In most laboratories, embryo transfer traditionally takes place on day 2 to 3 of culture, or during blastocyst stage, whilst embryo transfer on day 4 is an alternative that has not commonly been adopted into laboratory practice.
When compared with embryo transfer during early stages, blastocyst transfer is generally accepted as superior. This is mainly for reasons of improved synchronisation between the embryo and the endometrium and more objective embryo selection once the genome has been activated. It means that a smaller number of embryos can be transferred, thus avoiding the risk of a multiple pregnancy.
However, transfer is not always possible on day 5 and this is mainly due to objections raised by patients (work commitments, travel plans for returning home or other matters). In cases such as these, embryo transfer on day 4 could prove to be an appropriate strategy that allows for greater flexibility when programming transfer.
Existing publications and in-house data endorse the validity of transfer on day 4 and indicate that it allows for adequate embryo selection without a negative impact on pregnancy rates when compared with blastocyst transfer on day 5.