The endometrium versus embryonic quality in endometriosis-related infertility

In spite of a great deal of effort over many decades, the mechanisms that lead to infertility in women with endometriosis remain unknown. Moreover, controversial results in the literature add even more difficulties in the understanding of this issue. Since the introduction of IVF, we have had the opportunity to study the effects of endometriosis at specific stages of the reproductive process including folliculogenesis, fertilization, embryo development and implantation. Depending on the research group, there are conflicting data on the results of IVF in patients with endometriosis. Some researchers found impaired implantation in patients with endometriosis, but such a defect in implantation may be caused by either defective embryos or altered endometrium. The observation of a higher rate of arrested embryos in patients with endometriosis compared with disease-free women, and that women with this pathological situation undergoing ovum donation had the same chance of implantation as patients without endometriosis, initially suggested that impaired oocyte/embryo quality may be responsible for reduced implantation. Further investigations on follicular components have also found differences between women with endometriosis and those without the disease. In addition, recent advances on implantation research, mainly on markers of endometrial receptivity, also show features in the eutopic endometrium of women with endometriosis that are not found in endometrium of women without the disease, although there is no agreement on this point. In this review, we will focus on infertility-related endometriosis based on our own research and the available literature.