Can Placenta Accreta Occur Without a Previous Uterine Scar?
Yes, placenta accreta spectrum can develop in women without any previous uterine scar, though this is rare. While prior cesarean delivery is the most common risk factor, the condition does occur in unscarred uteri, including in nulliparous women without any history of uterine surgery or instrumentation 1.
Understanding the Epidemiology
The vast majority of placenta accreta spectrum cases (>90%) occur in women with prior cesarean delivery, particularly when combined with placenta previa 2. However, the guideline explicitly acknowledges that the standard etiology theory "fails to explain the rare occurrence of placenta accreta spectrum in nulliparous women without any previous uterine surgery or instrumentation" 1.
Risk in Unscarred Uteri
Even without prior cesarean delivery:
- Placenta accreta spectrum occurs in 3% of women with placenta previa and no prior cesarean deliveries 1
- In a large prospective study of 2,834 patients without placenta previa, 0.1% of cases with no placenta on a previous scar still developed placenta accreta 3
Other Risk Factors Beyond Uterine Scarring
Women without cesarean scars can still develop placenta accreta spectrum due to:
- Placenta previa (the most significant non-scar risk factor)
- Advanced maternal age
- Multiparity
- Prior uterine curettage
- Asherman syndrome
- Other prior uterine surgeries 1
Clinical Implications and Pitfalls
The critical pitfall is assuming that absence of a uterine scar excludes the diagnosis. Case reports document spontaneous uterine rupture from placenta percreta in completely unscarred uteri, including in multiparous women with only prior vaginal deliveries 4, 5, 6. These cases can present catastrophically with:
- Acute abdominal pain
- Hypovolemic shock
- Intraperitoneal hemorrhage
- Intrauterine fetal death
This can occur even in the second trimester 6.
Diagnostic Approach in Unscarred Uteri
Maintain clinical suspicion based on ultrasound findings rather than relying solely on risk factor assessment. The guideline emphasizes that "clinical risk factors remain equally important as predictors of placenta accreta spectrum by ultrasound findings" 1. Key ultrasound features include:
- Loss of retroplacental clear zone (33% of cases with this finding had accreta, versus 0.1% without) 3
- Multiple vascular lacunae within the placenta
- Abnormally increased vascularity 1
Even in the absence of classic risk factors, if ultrasound findings are suspicious, the patient requires tertiary-level care planning and multidisciplinary team involvement 1.