Clinical Review

Diagnosing placenta accreta spectrum with prenatal ultrasound

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Other US markers of PAS

Retroplacental–myometrial interface

Loss of the normal hypoechoic (clear) retroplacental zone, also referred to as loss of the clear space between placenta and uterus, is another marker of PAS (FIGURE 5). This finding corresponds to pathologic loss of the decidua basalis as trophoblastic tissue invades directly through the myometrium.15 This sonographic finding has been reported to have a detection rate of approximately 93%, with sensitivity of 52% and specificity of 57%, for PAS; the false-positive rate, however, has been in the range of 21% or higher. This marker should not be used alone because it is angle-dependent and can be found (as an absent clear zone) in normal anterior placentas.16

The strength of this US marker is in its negative predictive value, which ranges from 96% to 100%. The presence of a hypoechoic retroplacental clear space that extends the length of the placenta makes PAS unlikely.17 Of note, the clear zone may appear falsely absent as a result of increased pressure from the US probe.

Retroplacental myometrial thickness

Another US finding characteristic of PAS is a retroplacental myometrial thickness of <1 mm (FIGURE 6).15 This finding can result from trophoblastic invasion with minimal intervening myometrium. A thin myometrium also may be due to partial dehiscence (the so-called uterine window) of the uterine wall.18

Retroplacental myometrial thickness is difficult to assess because the lower uterine-segment myometrium thins in normal pregnancy as term approaches. This measurement also can be influenced by direct pressure of the US probe and fullness of the maternal bladder.18 In patients who have had a CD but who do not have PAS, the median myometrial thickness of the lower uterine segment in the third trimester is 2.4 mm.19

ON OUR US UNIT……we optimize transabdominal US imaging of the retroplacental–myometrial interface by applying minimal transducer pressure on the abdomen, minimizing image depth, and magnifying image display. We use linear sweeps to image the entire placenta.

Thinning of the myometrium in the upper uterine segment always should be of concern. Studies of this marker have reported sensitivity of US ranging from 22% to 100% and specificity from 72% to 100%.9,20 Given such variability, it is important to standardize the gestational age and sonographic approach for this marker.

AT OUR INSTITUTION……we typically approach retroplacental myometrial thickness transabdominally by applying minimal transducer pressure on the abdomen, minimizing image depth, and magnifying image display. We measure the myometrium at its thinnest point, taking the measurement perpendicular to the long axis of the wall of the uterus. In the presence of placenta previa or low-lying placenta, we take a transvaginal approach.

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