What causes endometriosis?
In endometriosis, the kind of tissue that lines the womb (endometrial tissue) also grows outside the womb. It is not clear why this happens. There are various theories about what causes endometriosis. Several factors probably play a role.
The inside walls of the womb (uterus) are completely lined with mucous membranes known as the endometrium. These are different to other mucous membranes in the body, particularly in their ability to change: Every month, new endometrial cells grow and the tissue thickens in case a fertilized egg settles in it. The thickened tissue can then provide the egg with everything it needs to grow. If fertilization doesn't take place and the woman doesn't become pregnant, most of the thick membrane tissue which has built up is shed and leaves her body during her period. The process of building up and shedding the lining of the womb is regulated by the female sex hormones estrogen and progesterone.