The Paradox of Endometriosis in Mayer-Rokitansky-Kuster-Hauser Syndrome: Applying Three Criteria to Discriminate Between Retrograde Menstruation/Implantation and Coelomic Metaplasia/Embryonic Cell Rests Theories

<b>Background/Objectives:</b> The scientific community is still divided between supporters of the implantation theory and researchers who advocate the theory of coelomic metaplasia/embryonic cell remnants to explain the initiation of endometriosis. A frequently cited argument in favor of the coelomic metaplasia/embryonic cell remnants theory is the occurrence of endometriosis in the Mayer-Rokitansky-Kuster-Hauser syndrome, since retrograde menstruation is not possible without endometrium. However, nearly all women with uterovaginal agenesis have uterine remnants that harbour islets of endometrium. <b>Methods:</b> To verify the validity of the coelomic metaplasia/embryonic cell rests theory, we analysed all reports of endometriosis in patients with Mayer-Rokitansky-Kuster-Hauser syndrome without endometrium, published between 1980 and 2025. Three criteria had to be met in order to clearly demonstrate the absence of endometrium and the presence of endometriosis: (i) preoperative imaging, (ii) surgical visualization, and (iii) histological examination. <b>Results:</b> None of the nine reports fully met all three criteria, and the presence of endometrium could never be ruled out. In addition, we used ten characteristics to assess the 'goodness' of a theory: testability, logical coherence, conceptual clarity and comprehensibility, external consistency, empirical validity, predictive power, parsimony, broad applicability, practical utility, and heuristic value. <b>Conclusions:</b> Overall, the implantation theory appears to fully satisfy all criteria to explain the onset of endometriosis in Mayer-Rokitansky-Kuster-Hauser syndrome. In contrast, the coelomic metaplasia/embryonic cell rests theory satisfies eight criteria only partly and does not satisfy two of them. Therefore, the null hypothesis that endometriosis can be present in the absence of endometrium in patients with utero-vaginal agenesis can be reasonably rejected.