There are varying degrees of endometriosis and staging is based on severity. The endometrial implants within the pelvis respond to oestrogen and thicken, grow and rupture each month. The spilled contents cannot escape from the pelvis and so causes irritation, inflammation, scarring and adhesions. Each time larger implants or cysts rupture, more endometrial cells are spread and thus replication of the disease occurs.

Stage 1 endometriosis is said to exist when there are relatively few implants where as Stage 4 endometriosis is said to exist when there are extensive implants and severe adhesive disease. Adjacent organs are usually involved such as the bladder and bowel. In some cases the normal anatomy of the pelvis can be grossly distorted. (see photograph) This photograph illustrates Stage 4 endometriosis with extensive adhesive disease. Active endometriosis cannot be seen deep to the adhesions. 

N.B. It is important to remember that the staging of the disease does not necessarily correlate with the severity of symptoms. Some women with mild disease may experience debilitating symptoms, while women with more severe disease may suffer little effect.