For selective embryo transfer, specialists rely on a series of criteria which help determining the quality of the embryos.
While embryos are being cultured, their quality is microscopically assessed. Embryo quality assessment implies analysing both embryo morphology and embryonic development.
The different sections of this article have been assembled into the following table of contents.
Understanding embryo grading
Cells or blastomeres are of equal size and no fragmentation is seen. Their texture is uniform and they are light-coloured. There is no evidence of multinuclear blastomeres (more than one nucleus per cell).
Minor fragmentation only (less than 15% of the embryo) and/or cells are mostly of equal size.
The fragmentation rate is 10-40% of the embryo. Blastomeres are of unequal size and/or their surface is less smooth.
The fragmentation rate is higher than 40%. Cells are now dark-coloured and less smooth.
Fragmentation is heavy and the embryo is dark-coloured.
As regards embryo morphology, the following aspects are taken into account: the zona pellucida thickness, the cytoplasmic vacuoles, the adhesion between blastomeres and the presence of a cytoplasmic filament.
If the zona pellucida is especially thick, embryo transfer can be performed previously to the assisted zona hatching (AZH) technique, a.k.a. assisted hatching (AH). AZH may facilitate the inner cell mass to break through the zona pellucida.
Embryonic development is another marker for embryo quality assessment.
This criterion can be summarised as follows: the greatest number of cells, the greater probability of implantation into the maternal uterus, always respecting the maximum of 4 cells two days after fertilisation and 8 cells for 3-day embryos.
Embryos with a cleavage faster than normal are considered as poor quality embryos, since they are more likely to develop a genetic alteration, usually aneuploidy.
Grade I and grade II embryos are considered good quality embryos.
Conversely, grade III embryos are considered as moderate quality embryos, although there are still chances for pregnancy. Embryo quality is related to embryonic development and embryo–endometrial linkage capacity. However, grade III embryos are not necessarily associated with a genetic alteration. Grade IV embryos present a poorer prognosis.
Grade V embryos are not transferred to the maternal uterus. They are cultured while their progress is monitored. Nevertheless, practically overall majority of grade V embryos do not continue its development. Their poor development may indicate that they are non-viable.
FAQs from users
What causes poor quality embryos?
This is a very common question but not an easy answer. First, the term “quality” is not well defined in reproductive medicine: is it based on appearance of the embryo?; is it the chromosomal competence of the embryo; or the structrual/genetic make-up? The first two metrics are definable – we can grade an embryo base on appearance; we can test the embryo for the chromosomal content. Neither of these qualifiers correlate well with the ability for the embryo to result in a live birth. Structural/genetic make-up is not able to be tested at this time.
Traditionally, we have considered a woman’s age as the dominant forcé contributing to egg “quality.” The percentage of chromosomally abnormal embryos increase as the woman ages. While male fertility declines with age, we do not have as much information on their contributions to embryo quality.
Can you pregnant with C and D scored embryos?
Yes, although the chances are quite low.
When we classify embryos according to their quality at the lab, we do so by evaluating their implantation potential, that is, trying to "guess" which ones have greater chances for attaching tot he uterus, and which don't. C and D scored embryos are embryos of moderate-to-low quality, which means that a C or D scored embryo has a reduced chance of implantation if compared to a B or A scored embryo. In any case, however, whenever we select an embryo for the transfer, it's because it has been observed that its implantation potential is a good one.
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