The transfer of blastocysts on Day 5 or 6 has the advantage of allowing better synchrony between the embryo and endometrium, as well as eliminating embryos that cannot develop after activation of the zygote genome due to genetic or metabolic defects.
Following the development of new generation sequential embryo culture media, single blastocyst transfer has been promoted as a means of improving the success of IVF, while at the same time reducing multiple birth rates. An important prerequisite for culturing blastocyst is the optimal environment for IVF laboratory cultivation. In extended culture there is no advantage unless satisfactory implantation rate is not achieved after culturing up to Day 2 or 3.
Published evidence indicates that a policy of careful embryo evaluation at cleavage stages can lead to success rates after Day 3 transfer that broadly match the success rates achieved with blastocyst transfer. Concern has also been expressed about the safety of prolonged culture to the blastocyst stage and the risk of potential aberrant epigenetic programming during extended in-vitro development. In the past, the poor survival rate for supernumerary blastocysts after slow freezing was a disadvantage of blastocyst culture, but blastocyst vitrification and its associated high survival rate has made single blastocyst transfer a more viable option; this also facilitates a policy for elective single embryo transfer (eSET).
The ability to identify healthy viable blastocysts is an important factor in the success of blastocyst transfer, and a grading system for their assessment takes into consideration the degree of expansion, hatching status, the development of the inner cell mass (ICM) and the development of the trophectoderm. Careful assessment of all morphological parameters available will optimize the chance of achieving satisfactory implantation rates after blastocyst transfer.
Potential Benefits of Blastocyst Transfer
True embryo viability can be assessed, post-embryonic genome activation,
Embryos with limited developmental potential are eliminated,
Embryonic stage is synchronized with the uterus, reducing cellular stress on the embryo,
Exposure of the embryo to a hyper-stimulated uterine environment is minimized,
Possibility of uterine contractions is reduced, minimizing the chance of embryo expulsion,
High implantation rate; reduces the need to transfer multiple embryos,
Single embryo transfer reduces the rate of multiple gestation.
Blastocysts are initially graded on the basis of volume/expansion (this can be performed under a dissecting microscope):
Early blastocyst: The blastocoel occupies less than half the volume of the embryo,
Blastocyst: The blastocoel occupies half the volume of the embryo or more,
Full blastocyst: The blastocoel completely fills the embryo, but the zona has not thinned,
Expanded blastocyst: The volume of the blastocoel is larger than that of the ICM, and the zona is thinning,
Hatching blastocyst: The trophectoderm has started to herniate through the zona,
Hatched blastocyst: The blastocyst has completely escaped from the zona.
Blastocyst transfer has also been used as a strategy or the treatment of patients who carry chromosomal translocations; chromosome translocations cause a delay in the cell cycle, and abnormal or slowly developing embryos are eliminated during in-vitro culture. Several normal pregnancies have been successfully established after transfer of healthy blastocysts in a group of patients carrying translocations.
After cleavage stage embryo transfer, remaining embryos of Grades 1 or 2 which show less than 20% fragmentation at the time of assessment may be cryopreserved on Day 2 or Day 3. Embryos of suboptimal morphology at this time can be further cultured until Day 6, with daily assessment. Those that develop to blastocysts on Days 5 or 6 can also be cryopreserved.