Endometriosis is defined as the presence of tissue identical to the endometrium outside of the uterine cavity. The tissue in endometriosis is usually found close to the uterus on the ovaries or Fallopian tubes, and more rarely on organs such as the bladder or bowel. No-one currently knows why these tissues leave their original site and become implanted elsewhere.
Amongst the various hypotheses proposed, the most widely accepted is migration of fragments of endometrium released during the time of a menstrual bleed that migrate to the abdominal cavity through the Fallopian tubes. However, 90% of women have reflux during menstruation and do not all develop endometriosis.
Another possibility is that the endometrial tissue is deposited outside the uterus well before birth.
Finally, it is possible that these fragments of tissues have "travelled" along the main means of transport in our body, i.e. in venous blood or lymph.