How PAS Is Diagnosed

Overview of Diagnosis

Placenta Accreta Spectrum (PAS) is diagnosed primarily through imaging during pregnancy, most commonly using ultrasound. Early and accurate diagnosis is one of the single most important factors in improving outcomes for both mother and baby.

Despite the proven benefits of prenatal detection, FIGO (the International Federation of Gynecology and Obstetrics) reports that 50–67% of PAS cases remain undiagnosed before delivery in general practice settings.1 This highlights a critical gap in screening, particularly for women without the classical risk factors of prior cesarean delivery and placenta previa (a low-lying placenta that covers the cervix).

Ultrasound Diagnosis (Primary Method)

Ultrasound is the first-line diagnostic tool for PAS. It is widely available, non-invasive, and — in experienced hands — highly accurate. Both grayscale (standard black-and-white) and color Doppler (which shows blood flow) imaging are used together.

When Ultrasound Is Done

PAS may first be suspected during the routine 18–20 week anatomy scan, especially in women with known risk factors such as previous cesarean sections or placenta previa. However, the Royal College of Obstetricians and Gynaecologists (RCOG) recommends that women at higher risk undergo a specialist assessment at 24–28 weeks of gestation, when the ultrasound signs of PAS become more apparent and reliable.2

Earlier scans (first trimester) may show suggestive signs — such as a low implantation into a cesarean scar — but definitive diagnosis typically requires second- or third-trimester imaging. However, emerging evidence suggests that first-trimester screening between 11 and 16 weeks can identify PAS with accuracy up to 93%, particularly in women with prior cesarean scars and low implantation.12

Ultrasound Signs and Markers

Radiologists and maternal-fetal medicine specialists look for specific markers that suggest the placenta has invaded beyond its normal boundary. Each marker has different diagnostic accuracy:

Data from: D'Antonio et al., Ultrasound in Obstetrics & Gynecology, 2013; Defined by International Federation of Placenta Accreta Disorders (IF-PAS) standardized descriptors.3

Overall Ultrasound Accuracy

When all markers are evaluated together by an experienced sonographer, ultrasound achieves impressive diagnostic accuracy:

• Pooled sensitivity: 83.3–90.7% (catches 83–91 out of every 100 cases)12
• Pooled specificity: 83.4–97.0% (correctly rules out PAS in 83–97% of unaffected pregnancies)12

Ultrasound Accuracy by PAS Subtype

Ultrasound accuracy varies depending on the depth of placental invasion. A 2023 SAR-ESUR review reported the following figures:12

Data from: Patel-Lippmann et al., SAR-ESUR Joint Consensus Statement, RadioGraphics 2023.12

Illustration: Simplified diagram for educational purposes only.

Expert vs. Non-Expert Assessment

One of the most striking findings in PAS research is the enormous difference between diagnostic accuracy at specialist centers versus general hospitals:

False Positives and False Negatives

No diagnostic test is perfect. Understanding the error rates helps set realistic expectations:

• False positive rate at non-expert centers: 68.5% — This means that among women referred to specialist centers with a suspected PAS diagnosis, roughly 68.5% did not actually have PAS upon expert re-evaluation.4
• False negative rate at expert centers: approximately 2.1% — Even at the best centers, a small number of PAS cases (~2 in 100) are missed on prenatal imaging.5
• Only 17% prenatal diagnosis rate without classical risk factors — The PACCRETA prospective study found that among women who lacked the typical risk factors (prior cesarean section, placenta previa), only 17% were diagnosed before delivery.6

Risk Escalation: Prior C-Sections and Placenta Previa

The risk of PAS increases dramatically with the number of prior cesarean deliveries, and escalates even further when placenta previa is also present. A single cesarean delivery triples the risk of PAS in the next pregnancy compared to vaginal birth.1112

Data from: Jauniaux et al., Nature Reviews Disease Primers 2025; Patel-Lippmann et al., RadioGraphics 2023.1112

2D vs. 3D Ultrasound

Most routine ultrasound assessments use 2D (two-dimensional) imaging, which is the standard of care. However, research has shown that 3D power Doppler ultrasound can improve diagnostic accuracy:

• 3D power Doppler sensitivity: 97%
• 3D power Doppler specificity: 92%

3D imaging provides a volumetric view of the blood supply to the placenta and uterine wall, which can reveal abnormal vessel patterns not visible on 2D imaging. However, 3D ultrasound availability is limited — it requires specialized equipment and significant operator expertise, so it is not routinely used in most hospitals.7

MRI Diagnosis (Adjunct Method)

Magnetic Resonance Imaging (MRI) is a powerful imaging technique that uses magnetic fields (not radiation) to produce detailed images of soft tissues. In the context of PAS, MRI is used as an adjunct (supplementary) tool rather than a first-line screening method.

When MRI Is Recommended

MRI is not necessary in all suspected PAS cases. It is typically recommended in the following situations:

• Inconclusive ultrasound: When ultrasound findings are ambiguous or borderline
• Posterior placenta: When the placenta is located on the back wall of the uterus, which is harder to visualize with ultrasound
• Assessing depth of invasion: Particularly when placenta percreta (invasion through the full uterine wall into adjacent organs) is suspected
• Surgical planning: To map the extent of invasion and involvement of surrounding structures (bladder, ureters, bowel) before surgery

The optimal timing for MRI is between 28 and 32 weeks of gestation. At this stage, there is sufficient contrast between placental tissue and the uterine wall to identify abnormalities clearly.8

MRI Findings in PAS

Data from Familiari et al. (2018) systematic review and meta-analysis.8

MRI Accuracy

When evaluating suspected PAS, MRI demonstrates strong but not perfect diagnostic performance:

• Sensitivity: 86.5–94.4% (catches 87–94 out of 100 true PAS cases)12
• Specificity: 79–86% for overall detection (correctly rules out PAS in 79–86% of unaffected pregnancies)
• Specificity for depth of invasion: 96–98.8% — MRI is particularly strong at distinguishing accreta from increta and percreta12
• AUC (Area Under the Curve): 0.91 — an overall measure of diagnostic performance where 1.0 is perfect and 0.5 is no better than a coin flip

These numbers reflect pooled data from multiple studies. A 2023 SAR-ESUR joint consensus confirmed that MRI's greatest added value is in characterizing the depth of invasion, where its specificity (96–98.8%) exceeds that of ultrasound.12 MRI accuracy depends heavily on the experience of the radiologist reading the images and the quality of the imaging protocol used.8

Ultrasound vs. MRI: Head-to-Head Comparison

One of the most common questions patients and clinicians ask is: "Which is better — ultrasound or MRI?" The evidence provides a nuanced answer.

Data from Defined by IF-PAS; D'Antonio et al. (2014); Defined by Familiari et al. (2018).89

When Ultrasound and MRI Disagree

In clinical practice, ultrasound and MRI do not always agree. Understanding what happens when they disagree is critical for making informed decisions about your care.

Research shows that in a meaningful minority of cases, the two imaging methods give conflicting results — one says PAS is present while the other says it is not, or they disagree on the severity of invasion.

What the Evidence Shows

• When ultrasound and MRI gave discordant (disagreeing) results, ultrasound was correct in 62% of disagreements (8 out of 13 cases in one key study).9
• 5 false-negative MRIs (MRI said no PAS, but PAS was actually present) were correctly diagnosed by ultrasound.
• In 5 other cases, MRI correctly identified that the ultrasound result was a false positive (ultrasound said PAS, but it was not actually present).

Other Diagnostic Methods

Serum Biomarkers (Blood Tests)

Several blood-based markers have been studied as potential diagnostic tools for PAS:

• AFP (Alpha-Fetoprotein): Elevated levels in the second trimester have been associated with PAS, but AFP is elevated in many other conditions, making it a poor standalone test.
• Beta-hCG (Human Chorionic Gonadotropin): Abnormal levels may be seen, but this is neither sensitive nor specific enough for PAS diagnosis.
• PAPP-A (Pregnancy-Associated Plasma Protein A): Low first-trimester levels have been loosely associated with PAS, but the predictive value is too low for clinical use.

Doppler Studies

Color Doppler and power Doppler imaging are not separate tests — they are integral components of ultrasound assessment for PAS. Doppler adds critical information by showing:

• Blood flow patterns within placental lacunae (turbulent flow is a hallmark of PAS)
• Uterovesical hypervascularity (abnormal blood vessels between the uterus and bladder)
• Bridging vessels crossing the uterine-bladder interface
• Overall vascularity patterns that help grade severity

Any comprehensive ultrasound evaluation for PAS should include Doppler assessment as standard practice.

The Diagnostic Journey

Understanding the typical diagnostic pathway can help reduce anxiety and ensure you advocate for thorough evaluation at each step.

What To Do If You Have Just Been Diagnosed

Receiving a PAS diagnosis can be frightening and overwhelming. Here are concrete steps you can take:

Immediate Steps

1. Get a specialist evaluation. If your initial diagnosis was at a general hospital, request a referral to a center with PAS experience. Remember that false positive rates can be very high at non-expert centers.
2. Ask about MRI. If your diagnosis is based on ultrasound alone and there are any questions about the severity, an MRI can provide additional information, especially about the depth of invasion.
3. Seek a center with a dedicated PAS team. Look for hospitals that have a multidisciplinary team and that manage PAS cases regularly (ideally 10 or more cases per year).
4. Do not assume the worst. Many suspected PAS cases turn out to be less severe than initially feared, and even confirmed PAS cases have excellent outcomes when managed by experienced teams.
5. Gather information. Understanding your diagnosis helps you participate in decision-making. Ask your providers to explain the specific imaging findings and what they mean for your case.

References