A 24-year-old woman at 10 weeks gestation presents with light vaginal bleeding and mild lower abdominal discomfort. Her last menstrual period was 10 weeks ago and she has had no previous pregnancies. She is haemodynamically stable. Transvaginal ultrasound shows an intrauterine gestational sac but no fetal pole. Serum beta-hCG is 1500 IU/L. Which factor most increases the likelihood of ectopic pregnancy and should be specifically sought in the history?