The endometrium is receptive when the embryo can adhere to the endometrial cells and then be introduced into their thickness. Classically, this process was said to occur between days 20 and 24 of a normal 28-day cycle.
In vitro fertilization treatments have helped to determine when the optimal time to perform the transfer is, the so-called implantation window. In a natural cycle, if ovulation occurs on day 14, the transfer of an embryo of between 4.5 and 7 days of age has to 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 an abrupt but a gradual process. The endometrium is gradually transformed from non-receptive to pre-receptive, receptive, post-receptive and non-receptive. Furthermore, it can be modified by endocrine, inflammatory, immunological factors, and it can even happen that a certain woman has an implantation window that is normally displaced with respect to the standard. If we are talking about in vitro fertilization treatments, this means that, in some cases, it is necessary to study the endometrium in greater depth before carrying out an embryo transfer to determine, in that particular patient, what is the optimum moment to consider her endometrium to be receptive.