Embryos and IVF

Published: 11/10/2022

How the quality of your embryo’s affect your levels of fertility and possibility of becoming pregnant through IVF.

Embryo grading and development

Ok, so you have done what some call the ‘hard part’ of the stimulation phase: your eggs have been fertilised and you face the wait of hearing from the embryologist about how your embryos are doing. 

But what does it all mean?

From fertilisation, embryos are kept in an incubator in a special media to be allowed to develop. Whilst this is happening there's a lot of medical jargon which you'll have to try and understand, here's the breakdown:

Embryo grading

The grading of the embryo refers to the appearance of the embryo. Embryologists check the embryos using a high-powered microscope looking at the structure of the embryo in detail to grade them. 

It is important to note that methods of grading can differ from clinic to clinic. Embryo grading is a tool that clinicians use to guide them on which embryo to transfer, what day to transfer, and the number of embryos to transfer. 

Day three embryos 

If you have a day three embryo, it means you’ve made it past egg collection and past the initial discussion about how many of your eggs fertilised. 

A day three embryo is also known as cleavage stage embryos. This means that the cells are dividing but the embryo itself is not yet growing. Day three embryos consist of 6-8 cells inside an outer covering called the zona pellucida. 

If you have one to two viable embryos, you may have embryo transfer on this day. 

What the embryologist expects to see

  • One-day post fertilisation the embryos should be between 2 and 4-cell stage
  • Two days post fertilisation they should be between 5 and 8-cell stage
  • Four – five days post fertilisation they would expect to see blastocyst formation

Day five embryos are known as Blastocysts and are graded according to a number and two letters. 

Not all embryos will develop to this stage, but research suggests that embryos that do are strong and likely have the best chance of successfully implanting in the uterus. 

Blastocysts are graded on 3 factors:

  • The degree of expansion
  • The appearance of the inner cell mass 
  • The appearance of the trophectoderm (the cells that make the placenta)

Degree of expansion 

The degree of expansion is graded from 1-5/6 in most clinics:

  • 1 means 1/3 expansion of the embryo (Early Blastocyst)
  • 2 means ½ expansion of the embryo (Blastocyst)
  • 3 means 80% expansion of embryo (Blastocyst)
  • 4 means 100% expansion of embryo (expanding Blastocyst)
  • 5 means fully expanded blastocyst beginning to hatch out of its shell (Hatching Blastocyst)
  • 6 means a full hatched embryo (Completely hatched Blastocyst) 

As the blastocyst develops, cells multiply and the outer casing begins to stretch and eventually pops/hatches releasing a ball of cells - your embryo. 

Before you panic… hatching is an important and necessary stage in the development of the embryo, as the embryo prepares itself for implantation In utero.

The appearance of the inner cell mass (ICM quality)

  • A means many cells, tightly packed 
  • B means several cells, loosely grouped
  • C means very few cells

Quality of trophectoderm 

  • A means many cells, forming a cohesive epithelium 
  • B means few cells, forming a loose epithelium
  • C means very few large cells

The results

The above outlines how embryologists grade an embryo that has reached the blastocyst stage. 

Five days post egg collection and fertilisation, embryos are checked and graded:

  • “Best embryo or a Top-Quality” embryo is a 5AA or 6AA or 3AA or 4AA
  • “Good Quality” embryo is 3AB/BA or 4AB/BA or5AB/BA or 6AB/BA
  • “Average Quality” embryo is 3BB or 4BB or 5BB or 6BB
  • “Poor Quality” embryo is 4BC/CB or 5BC/CB or 6BC/CB

Success rates - what does this all mean? 

Some embryos progress slightly slower and continue to develop and that is okay. An embryo not being graded the top grade when checked does not mean that it is not a good embryo or that it will not help you achieve pregnancy if transferred back in utero. 

Saying that it is likely that the grading of your embryo can indicate your chances of success. 

Choosing the embryo

Sometimes the hardest decision is selecting which embryo to transfer. Do you choose the embryo that is well developed but doesn’t have the best grade? Or do you choose an embryo that is graded AA?

It is likely that both these embryos have an equal chance of success, but it is still a nerve-wracking decision to make. Embryologists will liaise with consultants and yourself to reach a shared decision that is best for you.

Embryo grading is useful in determining which embryos have the greatest chance of success, however there is no way to know for sure what the outcome will be. 

Pre-genetic screening (PGTA) to check that the chromosomes are normal within the embryo can help increase chances of pregnancy and minimise chances of miscarriage, but this comes with its own risks.

Clinics often separate outcomes into pregnancy rate and live birth rate. The goal of having IVF and a transfer, after all, is to accomplish a healthy pregnancy and a live birth - a healthy baby held in your arms. 

Common questions about embryo transfer

The most common question is ‘can I transfer two embryos?’ 

Transferring two embryos can lead to multiple pregnancies. Not only can both embryos implant, but one or both can split, leading to twins and triplets in some cases. Multiple pregnancies come with their own set of risks and can jeopardise the pregnancy altogether.

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