Causes of Endometriosis

Endometriosis happens when tissue similar to the lining of the womb (the endometrium) grows in places outside it, such as the ovaries, the fallopian tubes, the lining of the pelvis, and sometimes the bowel or bladder. Like the womb lining, this tissue responds to hormones each month. It swells and bleeds, but the blood has nowhere to go, which causes inflammation, pain and scarring.

The exact cause is not known. Several explanations are being studied, and more than one may be involved:

• Retrograde menstruation. Some period blood flows backward through the fallopian tubes into the pelvis. This happens in many women, but only some develop endometriosis, so other factors must matter.
• Immune system differences. In some women the body does not clear these misplaced cells as it should.
• Hormones. Endometriosis depends on oestrogen, which is why it mainly affects women between puberty and menopause.
• Genes and family history. A mother or sister with endometriosis raises your own risk several times.
• Changed cells. Some cells in the pelvic lining may change into endometrial-like cells.
• Spread through blood or lymph, or growth at the site of a surgical scar, in rare cases.

Things that raise the chance

• Starting periods early or having short cycles and long, heavy periods.
• Never having been pregnant.
• A family history.