Which abnormal placental implantations extend beyond the endometrium?

Placenta accreta is characterized by an abnormal implantation of the placenta where the chorionic villi attach to the myometrium (the muscular layer of the uterus) instead of being confined only to the endometrial layer. This condition results from an insufficient decidual reaction, which is the modification of the endometrium during pregnancy that normally prevents deep placental attachment. In cases of placenta accreta, the placenta invades deeper, leading to potential complications during delivery, such as severe hemorrhage.

The other conditions listed, such as abruptio placentae, involve the premature separation of the placenta from the uterine wall, rather than an abnormal implantation. Placenta increta refers to a deeper penetration into the myometrium than accreta but does not extend beyond it, while placenta previa occurs when the placenta is implanted near or over the cervical os, but still maintains an appropriate attachment within the endometrium. Thus, placenta accreta uniquely extends beyond the endometrium, making it the correct answer.