Blastocyst culture

The term blastocyst describes a certain stage of embryo development, after 5-6 days of culture.

Theoretically, growth to the blastocyst stage is an indication of normal embryo development. In addition, it has been proven that embryos reaching the stage of hatching blastocyst have approximately a two-fold increase of implantation chances. The fully grown blastocyst is characterised by expansion, thinning of the zona pellucida and the accumulation of fluid among the cells. At this stage, the embryo contains 60-120 cells that form two distinct groups. The outer cell mass (trophoblast) that will give rise to the placenta, and the inner cell mass that will form the embryo.

The next stage of development is the hatching of the blastocyst from the zona pellucida. This is achieved through pressure on the zona from the expanding embryo and secretion of zona-digesting enzymes. This results in the rupture of the zona and the hatching of the blastocyst for the resulting opening. It is believed that hatching may be a mechanism of twinning; if the embryo is separated into two during hatching it is then possible for them to regenerate the missing cells and to develop as two different organisms (monozygotic twins) that share the same gender and genetic material.

The process of blastocyst transfer (5th or 6th day following egg collection) is similar to the embryo transfer on day 2 or 3, with the only difference being the number of the transferred embryos. Up to 3 embryos are transferred on day 2 and 3, while only one or two blastocyst are selected transfer.

Why blastocyst transfer is not recommended as a routine method

Only a few embryos reach the blastocyst stage under culture conditions. According to international studies, this percentage is estimated to be around 20-40% of fertilised eggs. This may be due to either the varying developmental potential of the embryos, or to suboptimal culture conditions that fail to fully support the increased metabolic requirements of a 60-120 cell embryo (blastocyst), while sufficiently covering the needs of 2-8 cell embryos. Therefore, we cannot identify with certainty the reasons why an embryo has not reached the blastocyst stage, as these may be due to reduced developmental competence or suboptimal culture conditions. This is why the application of blastocyst transfer has a limited scope in routine ART programs.

At Eugonia, we run a successful program of blastocyst culture and transfer, following certain indications, according to international criteria. The scientific team of Eugonia has the necessary knowledge and experience to inform you and suggest possible ways forward.