Causes:

Endometriosis occurs among women all over the world, but researchers have been unable to determine its cause. A combination of genetic, biologic, and environmental factors may work together to trigger the initial process, produce implantation, and cause subsequent reseeding and spreading of the implants.

Theories of the cause of endometriosis include:

Retrograde Menstruation. Retrograde menstruation occurs during a woman's period, when menstrual tissue flows backward through the fallopian tubes rather than out through the vagina. A theory is that, in some cases, the redistributed uterine tissue attached and grew in areas outside the uterus, forming endometriosis implants. This theory does not fully explain endometriosis, however. Many women have some retrograde menstruation, but not all of them develop endometrial cysts. Consequently, other factors must explain why uterine tissue becomes implanted and grows in areas outside the uterus.

Impaired Immune System. Another theory is that women who develop endometriosis have an impaired immune system that fails to identify and destroy endometrial tissue that grows outside of the uterus. Some researchers theorize that endometriosis represents an autoimmune condition, in which the immune system launches an attack on its own cells and tissue. There appears to be a relatively high incidence of other inflammatory autoimmune disorders (such as multiple sclerosis, rheumatoid arthritis, and lupus) among women with endometriosis. It is unclear, however, how this response relates to endometriosis itself and whether endometriosis should be treated as an autoimmune condition.

Inflammatory Response. The damage, infertility, and pain produced by endometriosis may be due to an overactive response by the immune system to the early presence of endometrial implants. The body, perceiving the implants as hostile, launches an attack. Levels of large white blood cells called macrophages are elevated in endometriosis. Macrophages produce very potent factors, which include cytokines (particularly those known as interleukins) and prostaglandins. Such factors are known to produce inflammation and damage in tissues and cells.