Placenta accreta

A 34-year-old, gravida 4, para 3, woman comes to the clinic for a 36-week antenatal visit. She has had three prior Cesarean sections. She is healthy and takes no medications other than prenatal vitamins. Family history is noncontributory. Her pregnancy has been uncomplicated and there have been no abnormalities on routine screening. Ultrasound shows that her placenta is "posterior and clear of the os." Which of the following conditions is this patient at greatest risk of having?