A multigravida, labor II, 36-37 weeks of gestation, has gone into labor. Her waters broke 8 hours ago, the labor activity continues for the last 4 hours, it is regular, with contractions that last 35 seconds and occur every 3-4 minutes. The child is in the cephalic presentation, with the head pressed to the entrance into the lesser pelvis. The parturient woman complains of a sudden sharp abdominal pain. Her pulse is 100/min., blood pressure is 110/70 - 100/70 mm Hg. The uterus is tense and does not relax between the contractions. Fetal heartbeat is muffled - 100/min. The amniotic fluid is blood-colored and continues to leak. What is the most likely diagnosis?