Retained Placenta: Causes and Complications
Definition and Pathophysiology
Retained placenta is diagnosed when the placenta fails to deliver spontaneously within 30 minutes after fetal delivery, though this timeframe is increasingly being challenged in favor of earlier intervention at 15 minutes to prevent catastrophic hemorrhage 1. The condition occurs when the retroplacental myometrium fails to contract adequately, preventing the normal shearing and detachment of the placenta from the uterine wall 2.
The underlying mechanism involves failure of the retroplacental myometrium to contract after delivery, which is necessary for placental separation. This may be caused by persistence of placental inhibitory factors (possibly progesterone or nitric oxide) that normally decrease before labor onset 2.
Primary Causes
Placenta Accreta Spectrum (PAS)
The most serious cause of retained placenta is PAS, which encompasses:
- Placenta accreta: Chorionic villi adhere directly to myometrium without intervening decidua (60% of PAS cases)
- Placenta increta: Villi invade into the myometrium (16% of cases)
- Placenta percreta: Villi penetrate through myometrium into serosa and adjacent organs (22% of cases) 3, 4, 5
The pathophysiology involves defects in the decidua basalis, often related to prior uterine scarring, leading to abnormally deep placental anchoring 3.
Uterine Atony
Inadequate uterine contractions prevent the normal shearing mechanism required for placental separation 6.
Cervical Closure
Premature closure of the cervix can trap the placenta before expulsion occurs 6.
Risk Factors
Strongest Risk Factors:
- Prior cesarean delivery: Risk increases dramatically from 7-fold after one cesarean to 56-fold after three cesareans 3
- Placenta previa: Present in 49% of PAS cases and the single most important risk factor 3
- Stillbirth: Increases risk 4.5-5.7 fold regardless of parity 7
- Prior retained placenta: Significant independent risk factor 8, 6
Additional Risk Factors:
- Advanced maternal age (≥30 years) 7
- High parity 6
- Prior uterine surgery (D&C, myomectomy) 8, 6
- Assisted reproductive technology/IVF 3, 9
- Congenital uterine anomalies (10-fold increased risk) 9
- Preterm delivery (24-27 weeks vs ≥34 weeks) 7
- Prolonged oxytocin use 6
- Labor induction 8
- Prior postpartum hemorrhage 3
Major Complications
Life-Threatening Hemorrhage
Postpartum hemorrhage is the most immediate and dangerous complication, with retained placenta accounting for a case mortality rate of nearly 10% in rural areas 2. The risk of severe hemorrhage is greatest at delivery when the placenta fails to separate normally 3.
- Blood transfusion requirements are significantly increased, particularly when general anesthesia is required for manual removal 10
- Massive hemorrhage may necessitate hysterectomy, particularly with PAS 11
Infection and Sepsis
- Endometritis develops in retained placental tissue 6
- Sepsis and septic shock: Occurred in 28% of patients attempting expectant management of PAS, with 70% of severe morbidity cases involving maternal sepsis 11
- Febrile morbidity is common with conservative management approaches 11
Delayed Complications
- Secondary postpartum hemorrhage: Can occur days to weeks after delivery if placental fragments remain 6
- Uterine necrosis and fistula formation: Rare but severe complications with expectant management 11
- Prolonged hospitalization: Significantly longer with complications (4.1 vs 3.5 days) 10
Surgical Complications
When manual removal or hysterectomy is required:
- Injury to adjacent pelvic organs (bladder, ureters) 11
- Loss of future fertility with hysterectomy 11
- Anesthesia-related complications including difficult intubation and aspiration (particularly with general anesthesia) 10
- Deep vein thrombosis and pulmonary embolism 11
Failed Conservative Management
For PAS managed expectantly (leaving placenta in situ):
- 22-94% ultimately require hysterectomy depending on severity 11
- Median time to placental involution is 13.5 weeks when successful 11
- Failure rate is 44% with placenta percreta vs 7% with less extensive disease 11
Clinical Pitfalls
The most critical pitfall is delayed recognition and intervention. Only 70% of PAS cases are identified antenatally despite known risk factors, leading to increased morbidity and mortality when diagnosis is unexpected at delivery 3.
Manual removal should not be routinely performed outside specialized facilities except in cases of severe uncontrollable hemorrhage 12. However, when hemorrhage occurs, intervention must be rapid—within hours rather than the traditional 30-minute window 2, 1.
Not all retained placenta represents PAS—18-29% of clinically diagnosed PAS cases lack histopathologic confirmation, and simple retention can occur from uterine atony, cervical closure, or other causes 5.