A 33-year-old woman presents to the emergency depament complaining of nausea and vomiting. She states that she has been having significant nausea that has been worsening over the past 2 weeks. Over the past 2 days, she has had 2 episodes of vomiting. She also notes increased fatigue. She has no abdominal pain or vaginal bleeding. She has no other complaints. Her past medical history is significant for occasional migraine headaches. She has never had surgery. She takes acetaminophen as needed for headache, and has no known drug allergies. She works as a lawyer at a local firm and lives with her husband for three years. She has no family history of cancer or hea disease. Her vital signs are stable. Examination is significant for a bluish-appearing cervix on speculum examination. The remainder of the examination, including the abdominal examination, is benign. Laboratory evaluation shows: Urine hCG: positive Leukocytes: 9,000/mm3 Hematocrit: 41% Platelets: 250,000/mm3 Pelvic ultrasound demonstrates a gestational sac with yolk sac and fetal pole surrounded by myometrium. There is a hea rate of 154 beats per minute. Which of the following is the MOST likely diagnosis?