Embryo grading becomes necessary when multiple embryos are produced during In Vitro Fertilization (IVF). Most IVF programs?are preceeded by?superovulation. It ensures that several embryos are available for implantation. Since most countries have a restriction on the number of embryos that can be transferred at a time, to avoid the risk complications during multiple pregnancies, the spare or supernumerary embryos are generally frozen for future use. Before the embryos are transferred for implantation or even considered for freezing, the embryos are assessed for the quality and their potential for implantation in the womb.
This process is carried out by specialized embryologist and is known as embryo grading. The process of embryo grading involves assigning grades to every embryo in order to identify and sort the best quality embryo to be selected for embryo transfer or embryo freezing (cyropreservation).
Factors of Embryo Grading
There are numerous embryo grading procedures or selection criteria that are followed to grade the quality of the embryos. The basic parameters on the basis of which early stage embryos are assessed and graded are as follows:
Number of Cells Present
Ideally, embryos must have 2 to 4 cells in the first 48 hours after the egg has been fertilized. This cell number should then grow to about 7 to 10 cells in 72 hours. At this stage, the cells are also known as blastomeres.
Even Cell Size
It is best if all the cells (blastomeres) of an embryo are similar in size. More often than not, if they are all not the same size, the cells should at least be close to the same size.
This refers to the speed at which the cells in the embryo divide and whether the division is even.
Degree of Fragmentation
Fragmentation is also known as blebbing. Certain parts of the blastomeres (embryo cells) are broken and are thus separated from the nucleated portion of the cell. Non-fragmented or least fragmented embryos are always preferred. Although fragmented embryos may lead to successful pregnancies, cells which display more than 25 percent fragmentation are not considered for potential implantation.
Number of Nuclei
If more than one nucleus is present in the blastomere, it is known as a multinucleated cell. A cell develops multi-nucleus after 2 or 3 days and usually after the third day it is difficult to ascertain multi-nucleation. Since majority of multinucleated embryos have displayed chromosomally abnormal in studies, they are not considered for ideal for implantation. In few cases, the implantation of multinucleated embryos has resulted in normal pregnancies, thus multinucleated embryos are transferred to the uterus only if that is the only embryo available.
Other aspects that affect the grading of embryos involve the appearance of the embryos under the microscope to check for the presence of granularity, thickness of the outer shell, vacuoles and other attributes.
Grades and What They Mean
- Grade A: Characterized by even and equal sized blastomeres and absence of cellular fragmentation
- Grade B: Few uneven and irregular blastomeres, whereas fragmentation of blastomeres is less than 10 percent
- Grade C: This grade of embryo is characterized by 25 percent fragmentation and blastomeres are granular (still viable)
- Grade D: Fragmentation is to the extent of 25 to 50 percent - blastomeres are highly granular
- Grade E: This grade of embryo is not considered viable.