A 28-year-old woman presents with first-trimester vaginal bleeding and lower abdominal cramping. Her last menstrual period was 8 weeks ago. She is haemodynamically stable. Speculum exam shows a closed cervix with scant bleeding. Beta-hCG is 35 000 IU/L and transvaginal ultrasound shows an intrauterine pregnancy with fetal cardiac activity. What is the most likely diagnosis?