During a period most of the menstrual blood comes out through the vagina. In women some blood also passes backward through the fallopian tubes into the abdominal cavity. Contained within the menstrual blood are fragments of endometrium, and these can seed and grow in the peritoneal cavity. We do not know why these cells seed in some women and not in others. This theory is called retrograde menstruation.
Early implants look like spots or pimples sprinkled on the pelvic surfaces.
The implants may remain unchanged, become scar tissue, or disappear over a period of months. In most women endometriosis grows slowly and can remain stable for years. Endometriosis can form benign cysts on the ovaries called endometriomas. These may be smaller than a pea, or larger than a grapefruit. A woman may suddenly feel pain when a large endometrioma bleeds or bursts.