By Neus Ferrando Gilabert BSc, MSc (embryologist) and José Muñoz Ramírez BSc, MSc, PhD (senior clinical embryologist).
Last Update: 04/15/2014

Couples who undertake assisted reproductive techniques are constantly filled with easy-to-understand questions which usually have unclear, long answers.

Once in vitro fertilisation is performed and they know there are embryos in the laboratory waiting to be transferred, couples start to ask themselves questions like: how are they? what is its quality?

This text is an attempt to answer these questions as concisely as possible. First thing to know are the stages that embryo development follows day by day.

Each of these steps or stages present some details revealing the embryo implantation potential to result in pregnancy, that is to say, its quality.

The different sections of this article have been assembled into the following table of contents.

Embryo development

The zygote is the “newly fertilised” embryo. It should be checked between 16 or 22 hours after insemination, being able to distinguish between its 2 polar bodies (PB) and its 2 male and female pronuclei (PN). The existence of both PN confirms that fertilisation has occurred. Any deviation on its number –1PN, 3PN, or more– may indicate an abnormal fertilisation and a non-viable embryo.

After the first two divisions, the embryo generates 4 cells, called blastomeres, whose aspect will be crucial for classification, just like its number, which should be appropriate. It should be checked between 44 and 47 hours after insemination, focusing on its 4 cells, which should be approximately the same size and not have more than 1 nucleus, as well as on the percentage of fragmentation between them.

Thus, in line with these criteria, together with an examination of the outer membrane, embryos can be classified in 4 categories –from A to D– ranging from best quality to least developed embryos.

The same procedural pattern is followed for embryos on day 3: they are checked between 67 and 71 hours after insemination. In addition, cell compaction may be seen if it begins early, preparing thus the transition towards next stage.

A morula is an embryo on day 4, between 94 and 98 hours after insemination. Since the compaction of its cells has occurred, distinctive features of these embryos cannot be observed. Thus, morula provides little information. But still we can check whether compaction has been fully completed or, conversely, any cell has fallen outside.

The last stage, blastocyst, should be observed between 112 and 120 hours after insemination. At this stage, the embryo has activated its own genes. Although not every “in vitro” embryo reaches this stage, those who are successful have a higher potential for implantation.

These embryos are classified depending on their expansion grade scale (from 1 to 6), their inner cell mass (A, B, C, D), and their trophoblast or outer cell layer (A, B, C, D). Therefore, a high quality blastocyst on day 5 should be based on grade “3AA”.

While this classification is mainly visual and rather subjective, today there are several types of studies whose main purpose is to detect the embryonic metabolism of high competence embryos, finding thus which ones are suitable for being transferred.

FAQs from users

Can you pregnant with C and D scored embryos?

By José Muñoz Ramírez BSc, MSc, PhD (senior clinical embryologist).

Yes, although the chances are quite low.

When we classify embryos according to their quality at the lab, we do so by evaluating their implantation potential, that is, trying to “guess” which ones have greater chances for attaching tot he uterus, and which don’t. C and D scored embryos are embryos of moderate-to-low quality, which means that a C or D scored embryo has a reduced chance of implantation if compared to a B or A scored embryo. In any case, however, whenever we select an embryo for the transfer, it’s because it has been observed that its implantation potential is a good one.

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Authors and contributors

 Neus Ferrando Gilabert
BSc, MSc
Bachelor's Degree in Biology from the University of Valencia (UV). Postgraduate Course in Biotechnology of Human Assisted Reproduction from the Miguel Hernández University of Elche (UMH). Experience managing Embryology and Andrology Labs at Centro Médico Manzanera (Logroño, Spain). More information
 José Muñoz Ramírez
BSc, MSc, PhD
Senior Clinical Embryologist
Bachelor's Degree in Biology from the University of Malaga. Master's Degree in Genetics by the University of Alcalá, and Master's Degree in Assisted Reproduction from the University of Valencia. He works as a clinical embryologist at Clínica Tambre (Madrid, Spain), in addition to being an Associate Professor at the University of Murcia. More information
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One comment

    1. san

      Hi… I had my ET on 29/06/2016 at 8.30 am. 13 Days after when I checked my BETA HCG the result was only 34.6 and the doctor said that it is a negative result. Embryologist’s opinion about the embryos used for ET is that they were in ‘A’ grade. Unfortunately, the result was negative. Why did this happen? I need an advise from you for the successful implantation on my next embryo.