A 27-year-old woman with a desired pregnancy presents at 8 weeks with mild vaginal bleeding but no pain. Transvaginal ultrasonography shows an intrauterine gestational sac measuring 19 mm, with no yolk sac or embryo; the mean sac diameter was measured by an experienced sonographer. Her serum human chorionic gonadotrophin is 31 000 IU/L, and she is haemodynamically stable. Which one of the following is the most appropriate next step?