How Embryos Make the Grade
Patients are often awestruck when they see a photo of their embryos for the first time on the morning of their transfer. That morning each embryo is carefully observed under a powerful microscope and the information about the quality of the individual embryos is given to patients prior to their transfer. There are three pieces of evaluative information patients receive about each of their embryos: the number of pro-nuclei, the number of cells present and the grade of the embryo. Using this information, decisions are then made about how many embryos to transfer and how many to freeze, and importantly, what to do with embryos that are not developing very well.
For the most part, the process of gauging the quality of an embryo involves a lot more than what meets the eye. Embryo evaluation, a standard practice in fertility clinics worldwide, allows embryologists to provide you and your physician with valuable information about your embryos, and about your fertility.
When eggs are retrieved from a patient’s ovaries, they are surrounded by thousands of helper cells that prevent us from seeing eggs directly or making any comments about quality. For patients having ICSI (Intra-Cytoplasmic Sperm Injection), we strip away the helper cells, but even then, very little information on quality can be ascertained. Only when eggs are of particularly poor quality are we able to observe obvious differences from healthy eggs. The vast majority of eggs do not show any characteristics to indicate quality. Therefore, the embryologist will not typically convey any information about egg quality.
For most patients, about 70% of their eggs will fertilize, regardless of egg quality (see Fertility Flash Vol 3 Issue 3 for more details). And fertilization is not necessarily an indication of things to come. A high rate of fertilization does not suggest better embryo quality.
The first step in embryo evaluation is examining fertilized eggs for the presence of two pro-nuclei (PN) in their centers. Each of these pro-nuclei contain the DNA from one parent, and in 95% of fertilized eggs they are unremarkable. In a small number of fertilized eggs however, we do see nuclear abnormalities such as asymmetry of size, any number other than 2, or failure to align in the egg center. These unusual abnormalities do suggest abnormal embryos, and we make patients aware of these. Each embryo will be noted for its number of pro-nuclei, e.g. 2PN or 3PN.
All of the fertilized eggs or embryos are kept in the laboratory for a minimum of 48 hours prior to the embryo transfer procedure. Only eggs that are grossly abnormal, such as those fertilized by more than one sperm, are discarded. Even eggs in which we see no evidence of fertilization are kept in case they fertilize late. No further observations are made until close to the time of transfer as the embryos must be left undisturbed in the laboratory incubator.
While incubating, the fertilized egg begins to divide. This first round of cell division happens within 12 hours of fertilization and thereafter, the cells in the embryo divide in two about every 16 hours. This continuous process of cell division is a very important indicator of embryo health as the embryos have only 4 to 5 days to make enough cells for implantation in the uterus. When we look at the embryos on their third day of life, we expect to see about 8 cells. Many embryos will have close to this number, and some will even have more, but embryos that are significantly delayed with 4 cells or fewer will have very little chance of establishing a pregnancy. Counting the number of cells in an embryo is the most important part of assessing their quality. We will give patients this cell count for each of their embryos.
A much less important aspect of determining quality involves observing the integrity of the cells. Some embryos will have uneven or asymmetrical cells and some will have one or more cells that are disintegrating. Cellular fragments that result from this disintegration are only an indicator of quality when they are severe, or when most or all of the embryo has broken up. Fragmentation is a normal feature in embryos and only about 20% of embryos have no fragments at all. However, the absence of fragments does not guarantee pregnancy, as there are many other factors involved in embryo quality.
This degree of fragmentation and cell asymmetry is given as a grade, usually 1, 2 or 3. Grade 1 embryos look beautiful and normal in every way. Grade 2 embryos will have a small degree of fragmentation and or unevenness, but are still considered high quality. Only if an embryo is in real trouble and has more fragments than cells, will we assign the dreaded Grade 3. These embryos very rarely implant after transfer and are not considered viable enough to freeze regardless of how many cells they contain.
While cell number is a very important predictor of embryo quality, grade is mostly most useful in deciding which embryos to transfer or freeze. Grade allows us to rank embryos when there are several embryos with the same cell number, and grade is only loosely associated with quality. Embryos that are given a poor grade are unlikely to implant after transfer, but other factors are much more important in establishing a pregnancy. A patient’s age for example is the best predictor of pregnancy, regardless of embryo grade. Also there is no correlation between embryo grade and genetic status. Genetically abnormal embryos are just as likely to be Grade 1 as genetically normal embryos. Moreover, a pregnancy from a Grade 3 embryo has no more increased risk of birth defects than a pregnancy from a Grade 1 embryo.
Joe Conaghan, PhD, HCLD and Carolyn Givens, MD,
Fertility Flash (SM), April 2005, Vol. 3, Issue 4
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