Other risk factors for ectopic pregnancy include Current use of estrogen / progestin oral contraceptives History of pelvic inflammatory disease or sexually transmitted infections (particularly due to Chlamydia trachomatis) Infertility Cigarette smoking Prior spontaneous or induced abortion Pathophysiology of Ectopic Pregnancy The most common site of ectopic implantation is a fallopian tube, followed by the uterine cornua (referred to as a cornual or an interstitial pregnancy).