Optimal Ultrasound Probe Selection for Ruling Out Placenta Accreta

Primary Probe Recommendations

The American College of Radiology recommends a combination of transvaginal ultrasound with transabdominal ultrasound using a high-frequency (5-9 MHz) linear probe when body habitus permits to rule out placenta accreta, with transvaginal ultrasound being essential for detailed evaluation of the anterior lower uterine segment and myometrial-placental interface 1, 2, 3
Transvaginal scanning must be used in conjunction with transabdominal scanning to provide the highest-resolution evaluation of the anterior lower uterine segment myometrium, placenta, and myometrial-placental interface 1, 2, 3
A high-frequency (5-9 MHz) linear probe should be used transabdominally if body habitus allows, as it permits focused evaluation of uterine and placental morphology with superior resolution 1, 2, 4, 5, 6, 7

The bladder must be at least moderately full (200-300 mL) to properly identify and evaluate the lower uterine segment and cesarean section scar area 1, 2, 3, 4
Assess the retroplacental clear zone without excessive probe pressure to prevent artifactual loss of this important diagnostic marker 1, 2, 4

Adding color Doppler and power Doppler imaging significantly improves detection of placenta accreta by identifying increased placental vascular flow, subplacental vascularity, and vessels bridging from placenta to bladder 1, 2, 4, 5, 6, 7
Turbulent lacunar blood flow is the most common and helpful Doppler finding for placenta accreta 8

Intraplacental lacunae (multiple, large, irregular sonolucent spaces with turbulent flow) have the highest sensitivity and positive predictive value for placenta accreta 2, 4, 6, 7, 8
Loss of the normal hypoechoic retroplacental clear zone is a diagnostic feature of placenta accreta 1, 2, 8
Myometrial thickness <1 mm is a diagnostic feature of placenta accreta 1, 2, 8

Ultrasound sensitivity ranges from 77% to 93% with positive predictive value of 65% to 93% when performed with appropriate technique 1, 2, 3
A systematic review of 3,707 pregnancies demonstrated average sensitivity of 90.72% and specificity of 96.94% for ultrasound diagnosis 8
Screening is typically performed at the second trimester anatomy scan at 18-22 weeks 1, 2, 4, 7