A 30-year-old woman presents with 7 weeks of amenorrhoea, mild lower abdominal pain, and light vaginal spotting. Her β-hCG is 1 500 IU/L, and transvaginal ultrasound shows no intrauterine gestational sac and no adnexal mass. She is haemodynamically stable. According to SOGC's pregnancy of unknown location guideline, what is the most appropriate next step?