有时没有正常的蜕膜形成，胎盘绒毛直接侵入子宫肌层，如图示，称为胎盘粘连。子宫切除可有效控制出血。
Sometimes no normal decidual plate forms in pregnancy, and placental villi invade directly into myometrium, as seen here. This condition in known as placenta accreta. A hysterectomy may be necessary to control bleeding.