Retained Fetal Membranes

For a mare, the birth of her foal is an explosive process, in the best of circumstances requiring less than a half-hour from the time her water breaks to the time the foal has been delivered through the birth canal. In a wilder existence, a pregnant mare would seek a secluded place to lie down and have her foal, trying to keep safe from predators. A newborn foal is a precocious creature, able to rise to his feet and quickly gaining enough coordination to run close on his mother's heels.

Yet the first few hours after birth are a critical period for both mare and foal. During this initial couple of hours while the mare and foal are resting and bonding with one another and while the foal is testing his long legs, the mare will undergo the final stage of delivery: Expulsion of the fetal membranes, which include the allantochorion and the amnion.

Some mares hardly notice this process as the fetal membranes slip loose in one quick moment. Other individuals appear mildly colicky, alternately lying down and getting up as uterine contractions create painful spasms. Because there is a high incidence of post-foaling colic in mares, it is critical to determine if her cramping is short-lived and related to uterine contractions or if her discomfort persists due to intestinal colic pain. Colic signs that persist for more than an hour post-foaling are cause for immediate veterinary intervention. The abrupt absence of the space-occupying mass of the foal allows for intestinal displacement, particularly of the large colon, sometimes necessitating surgical correction.

Fetal membranes are usually expelled within 15 to 60 minutes after birth of the foal, but might take two to three hours to pass. If the fetal membranes take longer than four to six hours to be expelled, this is a serious complication known as retained fetal membranes (RFM) that requires immediate veterinary care to avoid colic, laminitis, or infection.

Dangers of Retained Placenta

Usually after the foal's birth, the amnion hangs from the vagina, while the balance of placenta hangs partially in and partially out, restrained by its tight adherence to the uterus. A retained placenta occurs when all or part of the placenta is not passed from the uterus; this can endanger a mare's health in several ways. By hanging into the environment, the half-expelled placenta can provide a way for contaminants and bacteria to enter the uterus. Also, placental tissue retained for more than six to eight hours begins to digest itself (autolyze), putrefying both in and out of the uterus. Bacteria multiply rapidly in the uterus, giving rise to life-threatening complications.

Inflammation thins the uterine wall and increases blood flow to the area. This sets up conditions for absorption of bacteria and toxins from an inflamed and infected uterus (metritis) into systemic circulation.

As many types of bacteria proliferate in the infected uterus, many will die, among them the Gram-negative bacteria. Of the many bacteria that flourish in necrotic tissue, these are particularly dangerous. The external cell walls of these bacteria contain a substance known as endotoxin. A large concentration of endotoxin picked up in the bloodstream exerts profound changes upon the peripheral blood flow, particularly of the feet. Arterioles (small blood vessels) constrict within the hooves, and the hypertension (high blood pressure) that follows results in swelling and necrosis of the blood-rich laminae: Laminitis.

Acute laminitis is evident by bounding digital pulses, the foot pain causing a distressed mare to shift her weight from foot to foot. She might not be willing to move at all, and when forced to do so, she uses tentative steps, as if she is walking on eggs.

It seems crazy that an incident in the uterus can have such far-reaching consequences as foot disease, but this is one of the results of endotoxin overload or systemic sepsis from bacterial overgrowth in the uterus. Until the source of endotoxin is eliminated, the body keeps circulating this poison. Although the uterine infection (metritis) is eventually corrected, laminitis might be severe enough to permanently cripple the mare or warrant euthanasia.

Not all cases of retained fetal membranes result in such dire consequences as metritis or laminitis. However, up to 10% of retained fetal membranes persist more than 10 hours. Even in those cases, appropriate veterinary care can forestall such disasters.

Adverse Fertility Effects of RFM

Normally, strong contractions of the uterus continue up to 48 hours after foaling to empty residual fluids and to assist in involution (shrinking back to normal size) of the uterus. Any placental tissue remaining within the uterus will delay normal uterine involution. And the first postpartum estrous cycle might have to be passed over as a breeding opportunity.

A uterine infection potentially reduces future fertility by altering the functional characteristics of the uterine lining in supporting fetal growth. Replacement of glandular uterine tissue with scar tissue could interfere with nourishment of a future embryo or growing fetus, leading to early embryonic death or abortion.

What to Do and What Not to Do

To avoid such complications, call a veterinarian when the placenta doesn't pass after two to three hours. Although you might be tempted, don't pull on the placenta. This could lead to more serious complications, such as tearing of the uterine lining with subsequent scar tissue and/or uterine hemorrhage (bleeding), which can cause dire consequences.

Yet, leaving the placenta and amnion dangling presents risks. The mare might become agitated enough to kick at it, putting the foal in harm's way. Or, the swinging mass of fetal membranes might startle her, causing her to bolt, placing the foal at risk of being run over.

For safety, it is good management to tie together or wad the tissues into a plastic bag, leaving it to dangle about a foot from the vulva to raise it from contact with the ground or the mare's feet. It is best not to cut away portions of a dangling placenta as its weight exerts gentle traction to ease the rest of the placenta from the uterus. Don't add a weighted object to the tissues as this can cause them to tear away. Usually, the membranes remain intact unless a mare steps on them and shreds the tissue. When the placenta is passed, it should be retrieved and placed in a pail out of reach of domestic pets or wild animals that could shred it into impossible-to-read pieces.

It is common to call a veterinarian for a post-foaling exam soon after the birth. At that time, your vet should inspect the expelled placenta to ensure it has passed in its entirety. When spread on the ground, a normal placenta resembles a pair of baggy pantaloons, with each leg of the pantaloons representing one of the two uterine horns. Your veterinarian will look for missing pieces that might remain in the uterus; even the smallest tag of retained tissue can cause problems. Close scrutiny of the placenta also identifies any areas of focal infection (placentitis) that might mean the foal acquired infection while in the uterus and is in need of treatment.

Treatment of Retained Placenta

Treatment for retained fetal membranes attempts to loosen the adhered placenta from the uterus, and to limit infection and the effects of circulating endotoxin and infection. Your veterinarian, with his/her hand in a sterile glove, will gently massage adhered tags of placenta in the uterus in an attempt to loosen the attachments. At that time, the uterus will also be irrigated with sterile, antiseptic saline solutions to which a local antibiotic might be added. The fluid washes away cellular debris and bacteria to decrease the level of uterine contamination.

Local antibiotics can't reach high enough levels in the uterus, so they are supplemented with systemic administration of compatible antibiotics to achieve adequate blood and deep tissue levels. Non-steroidal anti-inflammatory drugs decrease the influence of endotoxin release (which initiates the changes in peripheral blood flow that can lead to laminitis).

Retention of fetal membranes can be a result of aberrations in endocrine signals at the time of foaling, with release of inadequate amounts of specific hormones such as oxytocin. A mare with retained membranes often responds favorably to injections of oxytocin, a hormone essential to the normal foaling process. Oxytocin is responsible for both milk let-down and stimulation of uterine contractions to expel the fetus and the fetal membranes.

Take-Home Message

While foaling is a natural process, often needing little human intervention, there can be life-threatening complications for mare and foal. When fetal membranes are retained, prompt and aggressive treatment can restore health and enhance the mare's chances of having another foal later.

Why Fetal Membranes are Retained

Retained fetal membranes occur in less than 5% of postpartum mares. Some of the reasons they do occur include:

  • The placenta adheres tightly to the uterine horn that does not contain the pregnancy. In this area, the placental membrane (allantochorion) is thicker, with increased folding and more deeply embedded attachments that do not release easily. 
  •  Obstetrical manipulation necessary to assist delivery can cause trauma that adversely affects uterine contractions, with subsequent retention of fetal membranes.
  • An excessively large foal might require assistance at delivery or a Caesarean section. Other crises might also necessitate a C-section prior to term, such as rupture of the prepubic tendon or a pelvic injury that has narrowed the birth canal. Both prolonged gestation and a C-section can lead to retained placental membranes.
  • Induction of labor increases the likelihood of retained fetal membranes. The chances of normal foaling improve if a mare is at least 320 days gestation and has a well-developed udder with colostrum present. A relaxed vulva, loose sacrosciatic ligaments around the tailhead, and softening of the cervix are important signals of hormonal readiness for foaling.
  • Endophyte toxicity associated with fescue grass increases risk of reproductive problems, including dystocia and retained fetal membranes.--Nancy S. Loving, DVM

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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