You are seeing a 31-year-old female a primary care setting who has presented with abnormal vaginal bleeding. This patient informs you during the history that she is experiencing heavy menstrual bleeding outside of her normal cycle, pelvic discomfort, and bloating. The patient also informs you that she experiences painful cramps when she has her period. During a pelvic examination you note the presence of a firm, enlarged and irregular shaped non-tender uterus. You can move the mass slightly from side-to-side. You suspect a uterine fibroid as there are no red-flag symptoms present. What is the most appropriate first-line investigation to perform in this case?