What is placenta accreta?

Accreta placentation is a clinicopathologic diagnosis at delivery when 1 or more placental lobules are abnormally attached into a myometrial scar or congenital defect requiring surgical removal. Over 90% of cases of placenta accreta spectrum are found in patients with a prior history of cesarean delivery presenting with a low-lying placenta or a placenta previa developing inside a lower uterine segment scar. Placenta previa accreta is a complex obstetric condition, and management strategies and clinical outcomes are directly linked to the quality of epidemiology data. We have recently questioned the concept of overinvasive placentation and placenta percreta, providing evidence that surgical manipulation of a dehiscent lower uterine segment covering a placenta previa is responsible for the extrusion of part of the placental tissue. Similarly, there is no evidence that villous tissue and extravillous trophoblastic cells can cross the entire uterine wall in accreta areas. Placenta accreta spectrum is the consequence of placental development at sites where the normal decidual and myometrial mechanisms limiting the migration of the extravillous trophoblastic cells are missing, rather than being due to inherently abnormally invasive villous tissue. Placenta accreta has also been increasingly reported in patients with no prior uterine surgery or pathology using clinical criteria similar to those used for uterine atonia and simple placental retention. Cases where a cleavage plane can be identified and the placenta fully detached manually at the time of birth or during gross examination of hysterectomy or partial myometrial resection specimens should not be reported as accreta. Traditional reliance on the absence of the decidua basalis with villous tissue simply apposed to the superficial myometrium to confirm the diagnosis of accreta placentation at histopathologic examination is inadequate and possibly misleading. Instead, pathologists should focus on other diagnostic features, such as deep villous attachment within the scar tissue and distortion of the uteroplacental interface with thick fibrinoid deposition on microscopic examination. There is a need to develop a new clinicopathologic classification based on a detailed topographic intraoperative description of the location and size of the accreta area and the changes associated with uterine remodeling postscarification.