Obstetric and Gynaecological · Placenta · Ultrasound 2021–2023; MRI 2020

Placenta Accreta Spectrum: Ultrasound and MRI Descriptors

Consensus imaging descriptors for suspected placenta accreta spectrum on ultrasound and MRI, designed for standardised recognition and reporting rather than numerical scoring.

Purpose

To standardise the description of prenatal ultrasound and MRI findings that raise suspicion for placenta accreta spectrum and help map its topography and possible extrauterine extension.

Modality	Descriptor	Standardised appearance	Reporting significance
Ultrasound	Loss of the clear zone	Focal loss or irregularity of the retroplacental hypoechoic zone between the placenta and myometrium.	Indicates an abnormal uteroplacental interface; assess with perpendicular insonation and minimal transducer pressure.
Ultrasound	Myometrial thinning	Focal thinning of the retroplacental myometrium at the placental bed, traditionally described as less than 1 mm.	Supports PAS when concordant with other signs; isolated thinning may occur over a uterine scar or in late gestation.
Ultrasound	Bladder-wall interruption	Focal interruption or irregularity of the echogenic posterior bladder wall, particularly where placental tissue extends beyond the uterus.	Raises concern for extrauterine extension; exclude artefactual echo dropout from engorged uterovesical vessels.
Ultrasound	Placental bulge	Placental tissue distorts the expected uterine serosal contour, producing a bulge-like lower-uterine-segment appearance.	Suggests severe myometrial thinning or scar dehiscence and possible extrauterine extension; it does not by itself prove a full-thickness defect.
Ultrasound	Uterovesical hypervascularity	Excessive, complex colour Doppler vascularity between the uterine serosa and bladder wall.	Supports PAS in an at-risk pregnancy but is not specific; lower-segment hypervascularity and bladder varices may accompany placenta praevia without PAS.
Ultrasound	Placental lacunae	Irregular intraplacental hypoechoic spaces containing vascular flow; concern increases when they are multiple, large, irregular and show high-velocity or turbulent flow.	A classic PAS marker in the appropriate risk setting; distinguish lacunae from smooth, central placental lakes with slow flow.
Ultrasound	Bridging vessels	Colour Doppler vessels extending from the placenta across the myometrium and beyond the uterine serosa.	A strong PAS descriptor; confirm placental origin and distinguish from non-placental bladder varices.
MRI	T2-dark intraplacental bands	One or more irregular, usually linear T2-hypointense bands within the placenta, often contacting its maternal surface.	A recommended SAR–ESUR MRI sign; report extent and distribution rather than calling normal septa or vessels dark bands.
MRI	Placental or uterine bulge	Deviation of the uterine serosa from its expected plane because placental tissue distorts the uterine contour, often towards the bladder or parametrium.	Supports deeper invasion, especially when accompanied by focal myometrial disruption.
MRI	Loss of the retroplacental T2-hypointense line	Focal interruption or absence of the thin low-T2 placental–myometrial interface behind the placental bed.	Supports PAS when combined with myometrial thinning or other concordant signs; it is not sufficiently specific in isolation.
MRI	Myometrial thinning or disruption	Myometrium over the placenta is thinned to less than 1 mm, becomes invisible or shows a focal breach.	Total loss or disruption of the low-T2 outer myometrial wall is more concerning than thinning alone.
MRI	Bladder-wall interruption	Irregularity, tenting or disruption of the normally low-T2 bladder wall; placental tissue may protrude into the bladder lumen.	Suggests bladder involvement; also report bladder-wall flow voids and the precise site of suspected invasion.
MRI	Focal exophytic placental mass	Placental tissue protrudes through the uterine wall and extends beyond it, commonly towards the bladder or laterally into the parametrium.	Highly concerning for placenta percreta and extrauterine extension; describe the involved adjacent compartment.
MRI	Abnormal vascularisation of the placental bed	Prominent, heterogeneous subplacental vessels disrupt the uteroplacental interface and may extend through the myometrium to the serosa, with perivesical or parametrial neovascularity.	A recommended sign that also helps map operative risk and suspected bladder or parametrial involvement.

How to use it

Ultrasound is the primary prenatal imaging test; combine transabdominal and transvaginal assessment with colour Doppler in an at-risk pregnancy.
MRI is an adjunct when ultrasound is equivocal or anatomically limited and is particularly useful for mapping the topography and possible extrauterine extension of disease.
Interpret descriptors as a pattern together with placental location and clinical risk factors; no single sign is diagnostic in every patient.
Report each descriptor as present, absent or indeterminate and state the suspected site and extent of invasion instead of assigning an unsupported imaging score.

Common mistake

Do not treat placental heterogeneity, asymmetric placental thickening, placental infarction or abnormal intraplacental vascularity as SAR–ESUR-recommended MRI signs; these four descriptors did not reach the consensus threshold and were categorised as uncertain.

Exam pearl

For rapid recall: ultrasound uses seven established signs—clear-zone loss, myometrial thinning, bladder-wall interruption, bulge, uterovesical hypervascularity, lacunae and bridging vessels; MRI uses seven recommended signs—T2-dark bands, bulge, retroplacental-line loss, myometrial thinning/disruption, bladder-wall interruption, exophytic mass and abnormal placental-bed vascularisation.

Viva questions

What is placenta accreta spectrum?: Placenta accreta spectrum is abnormal placental attachment, with a spectrum from abnormally adherent placenta to invasion into the myometrium and extension through the uterine serosa or adjacent organs.
Enumerate the seven established ultrasound signs of placenta accreta spectrum.: They are loss of the retroplacental clear zone, myometrial thinning, bladder-wall interruption, placental bulge, uterovesical hypervascularity, placental lacunae and bridging vessels.
How are placental lacunae described on ultrasound?: They are irregular hypoechoic spaces within the placenta containing vascular flow. Multiple large lacunae with irregular margins and high-velocity or turbulent flow are more concerning in an at-risk pregnancy.
What are bridging vessels, and what is their principal mimic?: Bridging vessels arise from the placenta and traverse the myometrium beyond the uterine serosa on colour Doppler. Their principal mimic is non-placental bladder varicosity.
Enumerate the seven SAR–ESUR-recommended MRI signs of placenta accreta spectrum.: They are T2-dark intraplacental bands, placental or uterine bulge, loss of the retroplacental T2-hypointense line, myometrial thinning or disruption, bladder-wall interruption, focal exophytic placental mass and abnormal vascularisation of the placental bed.
Which MRI descriptors were categorised as uncertain by the SAR–ESUR consensus?: Placental heterogeneity, asymmetric placental shape or thickening, placental ischaemic infarction and abnormal intraplacental vascularity did not reach the consensus threshold for recommended signs.
What is the role of MRI when placenta accreta spectrum is suspected?: MRI is an adjunct to ultrasound rather than the primary screening test. It helps when ultrasound is equivocal or limited and maps the topography, depth and possible bladder or parametrial extension for multidisciplinary planning.
Why should a single imaging descriptor not be used to grade placenta accreta spectrum?: Individual signs have imperfect specificity and may occur with placenta praevia, uterine scars or advancing gestation. Imaging should integrate multiple concordant signs, clinical risk factors and operative or pathological findings rather than assign an unsupported grade.