Discussion of Placenta previa
- Placenta previa is a condition in which the placental tissue lies abnormally close to or over the internal cervical os.
- Subtypes of placenta previa
- Complete: placenta covers the entire internal cervical os
- Partial: placenta partially covers 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.
- Risk factors for placenta previa include prior placenta previa, prior cesarean delivery, increased maternal age, large placentae (eg, multiple gestations or erythroblastosis), and a maternal history of smoking.
- As the lower uterine segment thins in late pregnancy, the margins of the abnormally implanted placenta are altered.
- Various degrees of placental detachment may develop, with ensuing maternal hemorrhage from the intervillous space.
- During labor, significant fetal hemorrhage also can occur as a result of disrupted villous placental vessels.
 Imaging Findings for Placenta previa
- US: If the internal cervical os can be visualized and if no placental tissue overlies it, placenta previa is excluded.
- MRI: Sagittal images best demonstrate the placental position in relation to the internal cervical os.
Patient #1: MRI demonstrates a complete placenta previa
 See also
 External Links
 References for Placenta previa
- E-medicine placenta previa article