How To Choose The Best Embryo? Characteristics & Techniques

By (embryologist), (embryologist) and (babygest staff).
Last Update: 12/31/2020

Before proceeding with a surrogacy treatment, it is necessary to carry out an assisted reproduction technique to generate the embryos. Specifically, the reproductive technique will be in vitro fertilization (IVF).

Generally, in an IVF process a high number of viable embryos are obtained, so it is necessary to classify them based on their quality and implantation potential. This embryo classification is done by evaluating different morphological aspects of the embryo. In particular, observation of the embryos on day 3 and day 5 of development is fundamental.

The selection of the best embryo for transfer to the woman's uterus is one of the great challenges for the IVF laboratory staff.

Number of embryos to transfer

The selection of the number of embryos to be transferred should be made taking into account medical recommendations.

Especially, in a surrogacy process, the decision on the number of embryos to be transferred will be taken jointly by the pregnant woman and the parents of intention, but always having adequate information about the risks and benefits, as well as paying attention to the medical recommendations.

The Society for Assisted Reproductive Technology recommends not transfer more than three embryos since a higher number of transferred embryos increases the risk of a multiple pregnancy which could cause harm to the woman carrying the pregnancy to term and to the fetuses. Most clinics, therefore, usually transfer between 1 and 2 embryos.

From all the viable embryos obtained after fertilization, one or two embryos of better quality will be selected. For this reason, it is necessary to classify the embryos throughout their development.

Embryo quality

In order to select the embryo with the highest quality and potential for implantation once it has been transferred to the uterus, a series of morphological aspects must be analyzed. This analysis will depend on the day of development in which the embryo is found.

Depending on the moment of development, the visualization of the embryo to evaluate its quality should present a series of characteristics. Below, each one of them is detailed according to the day of the embryo.

Day 1 selection

When approximately 24 hours have passed since the union of the egg and the sperm, it is vitally important to check if fertilization has occurred. To do this, the presence of the two pronuclei (NP), the male and the female, and the two polar corpuscles (CP) must be visualized, which is an indication that the embryo has been correctly fertilized.

All the embryos that present a deviation in the number of NPs or PCs are discarded for transfer since they are non-viable embryos as a result of errors in fertilization.

Embryo selection on the 2nd and 3rd day

On the 2nd and 3rd day of development, the embryo duplicates its cells. Thus, on day 2 of embryonic development, 4 cells or blastomeres should be seen and on day 3, about 8 cells. At this time, the following characteristics are analyzed to determine the viability of the embryo:

  • Symmetry of blastomeres: the cells should be about the same size.
  • Number of nuclei: Each cell must have a single nucleus. When there is a greater number of nuclei per cell, it can lead to errors in embryonic division.
  • Fragmentation: Both the number of fragments and their distribution within the embryo are studied.
  • Thickness and appearance of the zona pellucida: this layer should be neither too thin nor too thick.

Those embryos in day 3 of development that present some alteration of these properties are likely to present problems in the implantation in the uterus.

Embryo selection on day 4

At the end of day 3 or at the beginning of day 4 of development, depending on the rate of division, the embryo begins the so-called cellular compaction that will start the morula stage. Sometimes, some embryos show signs of early compaction, but it usually appears after 90-94 hours from fertilization.

This moment does not allow us to observe specific features of the embryo, but we can sense whether an embryo is better or worse depending on the degree and mode of compaction.

Blastocyst selection

From day 5 of development, the embryo expands and begins to form a liquid cavity inside called a blastocele, and differentiates its cells into cells of the inner mass or trophectoderm cells.

At this point, the embryo is named blastocyst and is selected based on the following characteristics:

  • Degree of expansion: a scale from 1 to 5 is established, with 1 being the lowest degree of expansion of the embryo.
  • Appearance of inner cell mass (ICM): number of cells, compaction, location... Its evaluation is done by assigning the letters A, B, C and D. A grade A blastocyst is considered an embryo of very good quality and with a high probability of implantation in the uterus.
  • Appearance of trophoectoderm cells: number, location, shape...

Based on all these characteristics, it is determined whether an embryo is of higher or lower quality. The embryos of higher quality will be the first ones selected for transfer and the rest of the viable embryos will be cryopreserved for use in later cycles.

Does Time-Lapse improve embryo selection?

As we have mentioned, until relatively recently, embryo selection was carried out taking into account only the morphological criteria observed with microscopic visualization. This caused a loss of information since the development of the embryos is something dynamic and not static.

Nowadays, Time-Lapse technology allows making videos of the embryonic development taking images every 10-15 minutes. Thus, it is possible to observe the evolution of the embryos in culture without removing them from the incubator and at any time.

Therefore, the Time-Lapse allows a better embryo selection for the moment of the transfer. Furthermore, this type of incubator provides the exact times when the changes in the embryo occur. For this reason, embryo selection will not be performed solely on the basis of morphological parameters, but also on the basis of morphokinetic selection.

By obtaining more information about the embryo and its development, implantation and pregnancy rates are increased. Therefore, the Time-Lapse system is a great advantage for improving the results of IVF treatments.

PGD and embryo selection

Pre-implantation genetic diagnosis (PGD) is a complex assisted reproduction technique that allows embryos to be analyzed genetically. This facilitates the transfer of healthy embryos without chromosomal alterations and, therefore, with greater potential for implantation.

This method of embryo selection is indicated for certain patients, such as people with a history of hereditary genetic diseases, cases of repeated miscarriages, previous implantation failures, women of advanced maternal age, etc.

We recommend you visit the following article to continue reading about it: What is the Preimplantation Genetic Diagnosis (PGD)?

FAQs from users

Are D-quality embryos transferred?

By Marta Barranquero Gómez (embryologist).

A D-quality embryo is an embryo with a low implantation rate into the uterus. This embryo will present signs of degeneration, morphological alterations, high percentage of fragmentation, etc.

However, if there are no more embryos, D-quality embryos can be transferred to a woman's uterus. Although the probability of pregnancy is low, it is true that any embryo that is transferred could implant in the uterus and lead to a gestation.

How to choose the best embryo?

By Marta Barranquero Gómez (embryologist).

Until relatively recently, embryo selection was done by taking into account only morphological aspects. In this case, the number of cells, the number of nuclei, whether it presented fragmentation, etc. were taken into account.

However, thanks to the revolution in the field of assisted reproduction, there are now incubators that allow us to observe the development of embryos in real time. This allows a morphokinetic study of the embryos, which improves embryo selection.

What quality does an embryo have to have in order to be frozen?

By Marta Barranquero Gómez (embryologist).

It is always preferred to freeze A and B quality embryos, although it is also possible to vitrify C quality embryos.

However, the decision to freeze the D quality embryos will depend on the center itself. These embryos have low quality, as well as the probability of survival after thawing. In these cases, it is usually decided to leave the embryos in culture until day 5-6 of development to study their evolution.

Suggested for you

If you want to learn more about how the embryo is transferred into the uterus, don't forget to visit the following article: How Does Embryo Transfer work?

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FAQs from users: 'Are D-quality embryos transferred?', 'How to choose the best embryo?' and 'What quality does an embryo have to have in order to be frozen?'.

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Authors and contributors

 Andrea Rodrigo
Andrea Rodrigo
Bachelor's Degree in Biotechnology from the Polytechnic University of Valencia. Master's Degree in Biotechnology of Human Assisted Reproduction from the University of Valencia along with the Valencian Infertility Institute (IVI). Postgraduate course in Medical Genetics. More information about Andrea Rodrigo
 Marta Barranquero Gómez
Marta Barranquero Gómez
Graduated in Biochemistry and Biomedical Sciences by the University of Valencia (UV) and specialized in Assisted Reproduction by the University of Alcalá de Henares (UAH) in collaboration with Ginefiv and in Clinical Genetics by the University of Alcalá de Henares (UAH). More information about Marta Barranquero Gómez
Adapted into english by:
 Romina Packan
Romina Packan
Babygest Staff
Editor and translator for the Babygest magazine in English and German. More information about Romina Packan

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