How does endometriosis affect infertility?

Endometriosis has been one of the most confusing gynecologic diseases since it was first described approximately a century ago. The rate of endometriosis in infertile women ranges from 4.5% to 33% [1]. The non-in vitro fertilization (IVF) clinical scenario confirmed the intricate relation between endometriosis and infertility by means of three approximations: (1) Prospective studies on the prevalence of endometriosis have shown that mild-to-moderate stages are more frequently found in infertile women and in women with pain or dysfunctional bleeding than in women who request tubal sterilization [2]. (2,) When women with endometriosis were treated randomly by surgery or managed expectantly, cumulative pregnancy rates significantly increased in treated patients [3], which demonstrated that mild-to-moderate lesions could interfere with fertility. (3) Artificial insemination with donor sperm and lack of other infertility factor endometriosis were factors against successful outcome [4]. Assisted reproductive technology has provided a diagnostic and therapeutic approach for endometriosis (Fig. 1). Some IVF-based basic and clinical research found alterations in all the steps of the normal reproductive physiology [5], including an altered embryo quality [6-8] and a higher in vitro embryo blockage [9]. Women with endometriosis display lower implantation capacity and further diminished pregnancy rates [5,10]; however, the pathophysiology by which endometriosis affects implantation is an unresolved medical question. We do not know much about the endometrium and the embryo intrinsic mechanisms in relation to the implantation process.