FCOG-0960

A 26-year-old G2P0+1 comes to the emergency department with vaginal spotting. She experienced periodic stomach pain related with the bleeding, but no fetal product passing, about 6 hours before presentation. She is now in the first trimester of her pregnancy and claims that her previous pregnancy was uneventful. She takes her prenatal vitamins regularly and does not use any other prescriptions or drugs.

Her vital signs are normal, and her physical examination reveals that she has a closed cervical os. Which of the following diagnoses is the most likely?