A 28-year-old woman at 7 weeks gestation presents with lower abdominal pain and light vaginal bleeding. She is haemodynamically stable. A urine pregnancy test is positive, but transvaginal ultrasound shows no intrauterine or ectopic pregnancy (pregnancy of unknown location). Serum β-hCG is 1,500 IU/L. According to Australian early pregnancy guidelines, what is the MOST appropriate next step?