Maternal-Fetal Medicine
Placenta Accreta Syndrome
Understanding the Optimal Timing for Diagnosis
A patient education guide from your MFM care team
What Is This?
Understanding Placenta Accreta
The Placenta
Normally, the placenta attaches to the uterine wall and separates cleanly after delivery.
Placenta Accreta
In PAS, the placenta grows too deeply into the uterine wall and does not separate normally at delivery.
Our Goal
Early, accurate diagnosis allows us to plan the safest possible delivery for you and your baby.
Core Concept
A Continuum, Not a Single Test
Diagnosing PAS is an ongoing process. As your pregnancy progresses, the uterus changes — and serial imaging at specific windows gives us the most accurate picture.
1
11–14 Wks
Early Risk
Stratification
2
18–22 Wks
Primary
Screening
3
24–30 Wks
Diagnostic
Mapping
4
32–34 Wks
Pre-Operative
Assessment
Stage 1 · 11–14 Weeks
Early Risk Stratification
What We Do
Transvaginal ultrasound (TVUS) to assess the lower uterus and the site where the gestational sac has implanted.
Why It Matters
A sac implanted near a prior cesarean scar requires close follow-up. Early identification allows us to build your specialized care team immediately.
High-risk patients include those with a prior cesarean delivery or uterine surgery.
Stage 2 · 18–22 Weeks
Primary Screening Window
Most cases of PAS are first suspected during the routine fetal anatomy ultrasound.
What We Look For
Irregular placental spaces (lacunae), thinning of the uterine wall, and abnormal blood vessels near the bladder.
Tools Used
Transabdominal and transvaginal ultrasound with Color Doppler to visualize blood flow patterns.
Next Step
If signs are present, we schedule a detailed diagnostic imaging session in the third trimester.
Stage 3 · 24–30 Weeks
Diagnostic Mapping
3D Ultrasound & Color Doppler
Provides the most detailed map of how deeply the placenta has grown and where abnormal vessels are located.
Pelvic MRI
Recommended when the placenta is posterior, or when ultrasound findings are unclear. Optimal timing is 24–30 weeks.
Goal of This Stage
Establish a definitive diagnosis and create a detailed surgical plan for your delivery team.
At 28–30 weeks, the lower uterine segment thins naturally, maximizing the accuracy of imaging.
Stage 4 · 32–34 Weeks
Pre-Operative Assessment
Serial ultrasounds monitor for changes and help us finalize your delivery plan.
Cervical Length
We measure cervical length at each visit. A shorter cervix (<25 mm) may indicate the need for earlier delivery.
Bleeding Monitoring
Any episodes of antepartum bleeding are carefully tracked and may prompt earlier intervention.
Your care plan is individualized. We adjust based on your specific clinical findings.
Delivery Planning
Scheduled Delivery: 34–35 Weeks
A planned cesarean delivery before labor begins is the standard recommendation for stable patients with PAS.
Standard Window
34 0/7 to 35 6/7 weeks of gestation for a stable patient. This prevents emergency hemorrhage.
Earlier Delivery
If cervical length is <25 mm with bleeding, delivery may be planned as early as 33–34 weeks.
Multidisciplinary Team
Your delivery involves MFM, urology, anesthesia, and interventional radiology working together.
At a Glance
Your Monitoring Timeline
| Gestational Age |
Imaging Tool |
Clinical Goal |
| 11–14 Weeks |
Transvaginal Ultrasound |
Identify low anterior implantation; screen for cesarean scar pregnancy |
| 18–22 Weeks |
Transabdominal / Transvaginal US |
Routine PAS screening; identify placental lacunae and myometrial thinning |
| 24–30 Weeks |
3D US / Color Doppler / MRI |
Definitive diagnosis; map depth of invasion; surgical planning |
| 32–34 Weeks |
Serial Ultrasound |
Cervical length monitoring; assess bleeding; finalize delivery timing |
Important Safety Information
Transvaginal Ultrasound Is Safe
Transvaginal ultrasound is a safe and essential part of PAS assessment, even with placenta previa.
⚠ Common Misconception
Withholding transvaginal ultrasound out of fear of provoking bleeding is a clinical pitfall. TVUS does not cause hemorrhage and provides the clearest view of the lower uterus and cervix — information that is critical for your safety.
Your physician will always discuss each imaging step with you before proceeding.
Your Care Team
We Are Partnering With You
Every imaging visit and every measurement is a step toward the safest possible delivery for you and your baby. Our multidisciplinary team is with you at every stage.
Please bring your questions to every visit. We are here to guide you.
Content consistent with ACOG Practice Bulletin and SMFM guidelines on Placenta Accreta Spectrum.