The exact cause of endometriosis is not known. Possible causes include the following:
- Your immune system may not be getting rid of endometrial cells outside of the uterus like it should.
- Heavy bleeding or an abnormal structure of the uterus, cervix, or vagina causes too many endometrial cells to go up through the fallopian tubes and then into the belly. (This is called retrograde menstruation).
- Blood or lymph fluid may carry endometrial cells to other parts of the body. Or the cells may be moved during a surgery, such as an episiotomy or a cesarean delivery.
- Cells in the belly and pelvis may change into endometrial cells.
- Endometrial cells may have formed outside the uterus when you were a fetus.
- It may be passed down through families.