What are the risk factors, diagnostic methods, and optimal management strategies for placenta accreta spectrum?

Placenta Accreta Spectrum: A Comprehensive Discussion

Definition and Classification

Placenta accreta spectrum (PAS) represents a life-threatening obstetric condition characterized by abnormal placental invasion into or through the myometrium, replacing the traditional categorical terminology (accreta, increta, percreta) with a descriptive grading system that parallels FIGO guidelines. 1

The pathophysiology centers on defects at the endometrial-myometrial interface from uterine scarring that prevent normal decidualization, allowing abnormally deep anchoring of placental villi directly to myometrial fibers without intervening decidua basalis. 1, 2

Grading System

• Placenta accreta (creta): Villi adhere directly to myometrium with invasion <50% of myometrial thickness, representing approximately 60% of cases 1, 3
• Placenta increta: Villi invade >50% into the myometrium 1, 3
• Placenta percreta: Complete transmural invasion through the myometrium with potential extension into bladder, bowel, parametria, or other pelvic structures—the most severe form with dramatically worse outcomes 1, 4

Epidemiology and Risk Factors

The incidence has increased dramatically from 1 in 2,510 deliveries in the 1970s-1980s to 1 in 272 by 2016 in the United States, directly correlating with rising cesarean delivery rates. 1

Primary Risk Factors

Placenta previa overlying a cesarean scar represents the single most important risk factor, present in 49% of all PAS cases. 5, 6

The risk escalates exponentially with multiple cesarean deliveries: 1, 5

• 3% risk with placenta previa alone
• 11% risk with previa plus one prior cesarean
• 40% risk with two prior cesareans
• 61% risk with three prior cesareans
• 67% risk with five or more prior cesareans

Additional Risk Factors

• Advanced maternal age and multiparity 1
• Prior uterine surgery, curettage, or Asherman syndrome 1, 4
• In vitro fertilization 1, 6
• Prior postpartum hemorrhage 1
• Uterine anomalies (congenital or acquired) 1
• Smoking and hypertension 1

Diagnostic Approach

Ultrasound as First-Line Imaging

Gray-scale ultrasound should be the first-line imaging modality for diagnosing PAS, with reported sensitivity of 90.72% and specificity of 96.94% in systematic reviews. 1, 5

Key Ultrasound Findings

Second and Third Trimester Features: 1, 5

• Loss of the normal hypoechoic retroplacental zone between placenta and myometrium
• Multiple placental lacunae (most strongly associated finding)
• Thinning or disruption of the hyperechoic uterine serosa-bladder interface
• Decreased retroplacental myometrial thickness (<1 mm)
• Abnormalities of the uterine serosa-bladder interface
• Extension of placenta into myometrium, serosa, or bladder

Color Doppler Findings: 1

• Turbulent lacunar blood flow (most common finding)
• Increased subplacental vascularity
• Gaps in myometrial blood flow
• Vessels bridging the placenta to the uterine margin

Critical Diagnostic Caveat

The absence of ultrasound findings does not exclude PAS; clinical risk factors remain equally important as predictors, with substantial interobserver variability in ultrasound interpretation. 1, 5

Role of MRI

MRI is not the preferred modality for initial evaluation of PAS, as it is unclear whether MRI improves diagnosis beyond ultrasound alone. 1 However, MRI may be helpful in select cases such as posterior placenta previa, suspected placenta percreta, or when ultrasound findings are equivocal. 5, 6

Management Strategy

Delivery Planning and Timing

Women with suspected PAS must deliver at a Level III or IV maternal care facility with a multidisciplinary team experienced in managing this condition. 1, 4, 5

Optimal Gestational Age

Planned cesarean delivery should occur at 34 0/7 to 35 6/7 weeks gestation in stable patients to balance neonatal outcomes against maternal hemorrhage risk. 1, 4, 5 This timing is critical because approximately 50% of women with PAS who wait beyond 36 weeks require emergent delivery for hemorrhage. 5, 6

Indications for Earlier Delivery

Earlier delivery may be required for: 1, 6

• Persistent bleeding
• Preeclampsia
• Spontaneous labor
• Rupture of membranes
• Fetal compromise
• Developing maternal comorbidities

Multidisciplinary Team Requirements

The delivery team must include: 4, 5

• Maternal-fetal medicine subspecialists
• Experienced pelvic surgeons (gynecologic oncologists preferred for Grade 3 percreta)
• Urologic surgeons when bladder involvement is suspected
• Interventional radiologists
• Obstetric anesthesiologists
• Neonatologists
• Blood bank with massive transfusion protocols
• Intensive care capabilities

Surgical Approach

The standard approach is cesarean hysterectomy with the placenta left in situ after fetal delivery; manual removal of the placenta should NEVER be attempted as it causes catastrophic hemorrhage. 4, 5, 6

Intraoperative Principles

• Make the uterine incision away from the placenta when possible 6
• After fetal delivery, leave the placenta in situ if abnormal attachment is evident 6
• Consider dorsal lithotomy positioning to allow vaginal access and optimal surgical visualization 6
• For suspected bladder involvement, consider ureteric stent placement preoperatively 6

Blood Product Management

In the setting of hemorrhage, transfuse packed red blood cells, fresh frozen plasma, and platelets in a ratio of 1:1:1 to 1:2:4. 1, 5, 6

Additional hemorrhage management: 6

• Maintain maternal temperature >36°C for optimal clotting factor function
• Re-dose prophylactic antibiotics if blood loss exceeds 1,500 mL
• Obtain baseline laboratory assessment including platelet count, PT, PTT, and fibrinogen at initiation of bleeding

Preoperative Optimization

• Optimize hemoglobin values during pregnancy; treat anemia with oral or intravenous iron 5, 6
• Notify blood bank in advance due to frequent need for large-volume transfusion 5, 6
• Administer antenatal corticosteroids when delivery is anticipated before 37 0/7 weeks 6

Conservative Management Considerations

Conservative or expectant management (uterine-sparing therapy) should be considered only for carefully selected cases after detailed counseling about risks, uncertain benefits, and efficacy, and should be considered investigational. 1

For placenta percreta specifically, conservative management has a 44% failure rate requiring hysterectomy and a 17% severe adverse complication rate. 4

Clinical Outcomes and Complications

Maternal Morbidity

Placenta percreta carries the highest risk: 4

• 44% failure rate requiring hysterectomy
• 17% severe adverse complication rate (sepsis, organ failure, or death)
• 28% infection and febrile morbidity rate
• 6% severe morbidity rate including sepsis, organ failure, or death

Common Pitfalls to Avoid

1. Failure to diagnose PAS in women with placenta previa and prior cesarean deliveries can lead to catastrophic hemorrhage 6
2. Attempting manual placental removal when PAS is encountered intraoperatively causes profuse hemorrhage 5, 6
3. Delivering at a facility lacking essential resources (blood bank, multidisciplinary team, ICU support) significantly increases maternal morbidity and mortality 4
4. Delaying delivery beyond 36 weeks in stable patients increases risk of emergent hemorrhagic delivery 5, 6

Pathological Diagnosis and Reporting

The pathology diagnosis uses a descriptive grading system rather than categorical terminology, with diagnostic criteria requiring absence of decidua basalis and direct placental attachment to or through myometrium. 1 Clinical diagnosis is acceptable when histologic confirmation is unavailable, particularly in conservatively managed cases. 4

A significant subset (18-29%) of cases clinically managed as PAS lack pathologic confirmation even after hysterectomy, though many show thinning of the uterine wall with <3 mm of intervening myometrium. 1

Quality Improvement and Surveillance

Pathology provides essential feedback to radiology and surgery as a quality improvement tool and is crucial for evaluating management strategy outcomes. 1 Enhanced antenatal clinical suspicion and surveillance in high-risk populations is justified given the increased morbidity and mortality when PAS is not suspected prior to delivery. 1