Accuracy of ultrasonography and magnetic resonance imaging in the diagnosis of placenta accreta.

Background: The incidence of placenta accreta has increased dramatically over the last three decades, in concert with the increase in the cesarean delivery rate. Optimal management requires accurate prenatal diagnosis. The purpose of this study was to determine the precision and reliability of ultrasonography and magnetic resonance imaging (MRI) in diagnosing placenta accreta.

Methods: A historical cohort study was performed with information gathered from our obstetric, radiologic, and pathology databases. Records from January 2000 to June 2005 were reviewed to identify patients with a diagnosis of placenta previa, low-lying placenta with a prior cesarean delivery, or history of a myomectomy to determine the accuracy of pelvic ultrasonography in the diagnosis of placenta accreta. The records of those considered to be suspicious for placenta accreta and subsequently referred for additional confirmation by MRI were also analyzed. The sonographic and MRI diagnoses were compared with the final pathologic or operative findings or with both.

Results: Of the 453 women with placenta previa, previous cesarean delivery and low-lying anterior placenta, or previous myomectomy, 39 had placenta accreta confirmed by pathological examination. Ultrasonography accurately predicted placenta accreta in 30 of 39 of women and correctly ruled out placenta accreta in 398 of 414 without placenta accreta (sensitivity 0.77, specificity 0.96). Forty-two women underwent MRI evaluation because of findings suspicious or inconclusive of placenta accreta by ultrasonography. Magnetic resonance imaging accurately predicted placenta accreta in 23 of 26 cases with placenta accreta and correctly ruled out placenta accreta in 14 of 14 (sensitivity 0.88, specificity 1.0).

Conclusion: A two-stage protocol for evaluating women at high risk for placenta accreta, which uses ultrasonography first, and then MRI for cases with inconclusive ultrasound features, will optimize diagnostic accuracy.