 1988.
  13. Berghuis, GA and Wesselink, HG. Behandelings- en drachtigheidsresultaten van merries met retentio secundinarum [Treatment and gestation results of mares with secondary retention]. Tijdschr Diergeneeskd. 1995. View Summary
  14. Haffner, JC. et al. Equine retained placenta: technique for and tolerance to umbilical artery injections of collagenase. Theriogenology. 1998. View Summary
  15. Meijer, M. et al. How to Use Umbilical Vessel Water Infusion to Treat Retained Fetal Membranes in Mares. International Veterinary Information Service (IVIS). 2015.
  16. Ishii, M. et al. Effects of vitamin E and selenium administration on pregnant, heavy draft mares on placental retention time and reproductive performance and on white muscle disease in their foals. J Equine Vet Sci. 2002.
  17. Busse, N.I. and Uberti, B. Uterine Inertia due to Severe Selenium Deficiency in a Parturient Mare. J Equine Vet Sci. 2020. View Summary
  18. Examining the mare’s placenta and keeping foaling records. OMAFRA. Accessed March 1, 2024.