Implantation by retrograde
menstruation - This is the most widely accepted theory which was postulated by Sampson. This theory suggests that the menstrual blood flows backwards through the fallopian tubes during menstruation and the endometrial cells get implanted into the peritoneal cavity.
Coelomic metaplasia (by Meyer) - This theory suggests that the potent cells in the abdomen and pelvis are transformed into endometrial tissue due to a variety of factors like inflammation, hormones and infection.
Dissemination through lymphatics and blood vessels (by Halban) – suggests that endometrial cells enter the blood vessels and lymphatics and are carried to different sites.
Genetic predisposition - It has been shown that endometriosis has a familial tendency, as the first-degree relatives of women with this disease are more likely to develop it as well.
Immunological theory - suggests that endometriosis is an autoimmune disease and is due to a decreased immunity.