Diagnostic Ultrasound Findings for Placenta Accreta Spectrum
Ultrasound is the primary diagnostic modality for placenta accreta spectrum, with gray-scale and Doppler findings demonstrating 90.72% sensitivity and 96.94% specificity, though no single ultrasound feature reliably differentiates accreta from increta or percreta. 1
Key Clinical Context
The most important ultrasound association is placenta previa, present in >80% of placenta accreta spectrum cases, particularly in patients with prior cesarean delivery. 1
Gray-Scale Ultrasound Findings
Primary Diagnostic Features
Multiple vascular lacunae within the placenta represent the most specific finding, appearing as irregular sonolucent spaces with the highest positive predictive value for diagnosis 1, 2, 3
Loss of the normal hypoechoic retroplacental clear zone between placenta and myometrium, though this finding has lower specificity and generates more false positives, particularly in the third trimester 1, 2, 3
Decreased retroplacental myometrial thickness <1 mm indicates deep placental invasion 1, 2
Abnormalities of the uterine serosa-bladder interface, including interruption or irregularity of the hyperechoic interface between uterine serosa and bladder wall 1, 4
Extension of placenta into myometrium, serosa, or bladder suggests deeper invasion 1
Color Doppler Findings
Essential Vascular Markers
Turbulent lacunar blood flow is the most common and diagnostically useful Doppler finding for placenta accreta spectrum 1, 5, 2
Increased subplacental vascularity with hypervascularity at the placental bed 1, 2, 4
Gaps in myometrial blood flow indicating disruption of normal uterine vasculature 1
Vessels bridging the placenta to the uterine margin or bladder 1, 2
High-velocity flow >15 cm/s within placental lacunae 4
Advanced Imaging Findings
3D Power Doppler Features
Hypervascularity of the entire uterine serosa-bladder wall interface is specific for placenta percreta 4
Irregular intraplacental vascularization with tortuous confluent vessels affecting the entire placental width is specific for placenta percreta 4
Critical Limitations
No ultrasound feature or combination of features reliably predicts the depth of invasion or accurately differentiates between accreta, increta, and percreta. 1, 6 This is because adherent and invasive placentation may coexist in the same placental bed and evolve with advancing gestation 6.
Diagnostic Approach Algorithm
Optimal Scanning Technique
Use combined transvaginal and transabdominal ultrasound with high-frequency (5-9 MHz) linear probe when body habitus permits 2
Ensure bladder is moderately full (200-300 mL) to properly evaluate the lower uterine segment and cesarean scar area 2
Apply minimal probe pressure when assessing the retroplacental clear zone to avoid artifactual loss 2
Add color Doppler and power Doppler imaging to significantly improve detection sensitivity 2, 4
Timing of Evaluation
Screen at 18-22 weeks during second trimester anatomy scan in at-risk patients 2
Features may be visible as early as 15-20 weeks gestation, with 86% detection rate in confirmed cases 3
Serial evaluations are recommended as findings evolve with advancing gestation 6
Diagnostic Certainty Based on Number of Findings
- 0 signs present: 5% likelihood of placenta accreta spectrum 7
- 1 sign present: 24% likelihood 7
- 2 signs present: 57% likelihood 7
- ≥3 signs present: 94% likelihood 7
Important Pitfalls to Avoid
The obliteration of retroplacental clear space alone is not reliable for diagnosis due to high false-positive rates, especially in the third trimester 3. Multiple lacunae with turbulent flow provide much higher diagnostic certainty 1, 3.
Distinguish true placenta accreta spectrum lacunae from benign placental lakes by assessing for associated findings: PAS-associated lacunae occur with loss of retroplacental clear zone, thinned myometrium <1mm, placenta previa, and turbulent flow on Doppler 5.
Clinical risk factors (placenta previa with prior cesarean delivery) remain equally important as ultrasound findings, as absence of ultrasound findings does not preclude diagnosis 1.
Management Implications
Women with suspected placenta accreta spectrum should be referred to a level III or IV maternal care facility with multidisciplinary expertise for delivery planning 1, 5. Planned delivery at 34 0/7 to 35 6/7 weeks gestation optimizes outcomes in stable patients 1, 8.