Placenta Previa & Placenta Accreta

Michael Cooney, MD
Carol Benson, MD

Presentation

Imaging Findings

Emergency obstetric ultrasound demonstrated a single live intrauterine gestation in breech presentation with unremarkable fetal survey. Sagittal midline image (Figure 1) shows placental tissue extending down to the region of the cervix. Translabial scan (Figure 2) demonstrates placental tissue partially covering the cervix. Transverse (Figure 3) and sagittal (Figure 4) transabdominal images demonstrate interruption of the normal hypoechoic rim of myometrial tissue beneath the placenta, suggesting abnormal placental attachment, placenta accreta, increta, or percreta.

Differential Diagnosis

• Placenta previa with normal placental attachment
• Placenta previa with placenta accreta
• Placenta previa with placenta increta
• Placenta previa with placenta percreta

Diagnosis

Discussion

Marginal or Partial - Placenta covers part of the cervix without completely covering the internal os

Complete - Placenta completely covers the cervical os

The incidence of placenta previa is about 1 in 200 pregnancies and increases with prior cesarean section, advanced maternal age and multiparity.

Translabial (transperineal) or transvaginal scanning may be helpful for better visualization of the lower uterine segment and cervix, when transabdominal findings are inconclusive.

Placenta accreta is abnormal attachment of the placenta to the uterine wall (decidua) such that the chorionic villi invade abnormally into the myometrium. It is thought to result from either a primary deficiency of or secondary loss of decidual elements (decidua basalis). Three grades are used, based on pathologic assessment of myometrial invasion by the chorionic villi:

1. Placenta Accreta - chorionic villi in contact with myometrium (80% of cases)
2. Placenta Increta - chorionic villi invade into myometrium (15% of cases)
3. Placenta Percreta - chorionic villi invade into serosa (5% of cases)

All three forms of abnormal placentation are associated with a history of prior cesarean section, history of uterine instrumentation or surgery, or placenta previa. Rarely, abnormal attachment is seen in the absence of prior surgery and in the absence of placenta previa.

Sonographic findings include:

• loss of the normal hypoechoic rim of myometrial tissue beneath the placenta
• loss of the normal hyperechoic uterine serosa-bladder wall interface
• presence of tissue of placental echotexture extending beyond the uterine serosa.

An associated finding that is sometimes seen is multiple or large placental venous lakes.

References

2. Hertzberg B, Bowie J, Carroll B, Kliewer M, Weber T, et al. Diagnosis of placenta previa during the third trimester: Role of transperineal sonography. AJR 159:83-87, 1992.

3. Hoffman-Tretin J, Koenigsberg M, Rabin A, Anyaegbunam A. Placenta Accreta: Additional sonographic observations. J Ultrasound Med 11:29-34, 1992.

4. Townsend R. Ultrasound evaluation of the placenta and umbilical cord. In: Callen Peter, editor. Ultrasonography in obstetrics and gynecology. Philadelphia: Saunders, 1994: 445-53.

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