Embryos quality and how they are classified

In all in vitro fertilisation cycles, the embryos created in the laboratory are classified according to their morphology, normally on the fifth day of development. The embryos will be transferred into the uterus, vitrified or discarded according to their classification. The choice of embryo destination is decisive in order to guarantee the success of the pregnancy and the birth of a healthy baby. This is why we would like to talk to you about the quality of embryos, how they are classified and how they develop.

During the days of embryo development, morphological changes happen that provide embryologists with information to analyse their quality and implantation potential. Poor quality embryos often lead to implantation failure or subsequent miscarriage. But how is embryo quality analysed? Different morphological characteristics such as cell division, cell evolution and subsequent organisation are assessed.

There are two options for studying these characteristics; either by observing the embryos under the microscope or by using a time-lapse system (a system for capturing images inside an incubator). The time-lapse system allows the embryologist to analyse the embryo in a more continuous way and to see its complete development. In addition, it allows greater stability of the culture conditions (temperature and gas concentration) of the embryo, reducing stress and improving its viability.

Embryo development:

• Day +1: Zygote. On day 1 of development, 16 to 18 hours after insemination, fertilisation is assessed. An oocyte is considered to be correctly fertilised when it has 2 pronuclei (one from the egg and one from the sperm) and 2 polar corpuscles. Oocytes that do not present these characteristics are considered abnormal or unfertilised and are discarded at this point.
• Day +2 and day +3: from day 2 cell division has already started and we can count the cells that form the embryo. Ideally we expect a day 2 embryo to have 4 cells, while a day 3 embryo will have between 7 and 9 cells. During these days we not only analyse the number of cells that form the embryo but also their symmetry (cells of the same size or not), the degree of fragmentation and the size and shape of the zona pellucida (the covering that envelops the embryo).
• Day 4: Morula. The embryonic cells begin to join together to form a compact mass called a morula.
• Day +5 and day +6: Blastocyst. The last stage of embryonic development in vitro is the blastocyst stage. After the compacting of the cells and formation of the morula, the organisation of the embryo takes place to reach the blastocyst stage. The compacted cells differentiate, forming a central cavity and giving rise to two structures, the inner cell mass (ICM), which will give rise to the foetus, and the trophectoderm, which will develop to form the placenta. Blastocysts are classified according to their degree of expansion, the quality of the ICM, the trophectoderm.

Embryos are expected to reach the blastocyst stage between day +5 and day +6, with the highest quality embryos reaching this stage at day +5. It is at this moment, with all the information provided by the blastocyst stage, that a decision is made as to which embryo will be transferred, which will be vitrified and which will be discarded.

How are embryos classified according to their quality?

The blastocyst classification consists of a number followed by two letters. The number refers to the degree of expansion, the first letter to the inner cell mass and the second letter to the trophectoderm. This classification is based on numerous published scientific studies on the implantation potential of embryos according to their morphology.

Expansion rate
As mentioned above, the embryos are covered by the zona pellucida. As the blastocyst expands, increasing in volume, the zona pellucida thins, until finally a small hole is created and the blastocyst begins to hatch from the zona pellucida.

Depending on the degree of expansion, blastocysts are classified as follows: 1 and 2 early blastocysts, they are still finishing organising and the zona pellucida is still thick; 3, expanding blastocyst, the blastocyst is already organised and increases considerably in size as the zona thins out; 4, expanding blastocyst, the zona pellucida is already very thin; 5, hatching blastocyst, the blastocyst has created a hole in the zona pellucida through which it begins to emerge; and, 6, hatched blastocyst, the blastocyst has completely emerged from the zona pellucida.

ICM and trophectoderm
Both the ICM and trophectoderm are classified into categories A to D, with A being the embryo category with the highest implantation capacity and quality and D being the lowest.

Category A. The most likely embryo to be implanted due to its high quality.
Category B. The embryo has a good quality and a high implantation rate.
Category C. The embryo has intermediate quality and the implantation rate is lower.
Category D. The embryo has poor quality and the implantation rate is nil or very low.

If you want to know a little more about the quality of embryos and how they are classified, from fertilisation to transfer, we encourage you to watch the webinar of our embryologist Aina Canyelles. Don't miss it!