A 37-year-old woman at 37 weeks has a small amount of vaginal bleeding shortly after her membranes rupture. She remains haemodynamically stable, with no abdominal tenderness or uterine rigidity, but the fetal heart rate falls from 140 to 80 beats per minute and does not recover with maternal repositioning. The placenta was posterior and clear of the internal os on the routine mid-pregnancy scan. Which one of the following is the most likely diagnosis?