A 30-year-old woman presents with lower abdominal pain and light vaginal bleeding at 7 weeks gestation by dates. She has a positive pregnancy test. Transvaginal ultrasound shows no intrauterine gestational sac and a complex right adnexal mass with free fluid in the pouch of Douglas. BP is 110/70 mmHg and pulse 92 beats per minute. What is the most likely diagnosis?