Placenta Previa

What are the various locations of placenta previa?

In placenta previa, the placenta is located over or very near the internal os.  There are four degrees of this abnormality and a related condition:

  1. Total placenta previa: placenta completely covers the internal os.
  2. Partial placenta previa: placenta partially covers the internal os.
  3. Marginal placenta previa: edge of the placenta is at the margin of the internal os.
  4. Low-lying placenta: placenta implanted in lower uterine segment so that the placenta edge does not actually cover the internal os but is close to it.
  5. Vasa previa: fetal vessels course through the membranes and present at the cervical os.
What is the clinical setting when you will consider placenta previa?   
What are the useful imaging modalities in evaluating placenta previa? What is the utility of the procedure, including limitations and accuracy?   
  1. Transabdominal sonography: simplest, most precise, and safest method of localization
  2. Ultrasound of the umbilical cord:
What are the radiologic findings of placenta previa?

A distance of greater than 2 cm from the os excludes the diagnosis of previa, although the placenta may be low lying.