A retained placenta occurs when the placenta is not expelled from the uterus within 30 minutes of childbirth. Risk factors include previous retained placentas, uterine injuries or surgeries, preterm births, induced labor, and multiple pregnancies. Causes can include failure of the placenta to separate fully from the uterine wall or abnormal placenta attachments like placenta accreta. Treatment involves controlled cord traction, manual removal in the operating room if needed, or hysterectomy for deeply embedded placentas. Complications can be life-threatening bleeding, infections, or shock if not properly managed.