This could occur before or after invagination, particularly of the ovary. The implantation theory is based on the principle that viable endometrium implants on the peritoneal surface. Therefore this theory requires three steps. First, retrograde menstruation has to occur. Secondly, retrograde menstruation should contain viable endometrial cells, and, thirdly, adhesion to the peritoneum has to occur with subsequent implantation and proliferation.
Menstruation is almost unique to human beings and a few other primates.
Menstrual effluent is composed of blood elements, endometrial cells and extracellular fluid. However, only in one of 8 cases they succeeded in finding endometriosis developing at the site of injection. The prerequisites for the implantation theory are discussed in further detail in the following section.