In preparation for ovulation, the lining of the uterus, or endometrium, thickens. If fertilization does not occur, the uterus sheds its lining through the vagina. This is known as menstruation.
In endometriosis, the endometrium grows outside of the uterus. One cause of this growth may be retrograde menstruation.
During retrograde menstruation, menstrual fluid flows backward into the fallopian tubes instead of leaving the body through the vagina. Because the fallopian tubes are open-ended, menstrual backflow can spill into the pelvic cavity.
Backflow of menstrual fluid may promote the transfer of clumped endometrial cells to other tissues in the pelvis. Or menstrual fluid in the pelvic cavity could transform parts of those tissues into endometrial cells.
In either process, clumps of endometrium may start to grow on the tissues lining your pelvic cavity, as well as on your fallopian tubes, ovaries and large intestine. This growth continues to act as it normally would during a menstrual cycle. It thickens, breaks down and bleeds each month.
Because there's nowhere for the blood from this displaced tissue to exit your body, it becomes trapped. Trapped blood may lead to cysts, scar tissue and adhesions, abnormal tissue that binds organs together.
Scarring from endometriosis can block your fallopian tubes. Blocked tubes may keep sperm cells from reaching and fertilizing the egg, causing fertility problems. The menstrual fluid could alter some types of cells in the pelvic cavity to change their structure, or clumps of endometrial tissue contained in the menstrual fluid could stick to the tissues it lands on and start to grow. In either case, this may be a factor in causing endometriosis.