Noyes dating endometrium
This interaction involves the embryo, with its inherent molecular programme of cell growth and differentiation, and the temporal differentiation of endometrial cells to attain uterine receptivity.
Implantation itself is governed by an array of endocrine, paracrine and autocrine modulators, of embryonic and maternal origin.
Once the blastocyst is oriented correctly (apposition), the zona pellucida is shed.
The blastocyst then comes into contact with the epithelial layer and adheres to the endometrial surface (adhesion).
Continuing refinements to ART protocols, such as optimizing ovarian stimulation regimens, the timing of human chorionic gonadotrophin injection, or the timing of embryo transfer, should help to increase implantation rates further. Tools are now available that enable the selection of high-quality embryos or assessment of endometrial status.
This strategy may be particularly relevant in the field of implantation because numerous factors are involved, many of these have multiple functions, and there is potentially a large amount of redundancy.
The endometrium becomes receptive to blastocyst implantation 6–8 days after ovulation and remains receptive for ∼4 days (cycle days 20–24) ( Bergh and Navot, 1992 ).
The importance of endometrial environment is highlighted by the observation that high-quality embryos transferred into women involved as embryo recipients in a surrogacy procedure have a higher probability of implanting than if they are transferred back into the donor women ( Check , 2005 ).
More is known about the fate of the embryo post-implantation.
Using markers of early pregnancy, such as human chorionic gonadotrophin (h CG), it has been demonstrated that one-third of post-implantation early pregnancy losses occur during the pre-clinical stages of pregnancy in fertile women ( Wilcox , 1999a ).