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:
- Total placenta previa: placenta completely covers the internal os.
- Partial placenta previa: placenta partially covers the internal os.
- Marginal placenta previa: edge of the placenta is at the margin of the internal os.
- 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.
- 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?
- Painless hemorrhage near the end of the second trimester or after.
- Frequently there is no warning in a woman who has had an uneventful prenatal course.
- Usually ceases spontaneously and then recurs.
- Have high index of suspicion in women with advanced maternal age, multiparity and post cesarian
What are the useful imaging modalities in evaluating placenta previa? What is the utility of the procedure, including limitations and accuracy?
- Transabdominal sonography: simplest, most precise, and safest method of localization
- Sagittal, parasagittal, and transverse sonographic views should be obtained.
- Able to visualize internal cervical os in all cases , in contrast to 70% using transabdominal equipment, PPV: 93%, NPV: 98%
- GOLD STANDARD
- 96 – 98% accuracy
- False-positive results are often a result of bladder distention; therefore, repeat after emptying bladder.
- Ultrasound of the umbilical cord:
- Identify umbilical cord's insertion into the chorionic surface of the placenta
- Without central cord insertion, splaying of the vessels at the cord's periphery of the placenta is suggestive of a velamentous insertion; also 12.5% of velamentous cords contain one umbilical artery, therefore, the presence a two vessel cord should increase suspicion of velamentous insertion
What are the radiologic findings of placenta previa?
- Identification of echogenic placental tissue overlying or proximal to the internal cervical os.
A distance of greater than 2 cm from the os excludes the diagnosis of previa, although the placenta may be low lying.