Extended culture of embryos is called blastocyst culture and involves keeping embryos in the laboratory for an extra two days until Day Five. By this stage, it is hoped that a proportion (30-40%) of embryos will have developed into blastocysts.
Blastocysts are considered to be advanced embryos, as they have undergone significant changes between Day Three and Day Five in preparation for implantation. Leaving the embryos to develop onto Day Five enables the embryologists to select the most advanced embryos for transfer, thereby increasing the chance of transferring the embryos most likely to implant.
To be eligible for blastocyst culture, patients need to have a good number of embryos (ideally more than four) that are at the right stage of development on Day Three. Once these criteria have been met, extended culture may be offered as a possible option.