Retained Placenta (Failure to Expel Fetal Membranes)

Retained placenta, also known as retained fetal membranes (RFM), is a postpartum condition in which the placenta or portions of the fetal membranes are not expelled within a species-specific normal time frame after parturition. In dogs and cats, the normal expulsion of fetal membranes typically occurs within 15 minutes to 1 hour after the delivery of each fetus, often as part of the afterbirth. Retention beyond this period, especially if accompanied by systemic signs, is considered pathological. The condition is classified as complete (entire placenta retained) or partial (fragments of chorionic villi or membranes remain adherent to the uterine wall). Retained placenta can lead to endometritis, metritis, septicemia, and potentially life-threatening toxemia if not promptly diagnosed and treated. The condition is more commonly recognized in cattle and horses, but in small animals, it is often underdiagnosed due to the natural passage of membranes during the expulsive phase of labor. In bitches and queens, retained placenta is frequently associated with uterine inertia, dystocia, or obstetric intervention, and may be accompanied by postpartum metritis. The clinical significance lies in the potential for bacterial overgrowth, systemic illness, and compromised future fertility. Accurate diagnosis relies on a combination of history, physical examination, ultrasonography, and laboratory findings, with treatment ranging from medical management to surgical intervention.

The etiology of retained placenta in dogs and cats is multifactorial. Primary causes include uterine inertia (primary or secondary), which may result from hypocalcemia, hypoglycemia, uterine overdistension (e.g., large litter size, fetal oversize), or exhaustion of uterine musculature after prolonged labor. Hormonal imbalances, particularly inadequate postpartum oxytocin release or altered prostaglandin F2α (PGF2α) secretion, can impair myometrial contractions and placental separation. Infectious agents, such as Brucella canis, Escherichia coli, Streptococcus spp., Staphylococcus spp., and Mycoplasma spp., can cause placentitis and weaken the fetal-maternal attachment, leading to retention. In cats, feline herpesvirus and feline leukemia virus have been implicated in reproductive failure, though their direct role in retained placenta is less clear. Genetic predispositions, such as brachycephalic breeds with conformational issues, may contribute to dystocia and subsequent retention. Iatrogenic factors include excessive exogenous oxytocin administration, which can cause tetanic uterine contractions and interfere with normal placental separation, or improper obstetric manipulation during dystocia management. Environmental stressors, such as overcrowding, poor hygiene, and inadequate nesting areas, may increase the risk of postpartum infections and retention. Additionally, nutritional deficiencies, particularly of calcium, selenium, and vitamin E, have been associated with uterine inertia and retained fetal membranes in other species, and may be relevant in small animals. The exact cellular mechanisms involve failure of the chorionic villi to detach from the endometrial crypts due to insufficient collagenolysis, impaired apoptosis of trophoblastic cells, or inadequate uterine involution.

Retained placenta is relatively uncommon in dogs and cats compared to ruminants, but it is a significant clinical entity in small animal obstetrics. The incidence in bitches is reported to be between 2% and 10% of all parturitions, with higher rates in breeds predisposed to dystocia, such as Bulldogs, Boston Terriers, and other brachycephalic breeds. In queens, the incidence is lower, estimated at 1% to 5%, but may be underreported due to the tendency of queens to consume the placenta. Age is a risk factor, with older primiparous or multiparous animals showing increased risk due to uterine fatigue and hormonal changes. Parity plays a role: primiparous animals are more prone to uterine inertia, while grand multiparous animals may have weakened uterine tone. Breed-specific genetic factors, such as those affecting uterine muscle contractility or placental structure, may contribute. Obesity and poor maternal body condition are associated with increased risk. Breeding management factors, including excessive human interference during parturition, improper use of oxytocin, and unsanitary whelping environments, elevate the likelihood of retention. Additionally, underlying systemic diseases, such as hypocalcemia or diabetes mellitus, can predispose to uterine inertia. In a clinical setting, retained placenta is often diagnosed concurrently with postpartum metritis, which occurs in approximately 3% to 5% of bitches. The condition is more frequently observed in dogs than cats, possibly due to differences in placental anatomy (zonary vs. diffuse) and parturition behavior.

The pathophysiology of retained placenta involves a failure of the normal separation and expulsion of fetal membranes. During parturition, the placenta undergoes a series of events: (1) hormonal changes, including a drop in progesterone and a surge in estrogen and oxytocin, trigger myometrial contractions; (2) the fetal membranes are expelled through the birth canal; (3) the chorionic villi detach from the endometrial crypts due to enzymatic degradation of collagen and apoptosis of trophoblastic cells. In retained placenta, this process is disrupted. Uterine inertia, whether primary or secondary, leads to weak or absent contractions, preventing the mechanical separation of the placenta. Hypocalcemia impairs smooth muscle contraction, while hypoglycemia reduces energy availability for uterine muscles. Inadequate oxytocin release or receptor downregulation can also impair contractions. Prostaglandin F2α, which is crucial for luteolysis and uterine involution, may be insufficient, leading to delayed placental separation. Infectious agents cause placentitis, which induces inflammation and edema, but may also interfere with the normal enzymatic breakdown of the placental attachment. The retained placenta becomes a nidus for bacterial colonization, leading to endometritis and metritis. Bacteria ascend from the vagina or are introduced during obstetric manipulation. The resulting infection can cause systemic signs due to endotoxin absorption, leading to septicemia and toxemia. In severe cases, uterine necrosis and peritonitis may develop. The inflammatory response involves infiltration of neutrophils, macrophages, and lymphocytes, with release of pro-inflammatory cytokines. If left untreated, the condition can lead to chronic endometritis, pyometra, and infertility.

Predisposing factors for retained placenta in dogs and cats include intrinsic and extrinsic elements. Intrinsic factors: (1) Age: older animals (over 6 years) have increased risk due to uterine muscle fatigue and hormonal imbalances. (2) Breed: brachycephalic breeds (e.g., English Bulldog, French Bulldog, Pug) are predisposed to dystocia and uterine inertia. (3) Parity: primiparous animals may have inadequate uterine contractions, while multiparous animals with repeated pregnancies may have weakened uterine tone. (4) Hormonal imbalances: hypocalcemia, hypoglycemia, and hypomagnesemia can impair uterine contractility. (5) Genetic anomalies: uterine malformations or placental abnormalities. (6) Obesity: excessive fat deposition can impede uterine contractions. (7) Systemic diseases: diabetes mellitus, hypothyroidism, or hyperadrenocorticism can affect uterine function. Extrinsic factors: (1) Exogenous steroid administration: use of progestins to prevent pregnancy can alter uterine physiology. (2) Improper breeding timing: breeding too early or too late may lead to abnormal placental development. (3) Poor hygiene: unsanitary whelping environment increases risk of infection. (4) Kenneling stress: stress can inhibit oxytocin release. (5) Obstetric intervention: excessive manual traction or improper use of oxytocin can cause uterine fatigue. (6) Nutritional deficiencies: inadequate calcium, selenium, or vitamin E in the diet. (7) Iatrogenic factors: cesarean section may increase risk due to surgical trauma and anesthesia effects.

Clinical signs of retained placenta in dogs and cats vary depending on the extent of retention and presence of infection. In mild cases, the only sign may be a persistent vaginal discharge that is dark green or black, which is normal for up to 24 hours postpartum, but if it persists beyond 24 hours or becomes purulent, it is concerning. The animal may show signs of systemic illness, including lethargy, depression, anorexia, fever (often >103.5°F or 39.7°C), and dehydration. Vomiting and diarrhea may occur due to toxemia. Abdominal palpation may reveal an enlarged, doughy uterus, and in some cases, the retained placenta may be palpable as a firm mass. Vaginal examination may reveal a portion of the fetal membranes protruding from the vulva, but often the placenta is entirely within the uterus. Behavioral changes include neglect of the newborn puppies or kittens, excessive licking of the vulva, and restlessness. In severe cases, septicemia can lead to shock, collapse, and death. In cats, signs may be more subtle, and queens may not show systemic illness until the infection is advanced. It is important to note that some animals may pass the placenta after 24 hours, but if clinical signs are present, immediate intervention is required. Chronic cases may present with persistent vaginal discharge, infertility, and recurrent metritis.

Differential diagnoses for retained placenta in dogs and cats include: (1) Postpartum metritis: inflammation of the uterine wall, often caused by bacterial infection, with signs similar to retained placenta, but metritis can occur without retained membranes. Diagnosis is based on ultrasonography showing thickened uterine wall and fluid, and culture of uterine discharge. (2) Uterine inertia: failure of uterine contractions, which may be primary or secondary, and can lead to retained placenta. It is diagnosed by lack of progression of labor and absence of fetal expulsion. (3) Dystocia: difficult birth, which may be due to maternal or fetal factors, and can result in retained placenta. Diagnosis is based on history of prolonged labor and imaging findings. (4) Pyometra: infection of the uterus with accumulation of pus, usually occurring later in the postpartum period, but can be confused with retained placenta. Ultrasonography shows a fluid-filled uterus, and systemic signs are similar. (5) Vaginal trauma or laceration: can cause bleeding and discharge, but no placental retention. (6) Uterine rupture: a rare but life-threatening condition that can occur during dystocia, leading to peritonitis. Diagnosis is based on ultrasonography and exploratory laparotomy. (7) Subinvolution of placental sites: a condition in which the placental attachment sites fail to heal, leading to prolonged bleeding, but not necessarily retention. (8) Retained fetus: a fetus that is not expelled, which can be detected by imaging. (9) Neoplasia: uterine tumors, such as leiomyoma, can cause abnormal discharge, but are rare in young animals. (10) Coagulopathy: bleeding disorders can cause persistent discharge, but are not associated with placental retention.

The diagnostic algorithm for retained placenta in dogs and cats begins with a thorough history and physical examination. Key questions include the number of fetuses delivered, time since last delivery, presence of any fetal membranes passed, and any signs of systemic illness. Physical examination should include assessment of temperature, heart rate, respiratory rate, and hydration status. Abdominal palpation may reveal an enlarged uterus, but is not definitive. Vaginal examination with a sterile speculum can visualize any protruding membranes, but is often inconclusive. The next step is imaging: abdominal ultrasonography is the preferred modality. It can confirm the presence of retained placental tissue as echogenic material within the uterine lumen, assess uterine wall thickness, and evaluate for fluid accumulation. Ultrasonography can also assess the ovaries for corpora lutea and detect any remaining fetuses. Radiography may be useful to rule out retained fetuses, but is less sensitive for placental tissue. Laboratory tests are essential: complete blood count (CBC) may show leukocytosis with a left shift, toxic neutrophils, and anemia. Serum biochemistry may reveal hypocalcemia, hypoglycemia, azotemia, and elevated liver enzymes. Vaginal cytology can be performed to assess for inflammation and bacteria, but is not diagnostic for retained placenta. Uterine culture and sensitivity should be obtained if infection is suspected, ideally via guarded swab or during surgery. In cases where the diagnosis is uncertain, a uterine biopsy may be performed, but this is rarely necessary. The diagnostic algorithm should also include assessment of serum progesterone levels to rule out retained luteal tissue, which can cause persistent progesterone and delay uterine involution. If the animal is systemically ill, blood cultures and imaging for septic foci may be indicated. The algorithm should be followed systematically to avoid misdiagnosis and to guide appropriate treatment.

Laboratory findings in retained placenta are variable and depend on the presence and severity of infection. Hematology: In uncomplicated cases, the CBC may be normal. With metritis, there is often leukocytosis (white blood cell count >20,000/μL) with a left shift (increased band neutrophils), toxic changes in neutrophils (cytoplasmic vacuolation, toxic granulation), and sometimes leukopenia in severe septicemia. Anemia may be present due to blood loss or chronic inflammation. Serum biochemistry: Hypocalcemia (total calcium <8.5 mg/dL) is a common finding, especially if uterine inertia is present. Hypoglycemia (glucose <70 mg/dL) may occur due to sepsis or poor nutrition. Azotemia (elevated BUN and creatinine) can result from dehydration or renal dysfunction secondary to sepsis. Liver enzymes (ALT, AST) may be elevated due to hepatic hypoxia or endotoxemia. Electrolyte imbalances, such as hypokalemia, may be present. Urinalysis: May show proteinuria, hematuria, or pyuria if there is concurrent urinary tract infection. Vaginal cytology: In the postpartum period, vaginal cytology typically shows red blood cells, neutrophils, and cellular debris. In retained placenta, there may be an increased number of neutrophils, bacteria, and possibly placental trophoblastic cells, which are large, multinucleated cells. However, cytology is not definitive. Uterine culture: Aerobic and anaerobic cultures of uterine discharge or tissue can identify the causative bacteria, with E. coli being the most common isolate. Sensitivity testing is essential for antibiotic selection. Serum progesterone: In normal postpartum bitches, progesterone levels should be <1 ng/mL. Elevated progesterone (>1 ng/mL) may indicate retained luteal tissue or luteal cysts, which can contribute to uterine inertia and retention. Prolactin levels may be elevated, but are not routinely measured. In cases of systemic infection, blood cultures may be positive for bacteria. Overall, laboratory findings help assess the severity of the condition and guide therapy.

Imaging plays a crucial role in the diagnosis and management of retained placenta. Abdominal ultrasonography is the primary imaging modality. Findings include: (1) Uterine enlargement: the uterus may be larger than expected for the postpartum period, with a thickened wall (normal wall thickness is <1 cm in dogs). (2) Intraluminal contents: echogenic material within the uterine lumen, which may represent placental remnants, blood clots, or purulent exudate. The placenta appears as a hyperechoic, irregular mass. (3) Fluid accumulation: anechoic or hypoechoic fluid may be present, indicating metritis or pyometra. (4) Uterine wall changes: the wall may be thickened and hypoechoic due to inflammation. (5) Ovarian findings: corpora lutea may be visible, and their size can be assessed. In cases of retained luteal tissue, the corpus luteum may be larger than normal. (6) Fetal remnants: ultrasonography can detect any retained fetuses, which appear as fetal structures with a heartbeat if viable. Doppler ultrasonography can assess uterine blood flow, which may be increased in inflammation. Radiography is less sensitive for placental tissue but can be used to rule out retained fetuses, which appear as mineralized skeletons after day 45 of gestation. Radiographs may also show uterine enlargement or free gas in cases of emphysematous metritis. Computed tomography (CT) and magnetic resonance imaging (MRI) are rarely used but can provide detailed images of the uterus and surrounding tissues. Vaginoscopy can be performed to visualize the vaginal vault and cervix, but is not routinely necessary. In summary, ultrasonography is the gold standard for confirming retained placenta and assessing the severity of uterine pathology.

Cytology and histopathology are valuable diagnostic tools in retained placenta. Vaginal cytology: In the postpartum period, vaginal smears typically show red blood cells, neutrophils, and cellular debris. In retained placenta, there may be an increased number of neutrophils, often with degenerative changes, and bacteria may be present. Placental trophoblastic cells, which are large, multinucleated cells, may be seen if the placenta is partially expelled. However, cytology is not definitive for retained placenta. Uterine cytology: Obtained via a guarded swab or during surgery, can show neutrophils, macrophages, and bacteria. Histopathology of the placenta or uterine biopsy can provide a definitive diagnosis. Placental histopathology: The retained placenta shows chorionic villi with necrotic trophoblastic cells, inflammatory infiltration, and bacterial colonies. Uterine biopsy: In cases of chronic metritis, histopathology reveals endometrial inflammation, with infiltration of plasma cells, lymphocytes, and neutrophils. There may be fibrosis and glandular changes. Special stains, such as Gram stain, can identify bacteria. In cases of subinvolution of placental sites, histopathology shows persistent trophoblastic cells and hemorrhage. Histopathology is also useful to rule out neoplasia, such as uterine leiomyoma, which can cause similar clinical signs. In summary, cytology and histopathology are important for confirming the diagnosis, identifying the causative agent, and assessing the extent of uterine damage.

Treatment of retained placenta in dogs and cats depends on the severity of clinical signs and the presence of systemic illness. In mild cases with no systemic signs, conservative management may be attempted, but medical therapy is often required. The goals of treatment are to promote expulsion of retained membranes, control infection, and support the animal systemically. Medical management: (1) Oxytocin: administered at a dose of 0.5-2 IU/kg IM or SC, up to a maximum of 20 IU per dog, every 30-60 minutes for 2-3 doses. Oxytocin stimulates uterine contractions and may aid in expulsion. However, it should be used with caution if there is evidence of uterine obstruction or if the cervix is closed. (2) Prostaglandin F2α (PGF2α): Dinoprost tromethamine (Lutalyse) at a dose of 0.1-0.25 mg/kg SC, or cloprostenol (Estrumate) at 1-2 μg/kg SC, can be used to promote uterine involution and expulsion. These drugs are more effective in the presence of luteal tissue. (3) Calcium gluconate: 10% calcium gluconate at a dose of 0.5-1.5 mL/kg IV, given slowly over 10-20 minutes, is indicated if hypocalcemia is present. It improves uterine contractility. (4) Antibiotics: Broad-spectrum antibiotics are indicated if there is evidence of infection. Amoxicillin-clavulanate (Clavamox) at 12.5-25 mg/kg PO q8-12h, or enrofloxacin (Baytril) at 5-10 mg/kg PO/IM q24h, are commonly used. Metronidazole at 10-15 mg/kg PO q12h can be added for anaerobic coverage. Antibiotics should be continued for 7-14 days. (5) Supportive care: Intravenous fluids, nutritional support, and antipyretics (e.g., meloxicam at 0.1-0.2 mg/kg PO q24h) may be necessary. Surgical intervention: Ovariohysterectomy (OHE) is indicated if medical therapy fails, if there is severe metritis, uterine necrosis, or peritonitis, or if the animal is a valuable breeding animal and the uterus is severely compromised. OHE is curative and prevents future reproductive issues. In cases of dystocia, cesarean section may be performed, and the placenta can be manually removed during surgery. Manual removal of the placenta is generally not recommended due to the risk of uterine damage and hemorrhage. In some cases, the placenta may be expelled spontaneously within 24-48 hours, but if not, treatment should be initiated. The choice of treatment depends on the individual case and the owner's wishes regarding future breeding.

The prognosis for retained placenta in dogs and cats is generally good with prompt and appropriate treatment. In uncomplicated cases where the placenta is expelled and no infection develops, the prognosis is excellent, and the animal can return to normal reproductive function. However, if metritis develops, the prognosis is guarded, especially if systemic signs are severe. With aggressive medical and surgical management, most animals recover, but there is a risk of chronic endometritis, which can lead to infertility. The prognosis is worse in cases of uterine necrosis, peritonitis, or septicemia, where mortality rates can be high. Factors that negatively affect prognosis include delayed treatment, presence of systemic illness, and underlying uterine pathology. Future fertility may be compromised if there is significant endometrial damage. In breeding animals, it is important to monitor for recurrence in subsequent pregnancies. The recurrence rate is not well documented, but may be higher in animals with uterine inertia. Overall, with early diagnosis and treatment, the prognosis is favorable, but close monitoring is essential.

Follow-up care for retained placenta is crucial to ensure complete resolution and to monitor for complications. After treatment, the animal should be re-examined within 48-72 hours. A complete physical examination, including temperature, should be performed. Abdominal ultrasonography should be repeated to confirm that the placenta has been expelled and that the uterus is involuting normally. The uterine wall thickness should decrease over time, and any fluid accumulation should resolve. Vaginal discharge should be monitored; it should become less purulent and eventually cease. A complete blood count and serum biochemistry should be repeated to ensure that infection is resolving and that calcium and glucose levels are normal. If antibiotics were prescribed, the full course should be completed. The animal should be kept in a clean, comfortable environment and should be monitored for signs of systemic illness. If the animal is a breeding animal, it is recommended to wait at least one estrous cycle before breeding again to allow the uterus to fully recover. During the next pregnancy, close monitoring is advised, and prophylactic measures, such as calcium supplementation, may be considered. In cases where ovariohysterectomy was performed, routine postoperative care is required, including incision care and pain management. Long-term follow-up should include regular reproductive health checks, especially if the animal is intended for breeding.

Clinical pearls: (1) In bitches, the normal time for placental expulsion is within 15 minutes after each puppy, but if the placenta is not passed within 1 hour, it is considered retained. (2) Ultrasonography is the most reliable diagnostic tool; look for echogenic material in the uterine lumen. (3) Serum progesterone levels should be <1 ng/mL postpartum; if elevated, consider retained luteal tissue. (4) Oxytocin should be used cautiously; if the cervix is closed, it can cause uterine rupture. (5) Calcium gluconate should be given slowly IV, with cardiac monitoring, to avoid arrhythmias. (6) Antibiotics should be based on culture and sensitivity, but broad-spectrum coverage is initially warranted. (7) In queens, the placenta is often eaten, so retention may be missed; monitor for signs of metritis. Pitfalls: (1) Do not manually remove the placenta, as this can cause hemorrhage and uterine damage. (2) Avoid excessive use of oxytocin, which can cause tetanic contractions and worsen retention. (3) Do not overlook systemic signs; retained placenta can lead to life-threatening metritis. (4) Do not assume that the placenta has been passed just because the animal appears normal; some animals may retain fragments. (5) In breeding animals, do not breed again until the uterus has fully recovered, as this can lead to infertility. (6) Do not use nonsteroidal anti-inflammatory drugs (NSAIDs) if the animal is dehydrated or has renal compromise. (7) Do not delay surgical intervention if medical therapy fails, as this can increase morbidity and mortality.

Current drug protocols for retained placenta in dogs and cats are based on Plumb's Veterinary Drug Handbook and theriogenology guidelines. Oxytocin: Dose: 0.5-2 IU/kg IM or SC, maximum 20 IU per dog, repeated every 30-60 minutes for up to 3 doses. Route: IM or SC. Interval: q30-60min. Duration: up to 3 doses. Prostaglandin F2α (Dinoprost tromethamine): Dose: 0.1-0.25 mg/kg SC. Route: SC. Interval: q24h. Duration: 2-3 days. Cloprostenol: Dose: 1-2 μg/kg SC. Route: SC. Interval: q24h. Duration: 2-3 days. Calcium gluconate (10%): Dose: 0.5-1.5 mL/kg IV, slowly over 10-20 minutes. Route: IV. Interval: once, may repeat in 6-12 hours if needed. Antibiotics: Amoxicillin-clavulanate: 12.5-25 mg/kg PO q8-12h for 7-14 days. Enrofloxacin: 5-10 mg/kg PO/IM q24h for 7-14 days. Metronidazole: 10-15 mg/kg PO q12h for 7-14 days. Supportive care: IV fluids (e.g., Lactated Ringer's solution) at maintenance rates (60-100 mL/kg/day) to correct dehydration. Antipyretics: Meloxicam: 0.1-0.2 mg/kg PO q24h for 3-5 days, but avoid in dehydrated animals. Anti-emetics: Maropitant (Cerenia) at 1 mg/kg SC q24h if vomiting. In cases of severe metritis, additional drugs such as dopamine agonists (cabergoline) may be used to reduce prolactin and support luteolysis, but are not primary therapy. Always adjust dosages based on patient status and renal/hepatic function.

Evidence-based literature on retained placenta in dogs and cats is limited compared to ruminants, but several studies and reviews provide guidance. Johnston, Kustritz, and Olson's 'Canine and Feline Theriogenology' (2001) is a cornerstone reference, detailing the pathophysiology and management of postpartum disorders. They recommend that retained placenta in bitches is often associated with uterine inertia and that oxytocin and calcium are first-line treatments. Noakes, Parkinson, and England's 'Veterinary Reproduction and Obstetrics' (2019) discusses retained fetal membranes in small animals, emphasizing the importance of early diagnosis and the role of prostaglandins. England and von Heimendahl's 'BSAVA Manual of Small Animal Reproduction' (2010) provides practical guidelines for diagnosis and treatment, including the use of ultrasonography and the indications for ovariohysterectomy. A study by Smith (2005) in the Journal of Small Animal Practice reported that retained placenta was diagnosed in 3% of bitches presented for postpartum problems, and that metritis was a common complication. Another study by Verstegen and Onclin (2002) in Theriogenology evaluated the use of PGF2α in postpartum bitches and found it effective in promoting uterine involution. The American College of Theriogenologists (ACT) and European College of Animal Reproduction (ECAR) have published consensus statements on postpartum care, recommending that retained placenta be treated aggressively to prevent metritis. Overall, the evidence supports a multimodal approach, including medical management with oxytocin, calcium, and antibiotics, and surgical intervention in severe cases. Future research is needed to establish standardized protocols for small animals.