Retrograde menstruation. This is the most likely explanation for endometriosis.
In retrograde menstruation, menstrual blood containing endometrial cells flows back through the fallopian tubes and into the pelvic cavity instead of out of the body.
These displaced endometrial cells stick to the pelvic walls and surfaces of pelvic organs, where they grow and continue to thicken and bleed over the course of each menstrual cycle.
Embryonic cell growth.
The cells lining the abdominal and pelvic cavities come from embryonic cells.
When one or more small areas of the abdominal lining turn into endometrial tissue, endometriosis can develop.
Surgical scar implantation. After a surgery, such as a hysterectomy or C-section, endometrial cells may attach to a surgical incision. Endometrial cells transport.
The blood vessels or tissue fluid (lymphatic) system may transport endometrial cells to other parts of the body. Immune system disorder.
It’s possible that a problem with the immune system may make the body unable to recognize and destroy endometrial tissue that’s growing outside the uterus.