The 28-year-old G2P1 patient presents after rupture of membranes 2 hours ago at 39 weeks gestation. She has a history of prior cesarean section for failure to progress but is otherwise healthy; this pregnancy is uncomplicated. Contractions are painful and labor is progressing. Abruptly, the patient complains of extreme abdominal pain; the nurse observes a small amount of blood coming from her vagina. Which assessment prompts the nurse to prepare the patient for an emergency cesarean section?