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Can an embryo's 'score' reveals its chances of fertilization?

Updated: 2020-04-07


At present, there is no uniform standard for embryo grading -- the method used to evaluate embryos for transfer in fertility clinics -- both at home and abroad -- and even different IVF labs in the same city have certain differences in the scoring the same embryo, experts say.

Embryo grading is based on the external morphology of the embryo and its rate of development, with subjective factors when assessed by embryologists, so there will be different scoring of the same embryo in two different reproduction centers in the same city.

There is no comparability in the score results of different fertility clinics, so it is not necessary to worry too much about the high or low embryo score. So what role does scoring play in the IVF lab?

Embryo grading, or scoring, is only used as a reference for laboratory technicians to select embryos and conduct statistical analysis of data.

Embryo development is a dynamic process and its grade mainly refers to the fertilization status after human sperm-egg binding and the development status of the embryo. A series of morphological changes occur from the beginning of sperm-egg association to the third or fifth day of development.

1. Scoring basis of cell stage: the main morphological parameters observed are cell number, cell size, fragments, etc.

Embryo growth and development has its own development speed. Usually the second-day embryo (43-45 hours after fertilization) should have four blastomeres of equal size and the third-day embryo (67-69 hours after fertilization) should have eight blastomeres of equal size.

Fragmentation is a common phenomenon during embryonic development. The degree of fragments varies from 5 percent to 100 percent and its proportion is related to the quality of the embryo. Mild fragmentation does not seriously affect the development of the embryo, but if the fragments exceed 50 percent, the embryo is considered to be unusable.

Another parameter of the morphological score is that cells are uniformly sized and normally divide equally, so each time they divide into two cells of equal size. Severe uneven cell size may indicate a low embryonic development potential.

2. Basis of a blastocyst stage score: mainly observe cell mass and trophoblast cells in blastocyst stage (blastocyst development rate).

If the cell-stage embryos continue to be cultured until the fifth or sixth day after fertilization, the embryos with the potential for continued development will reach the blastocyst stage. Blastocyst stage embryos are mainly staged by blastocyst expansion status.

According to the number of inner cell mass and trophectoderm cells, the inner cell and trophoblast are divided into three grades of A,B and C. 

Grade A: a large number of cells arranged tightly; Grade B: a small number of cells arranged loosely; Grade C: a very small number of cells.

The results of embryo scoring can be used as a reference for laboratory technicians to select embryos, but the embryo morphology scoring mainly evaluates the external characteristics of the observed embryos, does not fully reflect the implantation potential of the embryos -- nor can it judge whether the chromosomes of the embryos are normal; the intrinsic characteristics of these embryos cannot be observed by morphology.

In fact, embryologists mainly observe normal growth rates when choosing an embryo, such as whether the third day embryo is at the 7-8 cell stage and is uniformly sized with less than 35 percent fragments.

However, with the progress of related technologies, methods have been developed to automatically record the embryo growth process by time-lapse microscopy, combined with the ability of AI computer to analyze embryo implantation and select those embryos suitable for transplantation and cryopreservation. Therefore, traditional grading that only reflects embryonic morphology may disappear in the next few years.