Figure 1: A single intrauterine gestation is present. The placenta is located anteriorly. The inferior margin of the placenta overlies the internal cervical os.

Figure 2: There is irregularity at the interface between the superior aspect of the bladder and placenta, suggesting placental invasion through the uterine wall and serosa.

Placenta previa and placenta percreta.

Placenta previa is a condition where the placental tissue covers the cervical os. Subtypes of placenta previa include:

• Complete previa: placenta covers 360o of the internal cervical os
• Incomplete (or partial): placenta covers 0o-360o of the internal cervical os
• Marginal: placental tissue abuts but does not cover the internal cervical os
• Low lying: edge of the placenta lies abnormally close to but does not abut the internal cervical os

Placenta previa is present at term in 0.3-0.6% of live births. Risk factors include advanced maternal age, previous cesarean section, previous placenta previa, lower uterine surgical scars, and multiple previous pregnancies. Most patients present with painless vaginal bleeding in the third trimester. Bleeding is initiated by the effacement of the cervix and dilation of the cervical os, which disrupts the vascular bed of the placenta.

Placenta accreta is an abnormal adherence of the placenta to the uterine wall such that the chorionic villi invade into the myometrium. It is thought to result from either a primary deficiency of or secondary loss of decidual elements (deciduas basalis). Risk factors for placenta accreta include advanced maternal age, prior cesarean section, procedures (myomectomy, dilation and curettage), multiparity, miscarriage, prior placenta accreta or previa. Failure of the placenta to completely separate from the myometrium during labor results in hemorrhage, with resultant maternal and fetal morbidity and mortality. Myometrial invasion by the chorionic villi is categorized based on the pathologic assessment of invasion:

• Placenta accreta: myometrial invasion (76% of cases)
• Placenta increta: deep myometrial invasion (18% of cases)
• Placenta percreta: invasion through the uterine serosa with potential invasion of adjacent organs (6% of cases)

The placenta can be visualized as early as the 10th week of pregnancy. MR imaging allows for differentiation between the placenta and the myometrium. On T1-weighted MRI images, the placenta is of low to medium signal intensity and is slightly higher in intensity than the myometrium. On T2-weighted MRI sequences, the placenta has high signal intensity and the myometrium is hypointense. . In evaluation of placenta previa, sagittal T2-weighted images allow for assessment of the distance between the placental edge and internal cervical os. For evaluation of depth of placental invasion into the myometrium, images perpendicular to the boundary plane between the placenta and myometrium are most useful. Placenta accreta and increta are suggested by thinning, irregularity, or focal disruption of the sub-adjacent myometrium. However, minimal placental adhesion, such as in placenta accreta, can be difficult to identify. When irregularity or disruption of the normal bladder wall is seen, invasion of the bladder may be present, as demonstrated by this case.

References:

1. Brant WE and Helms CA. Fundamentals of Diagnostic Radiology, 2nd Edition. Philadelphia: Lippincott Williams & Wilkins, 1999. pp 831-832.
2. Nagayama M, Watanabe Y, Okumura A, Amoh Y, Nakashita S, Dodo Y. Fast MR Imaging in Obstretrics. RadioGraphics. 2002; 22: 563-582.
3. Marrinan G. Placenta Previa. eMedicine, 2002.