Key Takeaways
- Ultrasound is the primary tool for diagnosing PAS, with pooled sensitivity of 88–97% and specificity of 90–97% when performed by skilled operators18
- MRI is used as a complement when ultrasound is inconclusive or to assess depth of invasion, and is especially valuable for posterior placentas7
- An international panel identified 7 consensus MRI features to look for, with the best results at 28–32 weeks of pregnancy7
- Understanding what these findings mean can help you ask better questions and feel more informed during appointments
- Having imaging findings does NOT automatically mean the worst outcome — these findings exist on a spectrum
Why This Guide Exists
If you're reading this, you've likely had an ultrasound or MRI and your report mentions findings you don't fully understand. Terms like "placental lacunae," "loss of retroplacental clear zone," or "dark intraplacental bands" can feel overwhelming and frightening — especially when you're already worried about your pregnancy.
This guide is here to help you understand what those terms mean in plain language. It won't replace the conversation you need to have with your Maternal-Fetal Medicine (MFM) specialist, but it will help you walk into that conversation feeling more prepared and able to ask the right questions.
Not all imaging findings indicate severe PAS. Many of the signs described below are "soft" markers that can appear in normal pregnancies or pregnancies with only mild placental attachment issues. Your MFM specialist interprets these findings in context of your full clinical picture — including your surgical history, placental location, gestational age, and the combination of markers present.
Ultrasound Markers for PAS
Ultrasound is the first-line tool for evaluating PAS. During your scan, the sonographer looks for specific signs that the placenta may be growing too deeply into the uterine wall. Below are the seven key markers, explained in plain language.5
What it means: Normally, there is a thin dark gap (a "clear zone") visible on ultrasound between the placenta and the uterine wall. When this gap is absent, it may suggest the placenta is growing into the uterine muscle rather than sitting on top of it.
How important is it? This is considered a "soft sign" — it is most valuable for ruling out accreta. If the clear zone is present and intact, accreta is less likely. Its absence alone does not confirm PAS.
Report terms you may see: "loss of retroplacental clear zone," "absent retroplacental sonolucent zone"
What it means: Irregular, fluid-filled pockets within the placenta that give it a "Swiss cheese" look on ultrasound. These spaces often show turbulent blood flow on Doppler imaging.
How important is it? This is the single most predictive and most specific ultrasound sign for PAS. Lacunae are graded from 0 to 3 based on the number and size of these pockets — higher grades are more concerning. The number of lacunae directly correlates with the probability of PAS at birth and with surgical outcomes.8 Multiple large, irregularly shaped lacunae with turbulent flow are the most worrisome pattern.2
Report terms you may see: "placental lacunae," "intraplacental lacunae with turbulent flow"
What it means: The myometrium (uterine wall muscle) measures less than 1mm thick at the placental site. Think of it as the wall between the placenta and the outside of the uterus becoming paper-thin.
How important is it? This can be a sign that the placenta is eroding into the muscle layer. However, some myometrial thinning can be normal in late pregnancy or in the area of a prior cesarean section scar, so it must be interpreted in context.
Report terms you may see: "myometrial thinning," "thin myometrium"
What it means: When your doctor uses Doppler (color blood flow imaging), they see abnormally increased blood flow in the area between your uterus and bladder. Normally, this area is relatively quiet on Doppler.
How important is it? This sign suggests the abnormal placental tissue may be recruiting extra blood supply in the area where the uterus sits against the bladder. It becomes more concerning when seen alongside other markers.
Report terms you may see: "uterovesical hypervascularity," "increased vascularity at uterine-bladder interface"
What it means: The bright line representing the bladder wall appears jagged, irregular, or broken on ultrasound rather than smooth and continuous. Think of a smooth line that suddenly has notches or gaps in it.
How important is it? This is a highly specific finding — meaning when it's present, it strongly suggests deeper invasion (increta or percreta). It indicates the placental tissue may be reaching toward or into the bladder.
Report terms you may see: "bladder wall irregularity," "disruption of bladder-myometrium interface"
What it means: On Doppler imaging, a web of abnormal blood vessels is seen crossing from the placenta through the uterine wall. These vessels essentially "bridge" the gap between the placenta and the tissue outside the uterus.
How important is it? This is a very concerning sign when present, as it suggests the placenta has penetrated deeply enough through the uterine wall to establish blood supply beyond it. It is often associated with more severe forms of PAS.
Report terms you may see: "bridging vessels," "subplacental hypervascularity"
What it means: An outward bump or protrusion where the placenta appears to push the uterine wall outward beyond its normal smooth contour. The uterus should have a smooth, curved shape — a focal bulge disrupts this.
How important is it? Although it has relatively low sensitivity (it's not always present), when it is present it is highly associated with severe PAS — found in approximately 92% of severe cases. It suggests the placenta has grown through enough of the wall to physically deform the uterine shape.
Report terms you may see: "placental bulge," "focal myometrial bulge," "loss of normal uterine contour"
Ultrasound Marker Sensitivity Comparison
Data source: Collins et al. 2016 standardized descriptors;5 Elhawary et al. 20231
A marker with high sensitivity (like loss of clear zone at 82%) is good at detecting PAS when it's present. A marker with low sensitivity (like placental bulge at 35%) means many PAS cases won't show this sign — but when it does appear, it can be highly meaningful. Your doctor looks at the combination of markers, not any single one in isolation.
Ultrasound Accuracy by PAS Type
How well ultrasound performs depends partly on what type of PAS is present. A large meta-analysis found the following accuracy figures for ultrasound when diagnosing each PAS type:7
| PAS Type | Sensitivity | Specificity |
|---|---|---|
| Accreta (attached, not invaded) | 91% | 97% |
| Increta (invaded into muscle) | 93% | 98% |
| Percreta (grown through wall) | 81% | 99% |
Notice that specificity is very high across all types — meaning a positive ultrasound finding rarely leads you astray. Sensitivity is slightly lower for percreta (81%), likely because very deep invasion can be harder to visualize on ultrasound alone. This is one reason MRI may be recommended for suspected percreta.
The PAS Ultrasound Staging System
In 2019, researchers led by Cali and colleagues proposed a standardized staging system to help clinicians classify the severity of PAS based on ultrasound findings. This system combines multiple markers to give a more complete picture than any single sign alone.2
This ultrasound staging system helps your care team plan — but it is not a definitive diagnosis. The true extent of PAS can only be confirmed during surgery or by pathological examination of the placenta and uterus after delivery. Think of staging as your care team's best estimate to guide preparations.
MRI Findings for PAS
MRI (Magnetic Resonance Imaging) is not used for every PAS patient. It is typically ordered when ultrasound findings are inconclusive, when the placenta is in a location that's difficult to evaluate by ultrasound (such as a posterior placenta), or when your care team needs to better assess the depth of invasion and involvement of surrounding organs like the bladder.4
For assessing the depth of placental invasion, MRI has a sensitivity of 86.5–94.4% and a specificity of 96–98.8% — making it particularly good at telling your surgical team exactly how deep the placenta has grown.7
MRI does not use radiation — it uses magnetic fields and radio waves. It is considered safe during pregnancy. The scan itself is painless, though lying in the machine for 30–45 minutes can be uncomfortable. No contrast dye (gadolinium) is typically used during pregnancy.
The SAR-ESUR consensus recommends that MRI for PAS is best performed between 28 and 32 weeks of pregnancy. Before 28 weeks, the placenta may not be mature enough for certain features to appear reliably. After 32 weeks, some normal late-pregnancy changes in the placenta can overlap with PAS signs, making interpretation more challenging.7
The 7 SAR-ESUR Consensus MRI Features
In 2023, the Society of Abdominal Radiology (SAR) and European Society of Urogenital Radiology (ESUR) published a joint analysis identifying seven MRI features recommended for diagnosing PAS. These are the features your radiologist should be looking for when reading your MRI.7
What it means: On a type of MRI image called "T2-weighted," dark stripes or bands appear within the placenta. These dark areas represent fibrin deposits — essentially scar-like tissue formed from small areas of internal bleeding caused by the placenta's abnormal invasion into the uterine wall.
How important is it? This is the most sensitive MRI sign for PAS, with a sensitivity of approximately 89.7% (specificity 49.5–58.5%). The most concerning bands are those located on the maternal side (the side closest to the uterine wall) of the placenta, as these directly reflect the abnormal interface between placenta and uterus.7
Report terms you may see: "dark T2 intraplacental bands," "low-signal-intensity bands on T2," "intraplacental T2-dark bands"
What it means: The uterus loses its normal smooth, rounded shape. Instead, one area pushes outward (focal bulging — a single bump) or the entire lower segment narrows and then widens (diffuse bulging — an hourglass-like shape).
How important is it? This is the single most useful MRI sign when seen on its own, with 76.7% sensitivity and 62.5% specificity for increta. Research has shown it is an independent predictor of myoinvasive disease — meaning it provides strong evidence of deep invasion even without other features present.7 Focal bulging tends to indicate a localized area of deep invasion, while diffuse bulging may suggest more widespread involvement.
Report terms you may see: "focal uterine bulge," "uterine contour deformity," "hourglass appearance of lower segment"
What it means: The placenta appears mottled or uneven on MRI instead of having a relatively uniform appearance. Different areas show different brightness levels, creating a patchy or "mixed" look.
How important is it? This is a common finding but is not specific to PAS on its own. Many normal placentas, especially later in pregnancy, can show some heterogeneity. It becomes more meaningful when seen alongside other, more specific signs. Notably, the SAR-ESUR consensus classified placental heterogeneity as an "uncertain" feature that did not meet expert agreement for routine diagnostic use, along with asymmetric placental thickening, placental ischemic infarction, and abnormal intraplacental vascularity.7
Report terms you may see: "heterogeneous placental signal," "mixed signal intensity within the placenta"
What it means: On MRI, the uterine muscle wall normally appears as a continuous dark line surrounding the uterus. With PAS, gaps or breaks in this dark line indicate places where the placenta has breached through the muscle.
How important is it? This is a very helpful finding because it shows exactly where the placenta has penetrated the uterine wall. This information is valuable for your surgical team in planning the approach to delivery.
Report terms you may see: "focal myometrial disruption," "interruption of the hypointense myometrial line," "myometrial discontinuity"
What it means: The top of the bladder, which normally has a flat or gently curved surface where it sits against the uterus, appears to be pulled upward into a tent-like or peaked shape toward the uterus.
How important is it? This is a concerning sign that suggests percreta with bladder involvement — meaning the placenta has grown all the way through the uterine wall and is affecting the bladder. This finding helps the surgical team prepare for potential bladder repair during delivery.
Report terms you may see: "bladder tenting," "tenting of the bladder dome," "superior displacement of the bladder wall"
What it means: MRI shows placental tissue extending completely beyond the uterus and directly into an adjacent organ — most commonly the bladder, but potentially the bowel or other pelvic structures.
How important is it? This confirms placenta percreta (FIGO Grade 3) — the most severe form of PAS. While this is the most serious finding, knowing about it before delivery is actually very valuable because it allows the surgical team to assemble the right specialists (urologist, general surgeon) and plan accordingly.3
Report terms you may see: "direct invasion of the bladder," "placental tissue extending beyond the serosa," "extrauterine placental tissue"
The SAR-ESUR consensus also identifies two additional features to look for: a focal exophytic mass (placental tissue extending outward toward the bladder, which has a very high specificity of 98.9% but is present in only about 69% of cases) and abnormal vascularization of the placental bed (unusual blood vessel patterns in the area where the placenta attaches). Additionally, newer techniques like diffusion-weighted imaging (DWI) may help further delineate the boundary between the placenta and uterine muscle, though this is still an area of active research.7
What Multiple MRI Findings Mean Together
One of the most valuable aspects of MRI is counting how many features are present. Research has shown that the number of MRI features seen correlates with the likely complexity of your delivery:7
This is one reason your radiologist carefully counts and documents every finding — the total number helps your surgical team prepare the right resources, blood products, and specialists for your delivery.
MRI and Posterior Placentas
When the placenta is located on the back wall of the uterus (posterior placenta), ultrasound has a harder time evaluating it because the placenta is further from the ultrasound probe and partially obscured by the baby. In these cases, MRI provides a significant advantage:
If you have a posterior placenta and your doctor suspects PAS, asking about an MRI is particularly worthwhile.7
While MRI is a powerful tool, it is not perfect. Research has found that when MRI changes a diagnosis that was previously made by ultrasound, the new MRI-based diagnosis is often incorrect — this happens in more than one-third of cases where MRI changes the clinical assessment.8 MRI also has no role in screening for PAS — it is a follow-up tool, not a first-line test. The best outcomes come from combining ultrasound and MRI findings together, interpreted by experienced specialists, rather than relying on either test alone.
Ultrasound vs. MRI: How Do They Compare?
| Measure | Ultrasound | MRI |
|---|---|---|
| Role | First-line screening and diagnosis | Complementary; used for depth of invasion and complex cases |
| Overall sensitivity | 83.3–90.7%7 | 86.5–94.4% (for depth of invasion)7 |
| Overall specificity | 83.4–97.0%7 | 96–98.8% (for depth of invasion)7 |
| Best for | Initial detection; monitoring; placental lacunae | Depth of invasion; posterior placentas; bladder involvement |
| Limitations | Operator-dependent; harder with posterior placentas | May give incorrect revised diagnoses in >1/3 of changed cases8 |
FIGO Classification of PAS
The International Federation of Gynecology and Obstetrics (FIGO) established a classification system in 2019 to standardize how PAS severity is described worldwide. This is the system your doctor will use to communicate the grade of your condition.3
| FIGO Grade | Name | What It Means |
|---|---|---|
| Grade 1 | Accreta | Placenta is abnormally attached to the uterine muscle but has not grown into it. The placenta "sticks" but hasn't invaded. |
| Grade 2 | Increta | Placenta has grown into the uterine muscle wall. It has penetrated partway through the muscle layer. |
| Grade 3a | Percreta (serosa) | Placenta has grown all the way through the uterine wall and reaches the outer covering (serosa) of the uterus. |
| Grade 3b | Percreta (bladder) | Placenta has grown through the uterine wall and invaded the bladder. |
| Grade 3c | Percreta (other organs) | Placenta has grown through the uterine wall and invaded other pelvic organs (bowel, pelvic sidewall, or other structures). |
While imaging can suggest a likely grade, the definitive FIGO grade is determined by what the surgeon sees during delivery and by pathological examination of the tissue afterward. Imaging provides an estimate — sometimes the actual grade is better than predicted, sometimes worse. Your care team prepares for the full range of possibilities.
What to Do With This Information
Now that you understand what these imaging findings mean, here are concrete steps you can take:
- Bring your imaging reports to your MFM specialist — ask for a copy of the written report from your ultrasound and/or MRI so you can reference it during your appointment.
- Ask: "What stage of PAS do my findings suggest?" — your specialist can explain where your findings fall on the severity spectrum and what that means for your care plan.
- Ask: "Do I need an MRI as well?" — if you've only had an ultrasound, an MRI may provide additional information, especially if your ultrasound was inconclusive or suggests deeper invasion.
- Ask: "Should I be evaluated at a specialized PAS center?" — PAS outcomes are significantly better when managed by experienced multidisciplinary teams. If your current facility doesn't have a dedicated PAS program, a referral may be appropriate.6
It may not feel like it right now, but the fact that your PAS was detected on imaging before delivery is genuinely positive. Prenatal diagnosis of PAS is strongly associated with reduced blood loss, fewer emergency surgeries, lower ICU admissions, and dramatically improved maternal outcomes. You now have time to plan, assemble the right team, and deliver at the right facility. Many women with PAS who are diagnosed prenatally have excellent outcomes.
Where to Learn More
The following resources provide additional information about PAS imaging for patients and clinicians:
- Radiopaedia: Placenta Accreta Spectrum Disorders — comprehensive imaging reference with example images
- KJR Step-by-Step MRI Approach (PMC7817633) — a systematic approach to reading PAS MRI
- GLOWM Ultrasound Chapter — Global Library of Women's Medicine ultrasound reference for PAS
- GE Voluson Club Practical Guide — practical ultrasound imaging guide for PAS evaluation
- PubMed Central Pictorial Essays — searchable database of medical imaging articles with illustrative cases
References
- Elhawary TM, Dabees NL, Youssef MA. "Diagnostic value of ultrasonography and magnetic resonance imaging in pregnant women at risk for placenta accreta." SMFM Consensus. 2023. PMID: 33386103
- Cali G, Forlani F, Timor-Tritsch IE, et al. "Natural history of Cesarean scar pregnancy on prenatal ultrasound: the crossover sign." Ultrasound Obstet Gynecol. 2019. PMID: 30834661
- Jauniaux E, Ayres-de-Campos D, Langhoff-Roos J, Fox KA, Collins S; FIGO Placenta Accreta Diagnosis and Management Expert Consensus Panel. "FIGO classification for the clinical diagnosis of placenta accreta spectrum disorders." Int J Gynaecol Obstet. 2019;146(1):20-24. PMID: 31173360
- Familiari A, Liberati M, Lim P, et al. "Diagnostic imaging for placenta accreta spectrum: a joint SAR-ESUR consensus statement." Radiology. 2020. PMID: 32040730
- Collins SL, Ashcroft A, Braun T, et al. "Proposal for standardized ultrasound descriptors of abnormally invasive placenta (AIP)." Ultrasound Obstet Gynecol. 2016;47(3):271-275.
- American College of Obstetricians and Gynecologists; Society for Maternal-Fetal Medicine. "Obstetric Care Consensus No. 7: Placenta Accreta Spectrum." Obstet Gynecol. 2018;132(6):e259-e275.
- Patel-Lippmann KK, Planz VB, Phillips CH, Ohlendorf JM, Zuckerwise LC, Moshiri M. "Placenta Accreta Spectrum: Update on Imaging and Management from the Society of Abdominal Radiology and European Society of Urogenital Radiology." RadioGraphics. 2023;43(1):e220090. DOI: 10.1148/rg.220090
- Jauniaux E, Aplin JD, Fox KA, Afshar Y, Hussein AM, Jones CJP, Burton GJ. "Placenta accreta spectrum." Nat Rev Dis Primers. 2025;11. DOI: 10.1038/s41572-025-00624-3