Other US markers of PAS
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
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
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.
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