A 32-year-old woman presents at 7 weeks of pregnancy with left lower abdominal pain and light vaginal bleeding. The pregnancy followed in-vitro fertilisation, and transvaginal ultrasonography shows an intrauterine embryo with cardiac activity, as well as a separate 25-mm left adnexal gestational sac containing an embryo with cardiac activity and a small amount of free fluid. She is haemodynamically stable. Which one of the following is the most likely diagnosis?