Embryos are visually graded according to number of cells, size and shape of cells and amount of fragmentation of the embryo. In Thailand day 1 to day 3 embryos are graded 4,3,2,1, with grade 4 being excellent quality, grade 3 good quality, grade 2 embryos are average quality and grade 4 embryos are poor quality. Grade 3 and 2 embryos can, and do, create pregnancies and healthy babies, but chances for implantation are lower than transfer of a grade 4 embryo.
Day 1 – Zygote - 18-24 hours after insemination – embryologist looks for two well-defined structures called pro-nuclei in the centre of the egg. Pro-nuclei contains the DNA from the male and female.
Days 2-3 – Cleavage - now in the cleavage stage of development (cleaved meaning divided) the zygote has divided into two cells. On day 2 the embryo is expected to have divided into 2-4 cells; on day 3 cleavage to 8 cells is expected, though there can be some variable; any embryo with 7-12 cells is considered to have good prognosis. An embryo with only 2 cells on day 3 has a poor prognosis. Progression of development should follow a predictable rate, embryos which are lagging behind usually don’t catch up.
Day 4 – Morula – compaction stage. On day 4 the loosely associated cells compact to a tighter group of around 32 cells called a morula. The cells of the morula secrete a liquid that causes a fluid filled central cavity (blastocoele) to form.
Day 5 – Blastocyst – On day 5 the embryo should have divided to around 80-100 indistinguishable cells, which strain against the shell (zona) of the embryo. The straining is caused by cell division and fluid being pumped by the cells into the cavity in the embryo, causing it to expand, this is called an expanded blastocyst. Before creation of this inner space the embryo is consider non-expanded. Expansion of the blastocyst helps thin the zona so that it ruptures and the embryo can hatch – when this has occurred the embryo is called a hatching blastocyst.
Grading of blastocysts
A blastocyst is graded with one number and two letters, for example 5AA. The number 1-6 refers to the degree of expansion of the cells within the embryo, with 6 being a hatched embryo. The first letter refers to the inner cell mass, specialized cells inside the embryo which will go on to form the fetus, and the second letter refers to the cells that line the zona and surround and protect the inner cell mass, called the trophectoderm. These cells will form the fetal part of the placenta. The inner cell mass and the trophectoderm are graded from A-C, A being best quality with a high number of cells, to C being loosely packed cells. More expansion is better than less, and more cells are better than fewer.
Grade 1: the fluid-filled cavity takes up less than 50% the space of the embryo.
Grade 2: the fluid-filled cavity takes up more than 50% of the space of the embryo.
Grade 3: the blastocyst cavity has expanded into the entire volume of the embryo, pressing the trophectoderm cells tightly against the inside of the zona.
Grade 4: Expanded blastocyst, where the blastocyst has increased beyond the original volume of the embryo and caused the zona pellucida to become super thin.
Grade 5: Embryo has breached the zona and is hatching out of its shell
Grade 6: Embryo is completely hatched.
Inner cell mass grading
A: Many cells, tightly packed
B: several cells, loosely packed
C: very few cells
A: many cells, forming a cohesive layer
B: Few cells, forming a loose layer
C: Very few large cells.
You can see a fascinating video of embryonic development over five days here: http://stemcells.nih.gov/info/pages/cellmovie.aspx