A 32-year-old woman with irregular menstrual cycles presents with a positive home pregnancy test. She is unable to recall her exact LMP date but reports last menstruation was 'sometime in the last 2–3 months.' On examination, the uterus is palpable 2 cm above the symphysis pubis. Transvaginal ultrasound shows a gestational sac of 8 mm diameter with no yolk sac. What is the most appropriate next step in management?