A 29‑year‑old woman presents with amenorrhoea and lower abdominal pain at 7 weeks from her last menstrual period. She has a positive pregnancy test. Transvaginal ultrasound shows no intrauterine gestational sac and a 2.5 cm adnexal mass adjacent to the right ovary with free fluid in the pouch of Douglas. She is haemodynamically stable. What is the most likely diagnosis?