A 34-year-old woman, Gravida 2 Para 1, presents to the Early Pregnancy Assessment Unit at 9 weeks gestation with a 2-day history of light vaginal bleeding and mild lower abdominal cramping. On examination, her vital signs are stable. Pelvic examination reveals a small amount of blood in the vagina, but the cervical os is closed. A transvaginal ultrasound scan confirms a single viable intrauterine pregnancy with fetal cardiac activity appropriate for dates. What is the most likely diagnosis?