Abstract
Endometriosis, affecting 6–10% of reproductive age women, is more commonly found on the left side of the pelvis. This laterality phenomenon’s etiology remains unclear.We propose four different theories explaining the left laterality of endometriosis. The anatomical theory: based on the structural asymmetry of the pelvis. Organs surrounding the left adnexa serve as mechanical barriers and interfere with clearance of endometrial cells migrating to the pelvis, promoting in turn endometriotic implants. The Physiological theory: focuses on the role of the peritoneal fluid current, shown to be slower and weaker on the left side, allowing more time for adherence of endometrial fragments or cells. The hormonal theory rests on the right ovulatory predominance that may impose progesterone richer milieu on the right hemipelvis which in turn inhibits endometriosis on that side. The Vascular theory: shows that higher hydrostatic pressure in the left venous system, secondary to differences in anatomy, may favor local stasis and implantation of endometriosis.Although each theory can explain the phenomenon independently, they are likely to be inter-related. Whether the asymmetrical distribution of endometriosis is paralleled by asymmetric clinical manifestation is yet to be known. Further studies are required in order to elucidate our understanding of this common disease and its features.