Please discuss placenta accreta spectrum, including its risk factors, antenatal diagnosis, and management options.

Placenta Accreta Spectrum: A Comprehensive Discussion

Definition and Classification

Placenta accreta spectrum (PAS) represents a life-threatening obstetric emergency where the placenta abnormally invades the myometrium due to defective decidualization at the endometrial-myometrial interface, creating a vascular catastrophe where the placenta cannot separate from the uterus without catastrophic hemorrhage. 1, 2

The spectrum is classified into three grades based on depth of invasion:

• Placenta accreta (creta): Villi adhere directly to myometrium with invasion of <50% of myometrial thickness, accounting for approximately 60% of PAS cases 1
• Placenta increta: Villi invade >50% of myometrial thickness 1
• 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 maternal outcomes 1, 3

Epidemiology and Risk Factors

Rising Incidence

The incidence of PAS in the United States has increased dramatically from 1 in 2,510 deliveries (1970s-1980s) to 1 in 272 deliveries by 2016, directly paralleling the rise in cesarean delivery rates 1, 4.

Primary Risk Factors

The single most important risk factor is placenta previa overlying a prior cesarean scar, present in approximately 49% of all PAS cases. 1, 2

The risk escalates dramatically with increasing number of prior cesarean deliveries:

• Placenta previa alone: 3% risk of PAS 1
• Placenta previa + 1 prior cesarean: 11% risk (7-fold increase) 1, 2, 3
• Placenta previa + 3 prior cesareans: 56-fold increased risk 2, 3

Additional Risk Factors

• Advanced maternal age 1, 3
• Multiparity 1, 3
• Prior uterine curettage or Asherman syndrome 1, 3
• In-vitro fertilization 1, 4
• Short intervals between cesarean deliveries 4
• Smoking 4

Critical clinical caveat: PAS can occur in nulliparous women without any prior uterine surgery, though this is rare 5.

Antenatal Diagnosis

Why Early Diagnosis Matters

Antenatal diagnosis of PAS is absolutely essential because maternal and neonatal outcomes are dramatically improved when delivery occurs at a specialized center before the onset of labor or bleeding and with avoidance of placental disruption. 5, 3

Ultrasound as Primary Diagnostic Modality

Gray-scale ultrasound is the first-line diagnostic tool, with pooled sensitivity of 90.72% (95% CI: 87.2-93.6%) and specificity of 96.94% (95% CI: 96.3-97.5%) in systematic reviews. 5, 1

Key Gray-Scale Ultrasound Findings

Women with risk factors (especially placenta previa + prior cesarean) should be evaluated by experienced providers in the second and third trimesters 5:

• Placenta previa: Present in >80% of PAS cases in large series 5
• Multiple vascular lacunae within the placenta: The most strongly associated finding 5, 1
• Loss of the normal hypoechoic retroplacental zone between placenta and myometrium 5, 1
• Decreased retroplacental myometrial thickness (<1 mm) 5, 1
• Abnormalities of the uterine serosa-bladder interface 5, 1
• Direct extension of placental tissue into myometrium, serosa, or bladder (suggestive of percreta) 5, 1

Color Doppler Findings

Color flow Doppler imaging facilitates diagnosis 5:

• Turbulent lacunar blood flow: Most common Doppler finding 5, 1
• Increased subplacental vascularity 5, 1
• Gaps in myometrial blood flow 5, 1
• Vessels bridging the placenta to uterine margin 5, 1

Critical Diagnostic Caveat

The absence of ultrasound findings does NOT exclude PAS; clinical risk factors remain equally important as predictors of PAS, and this is a Grade 1A recommendation. 5, 1

This is because:

• Many ultrasound abnormalities associated with PAS are also common in normal placentas 5
• Studies reporting high sensitivity/specificity have selection bias (patients with known high-risk factors) 5
• Inter-observer variability affects interpretation 1
• No single ultrasound feature or combination reliably predicts depth of invasion or type of PAS 5

Role of MRI

MRI is NOT recommended as the initial imaging modality for PAS because its incremental diagnostic value over ultrasound alone is uncertain (Grade 1B recommendation). 5, 1

MRI may be considered in selected scenarios:

• Posterior placenta previa (difficult to visualize on ultrasound) 1
• Suspected percreta requiring surgical planning 1
• Equivocal ultrasound findings 1

Management Strategy

Delivery Location and Timing

Women with suspected PAS must be delivered at a Level III or IV maternal care center with considerable experience to improve outcomes (Grade 1B recommendation). 5, 1, 3

The optimal timing for planned cesarean delivery is 34 0/7 to 35 6/7 weeks gestation for hemodynamically stable patients (Grade 1A recommendation). 5, 1, 2, 3

This timing balances:

• Neonatal maturity against maternal hemorrhage risk 1
• Approximately 50% of patients who remain pregnant beyond 36 weeks require emergent delivery for hemorrhage 1, 2

Earlier delivery may be required for:

• Persistent bleeding 5
• Preeclampsia 5
• Labor 5
• Rupture of membranes 5
• Fetal compromise 5
• Developing maternal comorbidities 5

Multidisciplinary Team Requirements

Optimal management involves a standardized approach with a comprehensive multidisciplinary care team accustomed to PAS management (Grade 1B recommendation). 5, 1, 3

Required team members include:

• Maternal-fetal medicine physicians 1, 2, 3
• Experienced pelvic surgeons (gynecologic oncologists preferred for Grade 3 percreta) 1, 3
• Urologic surgeons (when bladder involvement suspected) 1, 2, 3
• Interventional radiologists 1, 2, 3
• Obstetric anesthesiologists 1, 2, 3
• Blood bank with massive transfusion protocols 1, 2, 3
• Neonatologists 3
• Intensive care unit capabilities 1, 2, 3

Performing delivery at a center lacking these essential resources significantly increases maternal morbidity and mortality. 3

Surgical Approach

The standard operative technique is cesarean hysterectomy with the placenta left in situ after fetal delivery; manual removal of the placenta is strictly contraindicated because it precipitates catastrophic hemorrhage. 1, 2, 3

Intraoperative Principles

• Make the uterine incision away from the placenta when feasible 1
• Consider dorsal lithotomy positioning for optimal pelvic exposure 1
• Place prophylactic ureteric stents if bladder invasion is anticipated 1
• Never attempt manual placental removal 1, 2, 3

Hemorrhage Management

In the setting of massive hemorrhage, transfuse packed red blood cells, fresh frozen plasma, and platelets in a ratio of 1:1:1 to 1:2:4 (Grade 1A recommendation). 5, 1

Additional hemorrhage management strategies:

• Consider tranexamic acid to reduce blood loss 3
• Maintain maternal core temperature >36°C to preserve coagulation factor activity 1, 3
• Re-dose prophylactic antibiotics if blood loss exceeds 1,500 mL 1

Pre-operative Optimization

• Correct anemia during pregnancy with oral or intravenous iron supplementation 1
• Ensure early notification of blood bank for anticipated large-volume transfusion needs 1
• Administer antenatal corticosteroids when delivery is planned before 37 0/7 weeks gestation 1

Conservative (Uterine-Sparing) Management

Conservative or expectant management should be considered only for carefully selected cases after detailed counseling about the risks, uncertain benefits, and efficacy, and should be considered investigational (Grade 2C recommendation). 5

Outcomes of Conservative Management for Placenta Percreta

The evidence for conservative management is sobering:

• 44% failure rate requiring subsequent hysterectomy 1, 3
• 17% severe adverse complication rate (sepsis, organ failure, or death) 1, 3
• 28% infection/febrile morbidity 1, 3
• 6% severe morbidity including sepsis, organ failure, or death 1, 3

One recent single-center series (2015-2022) reported leaving the placenta in situ in 11 patients, with 55% achieving uterine preservation, but 36% required minimally invasive hysterectomy and 9% required abdominal hysterectomy at a median of 5 weeks postpartum 6. This approach involved prophylactic intravenous antibiotics for up to 1 week and close outpatient follow-up until the uterus was empty, with median time to resolution of 18 weeks in successful cases 6.

Clinical Outcomes and Complications

Maternal Morbidity for Placenta Percreta

Placenta percreta carries the worst maternal outcomes:

• 44% failure rate (need for hysterectomy) 1, 3
• 17% severe adverse complication rate 1, 3
• 28% infection/febrile morbidity 1, 3
• 6% severe morbidity (sepsis, organ failure, or death) 1, 3

Pathologic Confirmation

Between 18-29% of clinically diagnosed PAS cases lack histopathologic confirmation after hysterectomy, though many demonstrate uterine wall thinning of <3 mm. 1

Pathologic diagnosis requires:

• Absence of decidua basalis 1, 3
• Direct placental attachment to or through the myometrium 1, 3

Quality Improvement

• Pathology feedback to radiology and surgery is essential for quality improvement initiatives and evaluating management outcomes 1
• Enhanced antenatal surveillance in high-risk populations (placenta previa with prior cesarean) is justified given increased morbidity when PAS is not suspected before delivery 1

Common Pitfalls and How to Avoid Them

1. Relying solely on ultrasound: Always integrate clinical risk factors even when ultrasound is negative 5, 1

2. Attempting manual placental removal: This causes catastrophic hemorrhage and is absolutely contraindicated 1, 2, 3

3. Delivering at an unprepared facility: Lack of multidisciplinary team and resources dramatically increases maternal morbidity and mortality 3

4. Waiting too long for delivery: 50% of patients beyond 36 weeks require emergent delivery for hemorrhage 1, 2

5. Overestimating conservative management success: The 44% failure rate and 17% severe complication rate for percreta make this approach investigational only 1, 3

6. Assuming ultrasound features predict depth of invasion: No combination of ultrasound findings reliably differentiates accreta from increta from percreta 5