Etiology

Advanced maternal age, multiple pregnancies, and tobacco use increase the risk of PP, but the mechanisms are uncertain.[12][13][14] With the exception of the risk factor of previous uterine scarring, it is likely that multiple risk factors contribute together to the occurrence of the condition.[6]

Pathophysiology

PP, and its variants, occurs when the blastocyst implants in the lower uterine segment near the cervical os. Most cases are probably accidental and simply the result of normal variation in placentation. The presence of a uterine scar in the lower segment is thought to interfere with the process of placentation in such a way as to increase the risk both of PP and of placenta accreta spectrum.[4]

By definition, in PP the placenta is completely covering the cervical os and is in front of the presenting part of the fetus.[1][2][3] Bleeding may occur spontaneously, from placental trauma (e.g., sexual intercourse, vaginal examination), or as the cervix opens at the onset of labor. As the presenting part moves into the lower uterine segment, the placenta may be torn or may separate (abrupt) from the uterus, in whole or in part.[5] Massive hemorrhage results from both the mother and the fetus. Untreated, this almost always results in the death of the mother and fetus.[1][15]

Classification

Anatomic classification[1][2][3]

Historically, PP was graded according to the relationship and/or distance between the lower placental edge and the internal os of the uterine cervix.[1][2][3] This caused challenges due to technical difficulties in differentiating between the grades, and the fact that separation between the opposing sides of the internal cervical os is not always present on ultrasound examination.[3] Updated consensus has recommended the following simplified classification criteria:[1][2][3]

  • Placenta previa (placenta is directly covering the cervical os)

  • Low-lying placenta (placental edge is <2 cm from the cervical os)

  • Normal placental location (placental edge is ≥2 cm from the cervical os).

Use of this content is subject to our disclaimer