Placenta accreta occurs when all or part of the placenta attaches abnormally to the myometrium (the muscular layer of the uterine wall). Three grades of abnormal placental attachment are defined according to the depth of attachment and invasion into the muscular layers of the uterus:

Accreta – chorionic villi attached to the myometrium, rather than being restricted within the decidua basalis. Increta – chorionic villi invaded into the myometrium. Percreta – chorionic villi invaded through the perimetrium (uterine serosa).Because of abnormal attachment to the myometrium, placenta accreta is associated with an increased risk of heavy bleeding at the time of attempted vaginal delivery. The need for transfusion of blood products is frequent, and surgical removal of the uterus (hysterectomy) is sometimes required to control life-threatening bleeding.Rates of placenta accreta are increasing. As of 2016, placenta accreta affects an estimated 1 in 272 pregnancies. Risk factors
An important risk factor for placenta accreta is placenta previa in the presence of a uterine scar. Placenta previa is an independent risk factor for placenta accreta. Additional reported risk factors for placenta accreta include maternal age and multiparity, other prior uterine surgery, prior uterine curettage, uterine irradiation, endometrial ablation, Asherman syndrome, uterine leiomyomata, uterine anomalies, and smoking. The condition is increased in incidence by the presence of scar tissue i.e. Ashermans syndrome usually from past uterine surgery, especially from a past dilation and curettage, (which is used for many indications including miscarriage, termination, and postpartum hemorrhage), myomectomy, or caesarean section.
MRI findings associated with placenta accreta include dark T2 bands, bulging of the uterus, and loss of the dark T2 interface.Although there are isolated case reports of placenta accreta being diagnosed in the first trimester or at the time of abortion <20 weeks gestational age, the predictive value of first-trimester ultrasound for this diagnosis remains unknown. Women with a placenta previa or "low-lying placenta" overlying a uterine scar early in pregnancy should undergo follow-up imaging in the third trimester with attention to the potential presence of placenta accreta. Complications
Damage to local organs (e.g., bowel, bladder, uterus and neurovascular structures in the retroperitoneum and lateral pelvic sidewalls from placental implantation and its removal;
Postoperative bleeding requiring repeated surgery;
Amniotic fluid embolism;
Complications (such as dilutional coagulopathy, consumptive coagulopathy, acute transfusion reactions, transfusion-associated lung injury, acute respiratory distress syndrome, and electrolyte abnormalities) caused by transfusion of large volumes of blood products, crystalloids, and other volume expanders;
Postoperative thromboembolism, infection, multisystem organ failure, and maternal death.The exact incidence of maternal mortality related to placenta accreta and its complications is unknown, but it is significant, especially if the urinary bladder is involved

Treatment
Treatment may be delivery by caesarean section and abdominal hysterectomy if placenta accreta is diagnosed before birth. Oxytocin and antibiotics are used for post-surgical management. When there is partially separated placenta with focal accreta, best option is removal of placenta. If it is important to save the womans uterus (for future pregnancies) then resection around the placenta may be successful.