"Ectopic pregnancy usually develops because your fallopian tubes have become damaged. This could have happened during an earlier operation on your abdomen or if you have had pelvic inflammatory disease, which is usually caused by a sexually transmitted infection, e. g chlamydia or gonorrhea. If your fallopian tubes have been damaged, they may become inflamed, narrowed or scarred, which makes it harder for the fertilised egg to pass down to your womb.
Other things that make an ectopic pregnancy more likely include smoking and being treated for infertility. However, for many ectopic pregnancies, the cause is unknown.".
Attribution: BUPA