Retensio Plasenta

01Pathophysiology

The third stage of labor begins after delivery of the infant and ends with delivery of the placenta. Normal placental separation occurs through a process of retroplacental clot formation at the decidua basalis. Uterine contractions compress the spiral arteries and create a plane of cleavage between the placenta and the uterine wall. Once this plane develops, the placenta shears off and is expelled by continued uterine contractions.

Retained placenta occurs when this process fails. There are three distinct mechanisms, each with different underlying pathology:

Placenta adherens is the most common type. The placenta has separated incompletely or not at all because the uterus fails to generate adequate retraction and contraction behind the placenta. The myometrium in the lower segment or at the placental site is hypotonic, so the retroplacental clot does not form effectively, and the mechanical shearing force is insufficient. This is functionally a problem of uterine atony localized to the placental bed.

Trapped placenta occurs when the placenta has fully separated from the uterine wall but cannot be expelled because the cervix closes or constricts before the placenta passes through. The detached placenta sits in the lower uterine segment or upper cervical canal. This variant is important because it produces a distinctive clinical picture: the cord may lengthen (a sign of separation), a gush of blood may occur (retroplacental clot escaping), yet the placenta does not appear at the introitus.

Placenta accreta spectrum represents the most dangerous mechanism. Here, the placenta has invaded beyond the normal decidual plane into the myometrium (accreta), through the myometrium (increta), or through the serosa to adjacent organs (percreta). Because there is no decidual cleavage plane, the placenta cannot separate. Attempting manual removal in this setting risks catastrophic hemorrhage or uterine perforation. The pathophysiology traces back to prior endometrial damage (from cesarean sections, curettage, or endometritis) that destroyed the decidua basalis, allowing trophoblastic invasion directly into the myometrium.

The clinical consequence of all three mechanisms is the same: the uterus cannot fully contract, the spiral arteries at the placental site remain open, and the patient develops postpartum hemorrhage. The connection between pathophysiology and presentation is direct: an undelivered or partially delivered placenta physically prevents the myometrium from clamping down on its own blood supply.

02Classification and Clinical Manifestation

Signs of placental separation (important for distinguishing trapped from adherent placenta):

If these signs are present but the placenta has not appeared, the diagnosis is a trapped placenta. If none of these signs occur within 30 minutes, the diagnosis is placenta adherens or accreta.

03Diagnostic Workup

Retained placenta is primarily a clinical diagnosis. The clock starts at the moment the infant is delivered. If the placenta has not been expelled within 30 minutes despite active management (controlled cord traction, oxytocin, uterine massage), the diagnosis is made.

When the placenta delivers but appears incomplete, the next step is careful inspection of the maternal surface. A normal placenta has a complete set of cotyledons forming a smooth, uninterrupted maternal surface. A gap, defect, or area of absent tissue indicates that a fragment remains in the uterus. Similarly, ragged membranes trailing off the placental edge suggest retained membranes. This inspection is the most commonly tested "bedside diagnostic" in exam vignettes.

Ultrasound is the best initial imaging modality when there is clinical suspicion of retained products. It can identify echogenic tissue within the endometrial cavity and help differentiate retained placental tissue from simple blood clot. However, in the acute setting of ongoing postpartum hemorrhage, ultrasound should not delay manual exploration if the clinical picture is clear.

There is no role for MRI in the acute evaluation of retained placenta. MRI may be used in the elective setting if placenta accreta spectrum is suspected antenatally, but during active third-stage complications, the workup is clinical and ultrasound-based.

Laboratory studies (hemoglobin, hematocrit, platelet count, fibrinogen, PT/INR) are drawn not to diagnose retained placenta but to quantify blood loss and to screen for consumptive coagulopathy. A fibrinogen level falling below 200 mg/dL in the postpartum period is an early warning of DIC and should prompt aggressive factor replacement.

04Management and Treatment

Active management of the third stage is the standard of care and reduces the incidence of retained placenta. The key components are: (1) oxytocin administration immediately after delivery of the infant, (2) controlled cord traction (not forceful pulling, which risks cord avulsion or uterine inversion), and (3) uterine massage after placental delivery.

If 30 minutes pass and the placenta has not delivered, the next best step is manual removal under anesthesia. This is the most tested management decision for retained placenta on licensing exams. The procedure involves passing a gloved hand through the cervix, identifying the lower placental edge, and gently sweeping the fingers between the placenta and the uterine wall in the plane of the decidua basalis. The other hand stabilizes the fundus abdominally to prevent uterine inversion.

A critical teaching point: if the operator's fingers cannot find a cleavage plane and the placenta seems inseparable from the myometrium, the immediate concern is placenta accreta. Continuing to force dissection in this situation risks tearing into the myometrium and causing uncontrollable hemorrhage. The correct response is to stop, call for surgical backup, and prepare for possible hysterectomy.

For trapped placenta, the cervix needs to be relaxed before extraction is possible. Nitroglycerin 50 to 100 mcg IV is the first-line agent for acute cervical relaxation. It acts within 60 to 90 seconds and has a short duration. If nitroglycerin fails, general anesthesia with a volatile halogenated agent (sevoflurane) provides both uterine and cervical relaxation, allowing manual extraction.

Uterotonic agents should be administered concurrently with manual removal to promote uterine contraction and limit hemorrhage:

Methylergonovine is contraindicated in hypertension because it causes vasoconstriction. Carboprost is contraindicated in asthma because prostaglandin F2-alpha induces bronchospasm. These two contraindications are tested repeatedly.

05Differential Diagnosis and Distractors

The classic vignette will ask you to differentiate the four T's of postpartum hemorrhage: Tone (atony), Tissue (retained placenta), Trauma (lacerations), and Thrombin (coagulopathy). The distinguishing feature for retained placenta is always that the uterus is not empty. The vignette will either state that the placenta has not delivered, that a cotyledon is missing on inspection, or that the uterus remains boggy despite oxytocin (because retained tissue prevents full contraction).

Uterine inversion is the highest-stakes distractor. It occurs when excessive cord traction is applied before placental separation, pulling the fundus through the cervix. The physical exam finding of a mass at the introitus with an absent uterine fundus on abdominal palpation is the discriminator. Management of inversion (manual replacement with Johnson maneuver, uterine relaxation) is the opposite of retained placenta management (uterine contraction, removal of tissue).

06Traps and High-Yield Pearls

The most frequent trap involves the sequence of management steps. When a vignette describes a patient with a boggy uterus and continued bleeding after delivery, students often jump directly to uterotonics or surgical intervention. The correct first response depends on whether the placenta has been delivered. If the placenta is out and complete, the diagnosis is atony and uterotonics are appropriate. If the placenta is still in utero or inspection reveals a missing cotyledon, the tissue must be removed before uterotonics alone can be effective. A uterus with retained tissue will not respond adequately to oxytocin because the myometrial fibers cannot fully contract around the foreign material. This is the central logic test-writers exploit: recognizing that the treatment for postpartum hemorrhage depends entirely on identifying the cause.

A second common error is confusing the trapped placenta with placenta accreta. In a trapped placenta, signs of separation (cord lengthening, gush of blood, firm globular uterus) are present, indicating that the placenta has already detached from the wall. The problem is purely mechanical: the cervix has constricted. The treatment is cervical relaxation with nitroglycerin, not hysterectomy. In accreta, there are no signs of separation, the placenta is fused to the myometrium, and attempting forceful removal is dangerous.

Students also underestimate the importance of inspecting the placenta after every delivery. The exam will test this by describing a patient who has "delivered the placenta" but develops delayed postpartum hemorrhage hours later. The clue will be a statement buried in the stem: "the placenta appeared intact but membranes were ragged" or "no formal inspection was performed." Retained fragments, even small ones, serve as a nidus for hemorrhage and infection (endometritis).

The core competency being tested is the ability to systematically evaluate the cause of postpartum hemorrhage by working through the four T's in order, and to correctly sequence the management of retained placenta from conservative (cord traction, uterotonics) to procedural (manual removal) to surgical (curettage, hysterectomy) without skipping steps or choosing interventions that do not match the underlying mechanism.