A 31-year-old woman at 9 weeks of pregnancy presents with vaginal bleeding and cramping after passing several clots. Her pulse is 88 beats/minute, blood pressure is 118/72 mmHg, and she has no fever. Transvaginal ultrasonography shows an intrauterine gestational sac with an embryo measuring 9 mm, but no cardiac activity; there is no adnexal mass. The cervix is closed and she wishes to avoid an intervention unless the diagnosis is certain. Which one of the following is the most appropriate next step?