MSFinals-3658

A 33-year-old woman who is 28 weeks pregnant arrives at the emergency department with painless vaginal bleeding. She had her second baby three years ago, which was delivered via a c-section, but otherwise was a normal pregnancy. Upon obstetric examination, her uterus was non-tender, however, her baby was in breech presentation. The foetal heart rate was also normal, and she denied experiencing any contractions during the bleeding episode. What is the recommended next investigation for the most probable diagnosis?