A blastocyst is a highly differentiated, highly developed embryo that has grown to the point where it is ready to attach to the uterine wall (implantation). Blastocyst transfer is claimed to be more physiological than pronucleate or cleaved-embryo since it mimics nature more closely. As the embryo advances in the development, after 5-6 days it becomes a blastocyst. The blastocyst has an outer thin layer of cells which will later form the placenta, and an inner cell mass which will develop into the fetus. A blastocyst has about 120 cells.

The usual embryo development is:
A single cell embryo is called a 2PN zygote (for 2 pronuclei). The 2PN zygote divides into an embryo (2-4 cells), the "morula" approx. 16 - 32 cells) and finally a "blastocyst" embryo. The embryo spontaneously "hatches" from its shell (zona pellucida) at the blastocyst stage of development and is ready to attach to the uterine wall.

Blastocyst culture is not a new technology. Indeed Cohen et al. reported, in the Lancet in 1985,  a birth following the transfer of replacement of hatching blastocyst cryopreserved at expanded blastocyst stage. However, with the use of a single medium formulation, with or without coculutre, blastocyst transfer did not significantly increase implantation rates compared to those obtained following embryo transfer on day 3, and has therefore not been embraced by the IVF community. In a paper published in Human Reproduction Journal Barnes and his colleagues introduce the sequential culture media (the embryos are cultured in different media according to their stage of growth) which led to increases in blastocysts implantation rates.  This new development represent a culture media that using the specific energy sources and amino acids with the specific intent of developing viable embryos. The development of this culture media does not necessarily resulted in higher numbers of blastocysts but allow the development of more viable blastocysts  as evidenced by the high implantation rates achieved

Day 4, early blastocyst Blastocyst Grade IAB

A high quality human blastocyst. The cells which will become the fetus are in the area marked as "ICM" (inner cell mass), the blastocoel cavity in the center is marked as "C", The trophectoderm cells that will form the placenta surround the cavity - one is marked with a "T"

There are several way to score the blastocysy.
Below can be find the one published by Michael J Tucker PhD FIBiol HCLD Georgia Reproductive Specialists, Atlanta, GA 30342
Published at: http://www.ivf.com/morphology.html

GOOD                        1. Fully expanded or hatching day-five
ADEQUATE                2. Fully expanded or hatching day-six
Moderate expansion day-five
MEDIOCRE                3. Moderate expansion day-six
Early cavitation day-five
POOR                        4. Early cavitation day-six
Morula day-five (?six)
Add to this two alphabetic scores to grade i.e.,
1st). Inner Cell Mass
2nd). Trophectoderm
GOOD                       A. high cell number with good cell/cell adhesion
MEDIOCRE               B. lower cell number with poorer cell/cell attachment
POOR                       C. no cells apparent (ICM) sparse granular (degenerate?) cells (trophectoderm)
For example, a good quality well-expanded blastocyst on day-six with good cell count in the ICM (>12 to 15), and good integrity of the trophectoderm would be scored as 2AA
A second example: a fully-expanded blastocyst on day-five with nice trophectoderm but non-existent ICM would be scored as 1CA
Another blastocyst scoring system was published by Gardner et al. Fertility and Sterility 73 1155-1158, and given below

What might be the advantage of using Blastocysts culture and transfer?
• The ability to identify those embryos that are well developed and choose one for transfer.
• More adequate synchronization of embryonic stage with the female endometrium.
• Uterine contractions are reduced by day 5 thereby reducing the chance of an embryo being expelled.
• The ability to undertake cleavage stage embryo biopsy which is easier and less harmful when the biopsy blastomere has to be taken.
• Increase implantation rate which need to transfer less number of embryos.
• Blastocyst stage embryos freeze as well, if not better, than cleavage stage embryos.
What might be the disadvantage of using Blastocysts for transfer?
• Patient may not have an embryo suitable for transfer.
• Increase rate of monozygotic twins.

The possibility to grow the embryos to the blastocyst stage should be offered to followings:

Patients with either a good response to gonadotropins  or with >4 eight-cell embryos on day 3.
If there is need for embryo biopsy at the cleavage stage for genetic analysis. The embryo can then be cultured to the blastocyst stage without compromising its viability while the genetic tests are performed.
Patients who were selected for one embryo transfer.
Patients who had repeatedly failed to achieve a pregnancy following the transfer of good quality cleaved embryos (If the embryo arrest and did not develop to blastocyst, this may indicate a potential problem).
Patient who do not wish to have their spare embryos frozen for whatever reasons may be advised to have blastocyst transfer.