What is the recommended antenatal surveillance strategy for a pregnant woman with risk factors for placenta accreta, such as prior cesarean delivery, other uterine surgery, placenta previa, multiple gestation, or advanced maternal age?

Antenatal Surveillance for Placenta Accreta Spectrum

Primary Screening Strategy

All women with placenta previa and any history of prior cesarean delivery or uterine surgery must undergo targeted ultrasound evaluation for placenta accreta spectrum (PAS), as this combination represents the highest-risk scenario for life-threatening hemorrhage. 1, 2

The surveillance protocol should be risk-stratified based on the presence of key clinical factors:

High-Risk Population Requiring PAS Surveillance

• Prior cesarean delivery combined with placenta previa – present in approximately 49% of all PAS cases and >80% of confirmed accreta cases 1, 2
• Multiple prior cesarean deliveries – risk escalates dramatically: 3% with no prior cesarean, 11% with one, 40% with two, 61% with three, and 67% with five or more cesareans 1, 3
• Other uterine surgery including myomectomy, dilation and curettage, or procedures causing endometrial-myometrial interface damage 1, 3
• Advanced maternal age, multiparity, in-vitro fertilization, Asherman syndrome, and prior postpartum hemorrhage are additional independent risk factors 1, 3

Ultrasound Surveillance Protocol

Gray-scale transabdominal and transvaginal ultrasound with color Doppler is the primary surveillance modality, demonstrating 90.7% sensitivity and 96.9% specificity for PAS detection. 1, 2

Key Ultrasound Findings to Document at Each Examination:

• Placenta previa (present in >80% of PAS cases) – the single most important sonographic association 1, 2
• Multiple placental lacunae – the most strongly associated gray-scale finding for PAS 1, 2
• Loss of the normal hypoechoic retroplacental zone between placenta and myometrium 1, 2
• Retroplacental myometrial thickness <1 mm 1, 2
• Disruption of the uterine serosa-bladder interface 1, 2
• Direct placental extension into myometrium, serosa, or bladder (pathognomonic for percreta) 1, 2

Color Doppler Assessment Should Include:

• Turbulent lacunar blood flow (most common Doppler finding) 1, 2
• Increased subplacental vascularity 1, 2
• Gaps in myometrial blood flow 1, 2
• Bridging vessels from placenta to uterine margin 1, 2

Timing and Frequency of Surveillance

• Initial detailed anatomic survey at 18–22 weeks should include placental location assessment 4
• If placenta previa is identified at mid-trimester, repeat ultrasound at 28–32 weeks to reassess position, as many cases resolve with advancing gestation 4
• For persistent placenta previa with risk factors, targeted PAS evaluation should be performed by 28–32 weeks by an examiner experienced in PAS diagnosis 1, 2
• In multiple gestations with prior cesarean or uterine surgery, assessment for PAS can be performed during routine growth surveillance 1

Role of MRI in Surveillance

MRI is not recommended as the initial or routine imaging modality for PAS surveillance because its incremental diagnostic value over ultrasound is uncertain. 2

MRI may be considered selectively in specific scenarios:

• Posterior placenta previa where ultrasound visualization is suboptimal 2, 4
• Suspected placenta percreta with possible bladder, bowel, or parametrial involvement requiring surgical planning 2, 5
• Equivocal ultrasound findings in high-risk patients 2, 4
• Maternal obesity limiting sonographic windows 4

Gadolinium contrast is contraindicated for routine fetal/placental imaging. 1, 4

Critical Clinical Caveats

The Negative Ultrasound Paradox

Absence of ultrasound abnormalities does NOT exclude PAS; clinical risk factors (placenta previa + prior cesarean) remain equally important predictors and mandate preparation for PAS at delivery. 2

This is a common and dangerous pitfall – approximately 18–29% of clinically diagnosed PAS cases lack definitive histopathologic confirmation after hysterectomy, yet many demonstrate uterine wall thinning <3 mm, suggesting the ultrasound findings were subtle or absent despite true pathology. 2

Limitations of Imaging

• No single ultrasound feature or combination reliably predicts depth of invasion (accreta vs. increta vs. percreta) 2
• Inter-observer variability affects diagnostic accuracy, emphasizing the need for experienced examiners 1
• First-trimester features may be present but most diagnoses occur in second/third trimester 1

Delivery Planning Based on Surveillance Findings

When PAS is suspected or confirmed on antenatal surveillance:

Mandatory Preparations

• Refer immediately to a Level III or IV maternal care facility with multidisciplinary expertise 1, 2, 4
• Assemble a multidisciplinary team including maternal-fetal medicine, experienced pelvic surgeons (gynecologic oncologist for grade 3E percreta), urologic surgeons (if bladder involvement suspected), interventional radiologists, obstetric anesthesiologists, blood bank with massive transfusion protocols, and intensive care capabilities 2, 5
• Notify blood bank early for anticipated large-volume transfusion needs 2, 4
• Optimize maternal hemoglobin with oral or intravenous iron supplementation during pregnancy 2, 4

Optimal Delivery Timing

Plan cesarean hysterectomy at 34 0/7 to 35 6/7 weeks gestation for hemodynamically stable patients with confirmed PAS. 2, 4

This narrow window balances:

• Neonatal prematurity risks before 34 weeks 2, 4
• Approximately 50% risk of emergent delivery for hemorrhage if waiting beyond 36 weeks 2, 4

Administer antenatal corticosteroids when delivery is planned before 37 0/7 weeks. 2, 4

Indications for Earlier Delivery

• Persistent bleeding 2, 4
• Preeclampsia 2, 4
• Onset of labor 2, 4
• Rupture of membranes 2, 4
• Fetal compromise 2, 4
• Development of maternal comorbidities 2, 4

Surgical Approach

The standard operative technique is cesarean hysterectomy with the placenta left in situ after fetal delivery; manual removal of the placenta is absolutely contraindicated because it precipitates catastrophic hemorrhage. 2, 4, 6

Intraoperative Considerations

• Make the uterine incision away from the placenta when feasible 2, 4
• Consider dorsal lithotomy positioning for optimal pelvic exposure 2, 4
• Place prophylactic ureteric stents if bladder invasion is suspected 2, 4
• Resuscitative endovascular balloon occlusion of the aorta (REBOA) is ideal for grade 3E disease when available 2, 5

Hemorrhage Management

• Activate massive transfusion protocol with packed red blood cells, fresh frozen plasma, and platelets in a ratio of 1:1:1 to 1:2:4 2, 4
• Maintain maternal core temperature >36°C to preserve coagulation factor activity 2, 4
• Re-dose prophylactic antibiotics if blood loss exceeds 1,500 mL 2, 4

Conservative Management Considerations

Conservative (uterine-sparing) management is considered investigational and should be offered only after thorough counseling regarding uncertain benefits and substantial risks. 2

For placenta percreta managed conservatively:

• 44% failure rate requiring subsequent hysterectomy 2
• 17% severe adverse complication rate (sepsis, organ failure, or death) 2

This approach may be considered in highly selected cases with grade 3E disease involving bladder, cervix, or parametria where immediate hysterectomy poses catastrophic hemorrhage risk, with planned delayed hysterectomy at approximately 4 weeks postpartum. 5

Quality Improvement and Pathology Correlation

• Pathology feedback to radiology and surgical teams is essential for evaluating diagnostic accuracy and management outcomes 2
• Enhanced surveillance in high-risk populations is justified given the markedly increased morbidity when PAS is not suspected before delivery 2