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In the course of a female cycle, there is a (temporary) change in the lining of the uterus after ovulation under the influence of female hormones, which allows the embryo to implant itself.

This period of time is called the so-called "implantation window".


Some patients do not become pregnant despite multiple transfers of good-quality embryos (after in vitro fertilisation) and despite comprehensive diagnostics. There are studies which suggest that in some of these cases the above-mentioned implantation window may be "shifted".


The so-called receptivity (probability of implantation) can only be insufficiently assessed with the classical examinations such as ultrasound, hormone determination and histological examination after biopsy of the uterine mucosa. A determination of the genes responsible for receptivity seems to be a promising approach. For this examination, a small amount of endometrium must be obtained and examined (see also: Endometrial biopsy).

This endometrial-reception array (ERA) can be used to determine the optimal time for embryo transfer more precisely.

In order to be able to carry out the subsequent embryo transfer successfully under standardised conditions, both the ERA and the subsequent transfers are carried out analogous to the conditions of a cryo-cycle (build-up of the mucous membrane with oestrogens and conversion of the mucous membrane with progesterones, followed by a time-defined thawing and transfer of previously frozen fertilised eggs or embryos).

However, further studies are required to conclusively prove the effectiveness of this method. A scientific discourse is still ongoing.