A 29-year-old woman presents at 7 weeks gestation with lower abdominal pain and light vaginal bleeding. Her last menstrual period was 7 weeks ago. She has a positive pregnancy test. Transvaginal ultrasound shows no intrauterine gestational sac and a 2 cm adnexal mass with free fluid in the pelvis. She is haemodynamically stable. What is the most likely diagnosis?