Embryonic quality for embryo transfer or freezing

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One of the stages of in vitro fertilization treatments (IVF) is the evaluation of embryo quality in the laboratory. Not all the embryos generated are apt to transfer or vitrify, since their viability may be compromised. Therefore, it is necessary to follow morphological criteria that help specialists in the choice of embryos of optimum quality.

The main purpose of the embryo selection is to identify the embryo with the greatest possible implantation capacity to be transferred to the uterus of the future mother.

Below you have an index with all the points that we will discuss in this article.

Index

Embryonic quality for embryo transfer or freezing

Parameters of embryonic quality

To determine which are the embryos of good quality during their development in culture, the parameters set by ASEBIR (Association for the Study of the Biology of Reproduction) are taken into account. These are the following:

  • Number of cells or blastomeres and division rate
  • Percentage and type of cellular fragmentation
  • Symmetry: size of blastomeres depending on the stage
  • Display of cores and multinucleation (more than one nucleus per cell)
  • Presence of cytoplasmic halo or vacuoles in the cytoplasm
  • Pellucid zone (ZP)
  • Degree of compaction

These parameters are routinely checked visually under the microscope on the second and third days of embryonic development (DAY 2 and DAY 3, respectively).

According to the embryologist Edurne Martínez of the Reproducción Bilbao clinic, she says:

There are some criteria that help us to select embryos. For the most part, these criteria are morphokinetic, which means that we have to verify that they are dividing at the moment they have to be divided and in the number of cells that have to be divided.



Classification of embryos

Embryologists classify embryos in different grades or categories according to the parameters observed, so that grade A is the highest quality and grade D is the worst quality.

Grade A

It corresponds to an embryo of excellent quality with maximum implantation capacity. Its morphological characteristics are the following:

  • Number of cells: 4 on DAY 2 or 7-8 on DAY 3
  • There is symmetry: blastomeres of equal size.
  • No fragmentation or less than 10%
  • There are no multinucleated blastomeres
  • Smooth texture and light color
  • Normal zona pellucida

Early compaction on DAY 3 is also considered a good prognostic factor, although there are specialists who prefer not to consider it for classification.

Embryonic quality: grade A

Grade B

The embryo remains of good quality with implantation capacity. Its morphological characteristics are the following:

  • Number of cells: 4-5 on DAY 2 or 7-10 on DAY 3
  • Slight asymmetry between blastomeres
  • Fragmentation between 10% and 25%
  • There are no multinucleated blastomeres
  • Small vacuoles in the middle of blastomeres
  • Abnormal zona pellucida

The alterations in the ZP can be several, such as pigmented or thickened ZP. In this case, the possibility of performing an assisted hatching before the embryo transfer is considered. You can know what this technique consists of in the Assited Hatching link.

Embryonic quality: grade B

Grade C

The embryo is considered regular, of intermediate quality, with a lower implantation probability. Its morphological characteristics are the following:

  • Number of cells: 2 or 6 blastomeres in DAY 2, or 6 or 12 blastomeres in DAY 3
  • Asymmetric blastomeres
  • Fragmentation between 25% and 35%
  • 1 or 2 multinucleated blastomeres
  • Vacuolas large in the middle of blastomeres
  • Rugged appearance
  • Abnormal zona pellucida

It should be noted that the category assignment to the embryo is based on its most negative characteristic. Therefore, if the embryo has multinucleated cells although it has good morphological parameters, it is assigned a grade C.

Embryonic quality: grade C

Grade D

The embryo is of poor quality with one with low implantation probability. Its morphological characteristics are the following:

  • Number of cells: 3, 6 or more blastomeres in DAY 2, or 3-5 blastomeres in DAY 3
  • Very asymmetrical blastomeres
  • More than 35% fragmentation
  • Several multinucleated blastomeres
  • Vacuoles in more than half of blastomeres
  • Large cytoplasmic alterations: dark color and rough appearance
  • Abnormal zona pellucida

Within this category there are several anomalies that make the probability of implantation very low. On the other hand, embryos with a faster or slower division than normal also do not have a good prognosis. They usually have a higher risk of genetic anomalies, such as aneuploidies, so they are not usually transferred or frozen.

Embryonic quality: grade D

Questions from users

What quality do frozen embryos usually have?

The quality of the embryos is the same at the time of freezing that once thawed, that is, they have the same morphological characteristics. What can happen is that, by continuing with their development after a few hours, they change. For example, compaction, multinucleation, fragmentation, etc. may occur, so the embryo should be assigned another category.

What is the quality of the embryos after an ICSI?

In general, the quality of the embryos is the same after an IVF or an ICSI. This depends more on the quality of the ovule than on the technique used for in vitro fertilization.

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