PAS overview and risk stratification

Placenta accreta spectrum (PAS) refers to abnormal placental adherence/invasion at the uteroplacental interface. Placenta accreta is defined as abnormal adherence of the placenta to the myometrium; increta is defined as invasion into the myometrium; and percreta is defined as penetration through the uterine serosa with possible involvement of adjacent organs, such as the bladder (American College of Obstetricians and Gynecologists [ACOG] & Society for Maternal-Fetal Medicine [SMFM], 2018; Jauniaux et al., 2018). The primary prenatal screening modality for PAS is ultrasound, but the examination is most clinically meaningful when sonographic marker recognition is interpreted in conjunction with the patient’s clinical risk profile (ACOG & SMFM, 2018; Shainker et al., 2021).

High-Yield Risk Framing

• The most important clinical risk factor is prior cesarean delivery (ACOG & SMFM, 2018; Silver et al., 2006).

• The highest-risk sonographic context is placenta previa or a low-lying placenta overlying a prior uterine scar (ACOG & SMFM, 2018; Jauniaux et al., 2018).

• When placenta previa is present, PAS risk rises steeply as the number of prior cesarean deliveries increases (ACOG & SMFM, 2018; Silver et al., 2006).

• Absence of a single sonographic marker does not exclude PAS in a high-risk patient (ACOG & SMFM, 2018; Shainker et al., 2021).

Figure 1. PAS spectrum schematic. The highest-value takeaway is not just depth—it is recognizing when a scarred lower uterine segment and placental overlap create a dangerous context.

Why does interpretation change in the context of risk? Risk context changes interpretation because PAS markers become more meaningful when they appear in the setting of placenta previa, a low-lying placenta, or scar overlap. Many findings may be seen in patients without PAS, especially in low-risk placentas; therefore, overcalling incidental findings should be avoided when the clinical pattern does not support the diagnosis (ACOG & SMFM, 2018; Shainker et al., 2021).

Figure 2. Approximate PAS risk when placenta previa is present, increasing with the number of prior cesarean deliveries.

Risk Tier	Common Profile	How to Think During the Exam

Highest

Placenta previa or low-lying placenta over a prior cesarean scar

Perform deliberate lower uterine segment assessment with transabdominal and transvaginal imaging. Assume the uteroplacental interface deserves scrutiny (ACOG & SMFM, 2018; Poder et al., 2020).

Increased

Anterior placenta near the scar, prior uterine surgery, or suspicious early implantation

Look for a cluster of markers rather than relying on a single sign (Collins et al., 2016; Shainker et al., 2021).

Contextual

No previa and no scar history

Avoid overcalling incidental placental lakes or lower-segment vascularity as PAS without a convincing pattern (ACOG & SMFM, 2018; Shainker et al., 2021).

2. Gray-scale markers that should change your level of concern

Gray-scale evaluation should answer three practical questions: Is the normal uteroplacental interface preserved? Are there abnormal intraplacental spaces? Is the myometrium extremely thin, distorted, or difficult to separate from the placenta? The most recognized gray-scale markers include loss of the clear zone, placental lacunae, myometrial thinning, placental bulge, and abnormality of the uterine serosa-bladder interface (ACOG & SMFM, 2018; Collins et al., 2016; Shainker et al., 2021).

Figure 3. Gray-scale findings of three core PAS markers: loss of the clear zone, placental lacunae, and myometrial thinning.

Marker	What it means	When concern rises	What to document
Loss of clear zone	Loss or irregularity of the retroplacental hypoechoic interface between the placenta and myometrium.	Focal abnormality along the posterior bladder wall or prior scar site.	State whether it is focal or extensive and where it is best seen (TA vs. TV).
Placental lacunae	Irregular hypoechoic vascular spaces within the placenta.	Multiple, large, irregular spaces—especially with turbulent/high-velocity flow.	Describe burden and distribution; note if clustered near the lower uterine segment.
Myometrial thinning	Marked thinning of the retroplacental myometrium, classically <1 mm in older descriptions.	The thinnest area is over the scar, coupled with other markers.	Document the thinnest site and whether the myometrium is still distinguishable from the placenta.

3. Doppler markers and scanning optimization

Color Doppler offers two big advantages. First, it helps to distinguish passive placental spaces from vascular lacunae. Second, abnormal vessels are visualized at the uteroplacental and uterovesical interface (Collins et al., 2016; Shainker et al., 2021). High-risk placentas: scan the lower uterine segment deliberately rather than quickly.

Figure 4. Doppler schematics for bridging vessels, subplacental/uterovesical hypervascularity, and contour distortion, with possible concern for deeper invasion.

Question: Does this image below confirm PAS? Chapter 2 teaches how to verify focal vascularity using topography mapping, reproducibility in two planes, and vessel tracing to avoid overcalling. This approach is consistent with standardized PAS ultrasound marker language and structured evaluation of pregnancies at risk for PAS (Collins et al., 2016; Shainker et al., 2021).

Doppler sign	Why it matters	Practical caution
Bridging vessels	Vessels appear to extend from the placenta across the myometrium and beyond the uterine serosa.	Differentiate true bridging vessels from bladder varicosities or pelvic vessels that are not placental in origin.
Subplacental / uterovesical hypervascularity	Dense or turbulent flow at the lower uterine segment raises concern for abnormal implantation.	Do not mistake color gain/bloom alone for a true abnormal interface. Correlate color Doppler with gray-scale anatomy.
Flow within lacunae	Flow supports that the spaces are vascular rather than simple placental lakes.	Concern is greater for high-velocity or turbulent flows than for a single low-flow space.

4. Differentiating PAS from common mimics and normal variants

A high-quality PAS scan is not only about finding abnormality. It is also about avoiding false reassurance and avoiding false-positive overcall. PAS markers are more trustworthy when they cluster together in a truly high-risk placenta (ACOG & SMFM, 2018; Shainker et al., 2021).

Clinical concept: PAS is high-stakes, but overcalling can cause unnecessary anxiety, additional imaging, transfers, altered delivery planning, and inconsistent care. Use proof-based checks before labeling an examination suspicious for PAS (ACOG & SMFM, 2018; Shainker et al., 2021).

High-yield mimics

• Bladder varicosities or pelvic vessels mimicking uterovesical hypervascularity
• Placental lakes mistaken for PAS lacunae
• Contractions or contour changes mimicking placental bulge or interface irregularity
• Echo dropout simulating loss of the retroplacental clear zone
• Cervical or lower uterine segment vascularity in placenta previa without true invasion

Potential mimic	Why can it fool the examiner	Features that pull you back toward the PAS concern
Placental lakes / isolated venous spaces	Can appear as dark intraplacental spaces in otherwise normal placentas.	Multiple large irregular lacunae, especially near the scar/lower uterine segment and with turbulent flow.
Thin lower uterine segment late in pregnancy	Physiologic thinning may occur without invasive placentation.	True concern rises when thinning is focal over the scar and accompanied by loss of clear zone, lacunae, or abnormal color flow.
Gain bloom or motion artifacts	Excessive color can simulate hypervascularity.	Persistent abnormal vessels are seen in orthogonal planes and are correlated with grayscale distortion.
Bladder varicosities	Can mimic bridging vessels at the bladder interface.	True bridging vessels track from the placenta across the myometrium; isolated bladder wall varices do not.
Low-risk patient with an incidental marker	Some markers may appear in unaffected pregnancies without a scar/previa history.	Clinical context remains the anchor; low-risk context should lower confidence in a PAS call unless the pattern is compelling.

Three Proof Checks That Prevent Overcalls

Trace the vessel.
If bridging vessels are suspected, trace the vessels back to the placental bed in two planes.

Change the plane and angle.
If apparent clear-zone loss disappears with probe angle or plane change, artifact should be considered.

Confirm clustering.
A single finding alone is weaker. Confidence increases when multiple findings cluster in the same high-risk region and the clinical context fits (ACOG & SMFM, 2018; Collins et al., 2016; Shainker et al., 2021). Figure 2.3: Mimics & overcall traps: quick decision tree

Table 2.2: Mimic matrix: what it looks like vs how to prove it

5. Reporting Language: Calibrate the Concern, Then State the next step.

For sonographers and interpreting clinicians, the report should do more than list markers. It should communicate the level of concern and make the next step clear. Use cautious language when findings are limited, and stronger language when multiple classic markers are present in a high-risk placenta (American Institute of Ultrasound in Medicine [AIUM], 2026; Shainker et al., 2021).

Figure 5. Example reporting ladder using escalating language tied to confidence and expected action.

Suggested structured report elements

• Placental location relative to the internal os, scar, and bladder

• Presence/absence of each major PAS marker

• Overall level of concern: not seen / cannot exclude / suspicious / highly suspicious

• Whether same-day physician communication occurred

• Recommended referral or follow-up plan

6. Escalation workflow and documentation checklist

Delayed recognition of PAS can contribute to avoidable hemorrhage, urgent unplanned surgery, and fragmented delivery planning. Antenatal suspicion allows for referral, delivery planning, multidisciplinary team involvement, and appropriate hemorrhage preparedness (ACOG & SMFM, 2018; Jauniaux et al., 2018). The workflow below is meant to standardize what happens after a concern is raised at the scanner or in the reporting room.

Optimization pearls from practice guidance

• Use both transabdominal and transvaginal views when the placenta is anterior, low-lying, previa, or located near a prior cesarean scar (ACOG & SMFM, 2018; Poder et al., 2020).

• Keep the maternal bladder partially filled for lower uterine segment and bladder-interface assessment when clinically appropriate and consistent with site protocol (Shainker et al., 2021).

• Use a low-velocity scale, low wall filter, and appropriate gain to maximize flow detection, then adjust for habitus and artifact (Collins et al., 2016; Shainker et al., 2021).

Key take-home points for the clinical team

• Ultrasound is the primary screening test for PAS, but risk factors matter as much as image signs.

• The strongest red-flag context is placenta previa or a low-lying placenta over a prior cesarean scar.

• Classic gray-scale markers include placental lacunae, loss of the clear zone, myometrial thinning, placental bulge, and abnormality of the uterine serosa-bladder interface.

• Classic Doppler markers include bridging vessels and uterovesical or subplacental hypervascularity.

• Do not confuse PAS communication with subtype certainty. When depth is unclear, report the concern honestly and escalate.

• Outcomes improve when suspected PAS is referred early to a center with multidisciplinary planning capacity (ACOG & SMFM, 2018; Jauniaux et al., 2018).

Placenta Accreta Spectrum (PAS): Early Recognition, Reporting, and Referral in Obstetric Imaging

Placenta Accreta Spectrum: Recognition, Reporting, and Referral Pathways