Pelvic inflammatory disease is caused by bacteria that cause sexually transmitted infections such as chlamydia and gonorrhea. The bacteria that cause PID are most commonly spread during sex when they travel through the reproductive tract. The bacteria may also enter the reproductive tract as a result of an IUD insertion, childbirth, miscarriage, abortion, or from an endometrial biopsy performed by a health care provider.
Birth control pills are thought in some cases to lead to a condition that allows easier access to tissue where bacteria may grow. However, the pill may also offer some protection against the development of PID by causing the body to create thicker cervical mucus, making it more difficult for bacteria to reach the upper reproductive tract.