The endometrium is receptive when the embryo can adhere to the endometrial cells and subsequently enter their thickness. This process was classically said to occur between days 20 and 24 of a normal 28-day cycle.
In vitro fertilization treatments have helped determine when is the optimal time to perform the transfer, the so-called implantation window. In a natural cycle, if ovulation occurs on day 14, the transfer of an embryo between 4.5 and 7 days old must take place between days 18.5 and 21 of the cycle for implantation to occur. Although this is not the case for all women. Like most biological processes, endometrial receptivity is not a sudden process but a gradual one. The endometrium gradually transforms from non-receptive, pre-receptive, receptive, post-receptive, and non-receptive. In addition, it can be modified by endocrine, inflammatory, immunological factors, it can even happen that a certain woman has an implantation window that is normally offset from the standard.