Occurs when an either an obstruction prevents sperm from reaching the egg or some factor interferes with fertilization. This may be a consequence of pelvic adhesions that are fibrous scars possibly arising from previous surgeries, past infections such as pelvic inflammatory disease, or infections following abortions or prior births. Endometriosis may also lead to pelvic adhesions. Other causes may be a smaller than normal cervical opening through which sperm pass from the vagina into the uterus or, the mucus produced by the cervix being the wrong consistency – too thick preventing sperm travel. In addition, sperm may be identified by the immune system of some women, as foreign invaders and so they are killed by it.