A 36-year-old woman at 8 weeks of gestation has vaginal bleeding and intermittent lower abdominal cramping. Transvaginal ultrasonography shows an intrauterine gestational sac with a yolk sac and an embryo measuring 6 mm from crown to rump; cardiac activity is not seen. There is no adnexal mass and she is haemodynamically stable. Which one of the following is the most appropriate next step?