A 29-year-old woman in early pregnancy presents to the emergency department with lower abdominal pain and light vaginal bleeding. She has had a positive pregnancy test, and her last menstrual period was 7 weeks ago. She is haemodynamically stable. Transvaginal ultrasound shows an empty uterus and a 2.5 cm adnexal mass with free fluid in the pouch of Douglas. What is the most likely diagnosis?