Blastocyst is a structure formed by human embryos on the 5th-6th day following IVF. It is made up of a large number (>70) of cells and characterised by a liquid-filled cavity in the centre, known as blastocoele or blastocystic cavity.

The improvement of culture media has enabled in vitro embryo culture to the blastocyst stage. Only a portion of fertilized eggs, often less than half, reach this stage. Therefore, the number of blastocysts available for embryo transfer or cryopreservation may be considerably lower than the initial number of embryos. In few cases, no embryo survives to the blastocyst stage. In such cases, no embryo transfer is performed.

It is noted that culture at the blastocyst stage does not improve the quality of embryos, but allows for a more objective assessment of embryos. This means that a good quality blastocyst has twice as many chances of successful implantation as a good quality embryo on Day 2 or 3. As a result it is possible to transfer less embryos achieving the same success rates and avoiding multiple pregnancies.

In BioDimioiurgia, 25% of the embryo transfers over the period 2008-2010 occurred at the blastocyst stage. Compared to embryo transfers on the 2nd and 3rd day, the biochemical pregnancy rate has been 12.5% higher. In conclusion, embryo culture to the blastocyst state may enhance the success rate in certain cases when there are reasons indicating the use of this method.