A 28-year-old woman is referred to your outpatient gynecology clinic after an initial ultrasound at her primary care physician's office failed to identify an intrauterine pregnancy despite her symptoms. She has been experiencing nausea, breast tenderness, abdominal distension, and amenorrhea for approximately four months. She and her spouse have been actively trying to conceive for over three years, experiencing significant distress due to recurrent failed attempts. She reports a home urine pregnancy test was 'faintly positive' about 12 weeks ago, though she admits to not checking it immediately due to anxiety. Her medical history includes well-controlled bulimia nervosa, stable for the past three years. On examination, her vital signs are stable: temperature 36.8°C, pulse 72/min, blood pressure 120/78 mmHg, respirations 14/min. Physical examination reveals mild abdominal distension, but it is soft and non-tender without any fetal heart tones detected. Her affect is tearful but cooperative when discussing the ultrasound findings. Given these findings, what is the most likely diagnosis and the most appropriate initial management approach?