Clear Zone

From: Collins SL, Ashcroft A, Braun T, Calda P, Langhoff-Roos J, Morel O, et al. Proposal for standardized ultrasound descriptors of abnormally invasive placenta (AIP). Ultrasound Obstet Gynecol. 2016;47(3):271-5.

Loss of the retroplacental clear zone

This is the echolucent space between the placenta and myometrium. The absence of this sign indicates a loss or deficiency of Nitabuch’s layer and the subsequent ‘fusion’ of placenta and myometrium into a single structure. This is currently the only direct marker for PAS. However, this sign is prone to false positive results. The retroplacental hypoechoic space is influenced by external compressive force, usually from pressing hard with the ultrasound probe (example here). Due to the susceptibility of the retroplacental hypoechoic zone to compressive effect, care must be taken to prevent/minimize compressive effect of the probe when assessing the uteroplacental bed for presence of the clear zone [19]. This sign becomes more prominent in advancing gestation due to myometrial thinning and prominent dilatation of the uteroplacental circulation [31]. To have a clear assessment on ultrasound, the dynamic range, chroma(tint) and focus must be utilized to improve contrast resolution and zoom feature, to enlarge the image for thorough assessment.