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Embryo selection criteria. How do embryologists select good quality embryos for ET?
Criteria for selecting an embryo for transfer
Embryologists visually assess the quality of embryos and the process of their development. On the day of the transvaginal puncture, the embryologists receive the eggs, which are then fertilized. This day is considered to be a zero one.
The next day, fertilization is evaluated. It is considered to be the first day of embryo development. With proper fertilization, the egg has two pronuclei (female and male).
On the second and third days of development, the process of embryo fragmentation into blastomeres begins. At this stage, the embryo score consists of a number (from 2 to 12), which is equal to the number of blastomeres, and one letter (A, B, C, D), which indicates the uniformity of the blastomeres and the presence or absence of fragmentation.
On the fourth day of development, the human embryo already consists, as a rule, of 16-18 cells. Intercellular contacts are gradually compacted, and the surface of the embryo is smoothed. This process is called compactization.
On the fifth day of cultivation, the embryo reaches the stage of blastocyst development. The cells of the embryo are divided into cells of the outer wall - trophectoderm (TE), which will form the placenta in the future, and into cells of the inner cell mass (ICM), from which the embryo itself will form in the future.
Embryologists evaluate the quality of blastocysts using numbers and letters. According to the blastocysts classification by David Gardner, the designation includes:
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a number (from 1 to 6) – it reflects the degree of the embryo development:
1 - a blastocyst cavity is less than half of the whole embryo
2 - a blastocyst cavity is larger than half of the whole embryo
3 – a complete blastocyst, a cavity fills almost the entire embryo
4 - a developed blastocyst, a cavity includes the entire embryo, a thin shell
5 - a blastocyst begins to hatch from its shell (ZP)
6 - a blastocyst that has completely left its shell
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the first letter (A, B, C) indicating the quality of the ICM:
A - many cells, densely compacted
B - several cells, loosely grouped
C - very few trophectoderm cells
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the second letter (A, B, C), which indicates the quality of the TE:
A - many cells, form a single layer
B - few cells, form a free epithelium
C - very few large stretched cells
Time-lapse technology
ecently, embryologists have gained an opportunity to observe the embryo development almost in a permanent way, thanks to the new technology of time-lapse microscopy. Time-lapse technology consists in continuous monitoring of the embryo development without their removal from the incubator for evaluation. This guarantees constant, stable conditions for the cultivation of embryos from the moment of fertilization to the moment of the blastocyst formation. The advantage of this cultivation is that it becomes possible to evaluate not only the appearance of the embryo but also to analyze the dynamics of its cleavage over time. This can help in selecting embryos with the greatest potential for pregnancy.
How many embryos to transfer?
The decision on the choice of the embryo number for transfer is always taken mutually: by the reproductive specialist, the embryologist, and the patient. The answer to this question in each specific case depends on the number of available embryos, the stage of their development, the assessment of the implantation risk of two or more embryos, and the analysis of previous transfers. Most often, a single embryo is transferred to reduce the likelihood of risks associated with multiple pregnancies. In rare cases, 2 embryos are transferred, but no more.
The introduction of imaging systems, together with preimplantation genetic testing (PGT), now allows the selection of an embryo with the greatest potential for implantation and normal development. All this contributes to the ultimate goal – the birth of a healthy baby. The selection of a quality embryo is a difficult task. The more modern methods and analysis tools are available to the embryologist at the clinic, the higher the likelihood that this choice will be the most accurate.
