Implantation window and ERA test

An embryo must implant in the endometrium (the layer that lines the inside of the uterus) for a pregnancy to occur. However, there is a certain period of time in which the endometrium is more receptive for implantation to occur. This period is known as the implantation window and usually corresponds to days 19-21 of the menstrual cycle.

However, this implantation window can be displaced and, thus, the moment of maximum endometrial receptivity may not coincide with the usual moment for embryo transfer. This is precisely what the ERA test evaluates.