What is it called when the placenta attaches improperly in the uterus?

Placenta accreta. Placenta accreta, percreta and increta are increasingly severe conditions where the placenta actually grows into the myometrium, or wall, of the uterus, making detachment at birth difficult if not impossible. Moms with this condition more often than not will require hysterectomies at the time of birth to avoid hemorrhage.
Abn placentation. There are several types of abnormal placentation. The most common is placenta previa, where the placenta sits over the cervix. There is also placenta accreta where the placenta has grown through the inner layer of the uterus into the outer muscular (myometrial) layer. Another type of placental abnormality is placental abruption, where the placenta separates from the uterus prematurely.