Unfortunately, there is no known cause for endometriosis. Several possible theories exist including: 

Retrograde menstruation: during menstruation, some of the endometrium lining (menstrual blood containing endometrial cells) escapes from the uterus via the fallopian tubes and enters the pelvic cavity. These cells then stick to surfaces of the organs in the pelvis eg bowels, ovaries, fallopian tubes, liver, even the lungs. Each time the uterine lining thickens in response to changing hormones in your menstrual cycle, these cells also grow and bleed. 

Immune system disorders: Another theory states that all women have some endometrial tissue outside of their uterus but mostly their immune system keeps it under check. However, in women with endometriosis their immune systems do not manage to control it.

Many women who have endometriosis also are more likely to suffer from other conditions like auto immune disease, rheumatism, fibromyalgia, etc.

Transformation of peritoneal cells. Some experts suggest that hormones or immune factors promote transformation of peritoneal cells — cells that line the inner side of your abdomen — into endometrial-like cells. 

  • Embryonic cell transformation. Hormones such as estrogen may transform embryonic cells — cells in the earliest stages of development — into endometrial-like cell implants during puberty.
  • Surgical scar implantation. Endometrial cells may attach to a surgical
    incision following a surgery, such as a hysterectomy or C-section causing
    endometriosis lesions.
  • Endometrial cell transport. Your body is constantly transporting blood and fluids around your body via the circulatory system and lymphatic system. One theory suggests  endometrial cells may be transported to other parts of the body via the blood or lymph.