The main cause of ectopic pregnancy occurs when the cilia in the uterine tubes are altered, removed, or deformed. The fertilized egg is transported into the uterus by the cilia in the uterine tubes. The cilia make a pathway by pushing the egg in the right direction. If the cilia are altered, removed, or deformed, this pathway is essentially changed, causing the fertilized egg to have a different pathway. Along with this, there are many other predictive causes of ectopic pregnancies, but there has not been enough experimental evidence to conclude why each have an effect. The other predictive causes are pelvic inflammatory disease, psychological stress, congenital abnormalities, blind tubal pouches, leiomyomata, endometriosis, poor semen quality, abnormal prostaglandin level in semen, smoking, intrauterine contraceptive devices, artificial reproductive technologies (including in vitro fertilization) and vaginal douching. This can also occur in uterine tubes that seem histologically normal due to previous inflammation, immobilized cilia, delayed ovulation, and embryo quality (Stabile, 1996).
The following picture shows what a tubal ectopic pregnancy looks like inside of the human body. As you can see it stretches the uterine tubes to the point of rupture which will cause the typical abdominal pains and hemorrhaging.