Embryo grading is an attempt to evaluate abnormalities of individual embryos as they relate to the potential to establish a pregnancy after transfer. They are graded 1 though 4, with Grade 1 being normal, and Grade 4 severely damaged or degenerated.
Abnormalities in the distribution of fluid are common and leaks involving the capsule are considered more important than those from the blastocoele.
Unfortunately, grading can only evaluate normal versus abnormal morphology and can in no way evaluate the health of the visible cells. An embryo can have a large number of dead cells and still structurally appear normal for some time until growth or deterioration take place.
Cells that have been dead or stressed for some time will appear darker, and are often extruded from the embryo. These are generally visible in the early blasocyst or morula beneath the zona (extruded blastomeres-seen in the upper left corner).
Grade 1 embryos have few if any detectable abnormalities and are expected to have the highest potential for a successful pregnancy.
Grade 2 embryos have mild abnormalities that are not expected to negatively impact establishment of a pregnancy, such as in this embryo where fluid has leaked from the blastocoele but is still contained within the capsule
Grade 3 embryos have significant abnormalities that are likely to result in a decreased chance for pregnancy.
Grade 4 embryos have such significant abnormalities that establishment of a pregnancy is unlikely.