In infertile patients with endometriosis, what is causing the infertility?

Multiple Factors: Endometriosis can cause scarring of the tubes or even blockage, inflammation of the uterus and ovaries, can affect the egg production of the ovaries, and can affect the uterine lining. Sometimes one source can be easily seen (usually the tubes, even fertilization and egg quality during ivf) but often times the exact cause in an individual can not be detected.

It's not clear: In early stage endometriosis , it is controversial as to whether it actually causes the infertility. In later stage disease, there may be a mechanical blockage. The current theories include inflammation, fertilization failure and abnormalities in the uterine lining.