Endometrial Receptivity in Natural and Controlled Ovarian-Stimulated Cycles

Abstract

The development of endometrial receptivity is a prerequisite for successful embryonic implantation in mammals. The receptive status is reached only during a short period of time in the midluteal phase, this being maximal 7 days after the endogenous peak of LH (LH+7), named as the window of implantation (WOI). At this time, the endometrial epithelium acquires the functional ability to support blastocyst adhesion. In ART, controlled ovar-ian stimulation (COS) induces lower implantation rates per embryo transferred than natural or ovum donation cycles, suggesting suboptimal endometrial development due to the abnormal endocrine/paracrine milieu. Researchers have investigated the functional genomics of endometrial receptivity in natural cycles and the impact of COS on the gene expression pattern of the endometrium, even with the use of different drugs such as gonadotrophin-releasing hormone (GnRH) agonists or antagonists. This paper reviews results obtained in different studies to elucidate the changes induced by the different clinical protocols with the objective to introduce a new molecular objective tool to analyze the endometrial receptivity status in natural cycles and to understand the impact of COS in the human endometrium.