A 32-year-old primigravida at 34 weeks gestation presents to the emergency department with sudden onset vaginal bleeding and severe abdominal pain. On examination, she is hemodynamically unstable (BP 90/60 mmHg, HR 110/min), with a rigid, tender uterus and vaginal bleeding. Fetal heart rate is 80 bpm with minimal variability. What is the investigation of choice to confirm the diagnosis of abruptio placentae?