Placenta accreta is characterized by what abnormal implantation?

Placenta accreta is primarily characterized by abnormal implantation that occurs when the placenta attaches too deeply into the uterine wall, particularly into the endometrium, which is the inner lining of the uterus. In this condition, the placental villi penetrate beyond the normal limit, leading to a situation where the placenta is firmly adhered to the uterine wall. This can result in complications during delivery, as the placenta may not detach properly, increasing the risk of hemorrhage.

While the other choices describe different aspects or complications of placental issues, they do not specifically define placenta accreta. The implantation itself is not typically classified as just into the muscle layer or involving invasion of nearby organs without the context of how deeply it penetrates. Therefore, the emphasis on the abnormal implantation into the endometrium as a defining characteristic is crucial in understanding the pathology of placenta accreta.