The exact cause of endometriosis is not known. Possible explanations include the following:

• The immune system normally destroys any endometrial cells outside of the uterus. But women with endometriosis may have a problem with the immune system that may impair this process.¹
• Menstrual bleeding (which contains endometrial cells) is carried up through the fallopian tubes into the abdomen (retrograde menstruation). This happens with most women. But it may be worse if you have heavy menstrual bleeding or were born with an abnormal structure of the uterus, cervix, or vagina that blocks or slows menstrual flow.²
• Endometrial cells may be carried to other locations in the body by the blood or lymph fluid circulation.
• Endometrial cells may be moved to another area during surgery, such as an episiotomy after childbirth or a cesarean delivery.
• Cells in the abdomen and pelvis, which are closely related to the cells of the reproductive system, may change into endometrial cells.
• Endometrial cells may be deposited outside the uterus before birth.
• Sometimes, the tendency to develop endometriosis is passed down through families (genetic cause).²