The precise cause of endometriosis is unknown, but there are several theories to explain it:
Retrograde Menstruation - It is known that menstrual blood can flow backwards along the fallopian tubes during a woman's period, carrying with it living endometrial cells. Then, instead of flowing out through the cervix and vagina as in normal menstruation, the menstrual fluid flows into the abdominal cavity instead. According to this theory, some of the endometrial cells in this fluid is implanted into the abdomen this way.
Some evidence for this theory comes from the observations that endometriosis is uncommon following a sterilization procedure in which the fallopian tubes are blocked. Endometriosis is associated with conditions in which the normal flow of menstrual blood is blocked, such as in women whose cervix did not develop properly.
Lymphatic System Distribution - There is also a similar theory which suggests that wayward bits of endometrial tissue are distributed from the uterus to other parts of the body not necessarily through the fallopian tubes, but possibly through the lymphatic system. The lymphatic system consists of channels throughout the body that help to drain away excess fluids.
Formation in Situ - According to this theory, inflammation or a hormonal change can cause normal tissue to transform itself into endometrial tissue.
Immune System Defects - Endometriosis may result from a specific defect in the immune system (the body system that fights disease and infection), which normally would eradicate such stray cells migrating outside the uterus.
Genetic Theory - Since endometriosis appears to run in families, this theory suggests that the likelihood for developing endometriosis may be contained in a woman's genes.
Embryonic Tissue Theory - This theory suggests that endometriosis is caused by microscopic remnants of a women's embryonic tissue (from when she was an embryo), that later develops into endometriosis.
Estrogen theory - Because endometriosis is linked to the activity of hormones, substances called "organochlorines" have come under scrutiny. Contained in pesticides, these man-made chemicals are known, as "environmental estrogens" for their abilities to act like estrogen.
Organochlorines are suspected as the reason for an increase in estrogen-related conditions, including endometriosis. They also are suspected as one of the causes of a variety of cancers, such as reproductive cancers and some forms of breast cancer that respond to hormonal influences.