PAS Screening & Evaluation Tool

Evidence-based clinical decision support for placenta accreta spectrum

What This Tool Does

Placenta accreta spectrum (PAS) affects 2-3 per 1,000 pregnancies, an 8-fold increase since the 1970s driven largely by rising cesarean delivery rates. Undiagnosed PAS remains a leading cause of catastrophic postpartum hemorrhage, emergency hysterectomy, and maternal death. Yet prenatal detection rates vary widely, from under 50% in community settings to over 95% at specialized centers using standardized approaches.

This tool guides clinicians through a structured, evidence-based PAS evaluation in two phases:

Phase 1: Risk Calculation

Enter clinical history (prior cesareans, placental location, additional risk factors) and the tool calculates the patient's a priori PAS probability using population-based incidence data from Silver et al. (2006) and White et al. (2025). This tells you the baseline risk before any imaging.

Phase 2: Ultrasound Guidance

For patients who screen positive, the tool provides the PAS2 expert consensus imaging protocol, a structured checklist of the 7 validated ultrasound markers, and the Placenta Accreta Index (PAI) calculator (Rac et al., AJOG 2015, AUC 0.87) that converts ultrasound findings into a 2-96% invasion probability.

The final summary integrates both phases: clinical risk percentage, PAI score with probability, individual marker findings, and an evidence-based management recommendation. Every calculation in this tool is derived from peer-reviewed publications listed below. No risk estimates are assumed or extrapolated beyond published data.

Publications Used in This Tool

1 Rac MWF, Dashe JS, Wells CE, Moschos E, McIntire DD, Twickler DM. Ultrasound predictors of placental invasion: the Placenta Accreta Index. Am J Obstet Gynecol. 2015;212(3):343.e1-7. PAI Score
Original derivation of the Placenta Accreta Index. Logistic regression model combining 5 clinical and US parameters into a 0-9 point scale (AUC 0.87). Score maps to 2-96% probability of placental invasion. N=184, 54 histologically confirmed.

2 Happe SK, Yule CS, Spong CY, Wells CE, Dashe JS, Moschos E, Rac MWF, et al. Predicting Placenta Accreta Spectrum: Validation of the Placenta Accreta Index. J Ultrasound Med. 2021;40(8):1523-32. PAI Validation
Independent validation. PAI score ≥4 predicted 100% of PAS in patients with ≥1 prior CD. Improved sensitivity from 34% to 60% over non-standardized assessment.

3 Silver RM, Landon MB, Rouse DJ, Leveno KJ, Spong CY, Thom EA, Moawad AH, et al. Maternal morbidity associated with multiple repeat cesarean deliveries. Obstet Gynecol. 2006;107(6):1226-32. A Priori Risk
Landmark MFMU Network study establishing PAS incidence by number of prior CDs with and without placenta previa. Used in Step 2 risk table.

4 White A, Malik M, Pruszynski JE, Do QN, Spong CY, Herrera CL. Contemporary Placenta Accreta Spectrum Disorder Incidence and Risk Factors. Obstet Gynecol. 2025;145(6):665-73. A Priori Risk
Updated PAS incidence data, stratifying by anterior vs posterior previa and anterior low-lying placenta. Used in Step 2 risk table.

5 Jessel RH, White A, Yu HY, Hollard AL, Khandelwal M, Zelop CM, Philips J, Shamshirsaz AA, Kingdom JC, Abuhamad A, Herrera CL; Pan American Society for Placenta Accreta Spectrum (PAS2). Importance of placental evaluation in pregnancies at high risk for placenta accreta spectrum: Expert clinical perspective. Pregnancy. 2026;2:e70266. Screening Protocol
PAS2 expert consensus on who to screen, imaging protocol, and the 7-marker ultrasound checklist used in Steps 1 and 3.

6 Jacobsson B, Svanvik T. Placenta accreta spectrum: From predictable risk to preventable crisis. Pregnancy. 2026;2:e70259. Editorial
Accompanying editorial framing PAS screening as a health system imperative, emphasizing rural/low-resource implications.

7 Jauniaux E, D'Antonio F, Bhide A, Prefumo F, Silver RM, Hussein AM, Shainker SA, et al. Modified Delphi study of ultrasound signs associated with placenta accreta spectrum. Ultrasound Obstet Gynecol. 2023;61(4):518-25. US Markers
Delphi consensus establishing the 7 conventional PAS markers for targeted evaluation. Used in Step 3 checklist.

8 Shainker SA, Coleman B, Timor-Tritsch IE, Bhide A, Bromley B, Cahill AG, Gandhi M, et al. Special Report of the Society for Maternal-Fetal Medicine Placenta Accreta Spectrum Ultrasound Marker Task Force. Am J Obstet Gynecol. 2021;224(1):B2-B14. US Markers
SMFM multi-society consensus on marker definitions and ultrasound approach. Used for marker descriptions in Step 3.

9 Rosenthal EA, Lafferty AK, Pruszynski JE, Uddin N, Spong CY, Herrera CL. Placenta Accreta Spectrum Outcomes With a Multidisciplinary Team and Standardized Ultrasound Approach. Am J Perinatol. 2026;43(4):461-71. Outcomes
Demonstrates 97% prenatal detection rate with structured PAI-based referral network.

How Risk Is Calculated

Step 2 (A priori risk): Uses population-based PAS incidence from Silver et al. (2006) and White et al. (2025), stratified by number of prior cesareans and placental location.

Step 3 (PAI score): Calculates the Placenta Accreta Index (Rac et al., 2015) from 5 weighted parameters. The logistic regression model maps scores 0-9 to invasion probability of 2-96%. This is the only validated, ACOG/FIGO-endorsed multiparametric prediction tool for PAS.

Step 3: Ultrasound Evaluation & PAI Score

PAS2 imaging protocol + Placenta Accreta Index calculation

Imaging Protocol (PAS2 Consensus)

Partially full bladder. TA + TV imaging.

TA first. Sagittal + transverse sweeps (cine), then with color Doppler.

TV to complete. LUS/bladder wall 2D + color cine. Zoom areas of concern.

Color Doppler <10 cm/s. Low-flow for hypervascularity, feeder & bridging vessels.

Minimal pressure. Perpendicular insonation. Multiplanar assessment + cine clips.

Placenta Accreta Index (PAI) Calculator

Rac et al., AJOG 2015 | AUC 0.87 (95% CI 0.80-0.95) | Endorsed by FIGO & ACOG

Prior Cesarean Deliveries

Placental Lacunae (Finberg-Williams)

Smallest Sagittal Myometrial Thickness

Anterior Placenta Previa

Bridging Vessels

PAI Score (0-9)

~2% estimated probability of PAS

Probability from logistic regression: score 0→~2%, 1→~5%, 2→~11%, 3→~21%, 4→~36%, 5→~54%, 6→~71%, 7→~83%, 8→~91%, 9→~96%. Cutoff ≥5: sensitivity 73-93%, specificity 76-95% (multiple validation studies).

7 Consensus US Markers (Delphi 2023)

Expand each marker for clinical guidance. Record findings.