How do I diagnose placenta accreta spectrum in a pregnant woman in the second or third trimester with risk factors such as prior cesarean delivery, uterine surgery, or placenta previa?

How to Diagnose Placenta Accreta Spectrum

Diagnose placenta accreta spectrum using combined transvaginal and transabdominal ultrasound with color Doppler in the second or third trimester, focusing on multiple placental lacunae (the most specific finding), loss of the retroplacental clear zone, and turbulent lacunar blood flow in women with prior cesarean delivery and placenta previa. 1

Identify High-Risk Patients Requiring Dedicated Imaging

Prior cesarean delivery combined with placenta previa is the single most critical risk factor requiring systematic evaluation for placenta accreta spectrum. 1, 2, 3

• The risk escalates dramatically with each prior cesarean: 3% with no prior cesarean, 11% with one, 40% with two, 61% with three, and 67% with five or more prior cesareans 4
• Other major risk factors include prior uterine surgery (curettage, myomectomy), Asherman syndrome, endometrial ablation, prior cesarean scar pregnancy, and uterine artery embolization 2, 3
• Placenta previa is present in >80% of placenta accreta spectrum cases, particularly with prior cesarean delivery 1

Ultrasound Technique and Timing

Perform initial screening during the second trimester anatomy scan (18-22 weeks), with follow-up evaluation at 28-32 weeks to reassess placental position and invasion features. 5

Technical Requirements for Optimal Evaluation

• Use combined transvaginal and transabdominal ultrasound with high-frequency linear probe (5-9 MHz) when body habitus permits 1, 5
• Ensure moderately full bladder (200-300 mL) to properly evaluate the lower uterine segment and cesarean scar area 1, 5
• Apply minimal probe pressure when assessing the retroplacental clear zone to avoid artifactual loss 1, 5
• An empty bladder impedes adequate evaluation of bladder wall interruption and uterovesical hypervascularization 5

Gray-Scale Ultrasound Diagnostic Findings

Multiple vascular lacunae within the placenta represent the most specific finding with the highest positive predictive value for diagnosis. 1, 5

• Lacunae appear as irregular sonolucent spaces within the placenta and are the finding with highest sensitivity and positive predictive value 1, 5
• Loss of the normal hypoechoic retroplacental clear zone between placenta and myometrium is a key finding, though it has lower specificity (52% sensitivity, 57% specificity) and generates more false positives, particularly in the third trimester 1, 5
• Decreased retroplacental myometrial thickness <1 mm indicates deep placental invasion 1, 5
• Abnormalities of the uterine serosa-bladder interface, including interruption or irregularity of the hyperechoic interface between uterine serosa and bladder wall 1, 5
• Extension of placenta into myometrium, serosa, or bladder suggests deeper invasion 1

Critical Pitfall to Avoid

Isolated loss of the retroplacental zone has a false positive rate of 21% and can be absent in normal anterior placentas; do not rely on this finding alone. 5 Excessive probe pressure during transabdominal evaluation causes artifactual loss of the retroplacental zone 5.

Color Doppler Diagnostic Findings

Add color Doppler and power Doppler imaging to significantly improve detection sensitivity. 1

• Turbulent lacunar blood flow is the most common and diagnostically useful Doppler finding 1, 5
• Increased subplacental vascularity with hypervascularity at the placental bed 1, 5
• Gaps in myometrial blood flow indicating disruption of normal uterine vasculature 1
• Vessels bridging the placenta to the uterine margin or bladder 1, 5
• Increased vascularity in the inferior part of the lower uterine segment and parametrial region is associated with more severe disease 6

Role of MRI as Adjunct Imaging

MRI may be helpful in difficult cases or when placenta accreta is suspected based on ultrasound findings, but is not routinely recommended. 5, 2

• MRI parameters that correlate with surgical outcomes include: uterine bulge (odds ratio 7.432), placenta previa (odds ratio 7.283), low-attenuation T2 linear bands (odds ratio 5.985), placental heterogeneity near the scar (odds ratio 4.384), and fibrin deposition (odds ratio 4.322) 7
• Additional significant MRI variables include interruption of the bladder-serosa interface and largest dimension of invasion 7
• MRI is particularly useful for evaluating depth and topography of placental invasion, which are major determinants of maternal morbidity 6, 8

Clinical Context: When Ultrasound Findings Are Absent

The absence of ultrasound findings does not preclude a diagnosis of placenta accreta spectrum; clinical risk factors remain equally important as predictors. 3

• A large subset of cases clinically managed as placenta accreta spectrum are not confirmed on histology, with 18-29% of cases lacking pathologic confirmation despite clinical diagnosis 9
• Ultrasound may help predict depth and topography of placental invasion, which are major determinants of maternal morbidity 6
• Prenatal ultrasound markers of placenta accreta spectrum can be present as early as the first trimester 6, 2

Immediate Management After Diagnosis

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, 3

• Planned delivery at 34 0/7 to 35 6/7 weeks gestation optimizes outcomes in stable patients 1
• Antenatal diagnosis is highly desirable because outcomes are optimized when delivery occurs before the onset of labor or bleeding and with avoidance of placental disruption 3
• Optimal management involves a standardized approach with a comprehensive multidisciplinary care team including experienced pelvic surgeons, gynecologic oncologists, and access to blood bank protocols for massive transfusion 8, 3