Endometriosis is an embryological condition, like a birth "spot" of endometrium, which is the inner layer of the uterus, outside the uterus, mainly amidst the collagen or connective tissue of the pelvis. Each time the woman menstruates, she will have a hematoma in the endometriosis. This determines a very strong local and systemic immune reaction. Over the time of her reproductive life the endometriosis will be transformed into fibrosis, which leads to retraction of the pelvic nerves and consequent chronification of pain. It is also the leading cause of infertility in women. Developments in the study of genetics and embryology have unveiled the origin of endometriosis.