Embryo grading explained – what do those numbers and letters really mean?

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Here, we break down what embryo grading really represents, how it is used in IVF laboratories, and what it means – and does not mean – for your chances of success.

What is embryo grading?

Embryo grading is a system embryologists use to describe the appearance of an embryo under the microscope at a specific moment in time. It allows us to make informed decisions about which embryos to transfer or freeze, and to communicate clearly within the laboratory team when assessing embryo development.

Importantly, embryo grading is a descriptive tool rather than a guarantee of outcome. IVF laboratories may use slightly different grading systems, which means embryo grades can vary between clinics – even clinics located within the same state – and are not always directly comparable. Understanding what grading does and does not tell us can help you feel more informed and reassured throughout your IVF journey.

Why is so much attention given to embryo grading?

It’s natural to want something tangible to hold onto during IVF, and embryo grades can feel like a clear point of reference. Much like our early educational experiences, receiving a ‘grade’ can feel like an assessment of performance. Many people assume that a higher grade means a better embryo (and therefore a higher chance of success), while a lower grade suggests the opposite.

While this interpretation is understandable, it isn’t entirely accurate. There are many factors involved in embryo development and implantation, and grading alone cannot capture them all.

Embryo grading is not a prediction tool; it is an observation tool. It tells us what an embryo looks like at a particular point in time, not whether it will implant or result in a pregnancy. Every viable embryo, regardless of grade, has the potential to result in a pregnancy.

In our many years of experience in IVF laboratories, we have seen high-grade embryos that did not implant, and lower-grade embryos that went on to become healthy babies. While grading helps us prioritise embryos for transfer or freezing at that moment, it is only one piece of a much larger picture.

When do we grade embryos?

Embryos are grown in the Newlife IVF laboratory for up to six days after fertilisation. By day five or six, embryos ideally reach the blastocyst stage, which marks a critical milestone in development.

At this stage, the embryo has developed into many interacting cells, and for the first time, we can clearly identify three distinct structures:

  • Inner cell mass (ICM): A cluster of cells inside the embryo that will eventually form the baby
  • Trophectoderm (TE): The outer layer of cells that will develop into the placenta, which supports the baby during pregnancy
  • Blastocoel: A fluid-filled cavity that allows the embryo to expand.

A photographic image of a blastocyst (left) positioned next to a graphic representation of a blastocyst (right). The images show the types of cells that go on to become an embryo proper (foetus), including the zona pellucida (shell), cavity, trophectoderm cells (TE) and inner cell mass (ICM).

How does blastocyst grading work?

Blastocyst grading assesses three main features: the stage of expansion, the characteristics of the inner cell mass, and the characteristics of the trophectoderm. Each component is graded separately, as outlined below.

Expansion stage

The expansion stage refers to the size of the blastocyst and how far it has progressed in breaking free from its protective outer shell, known as the zona pellucida. As the blastocyst grows, it needs to thin and eventually break through this shell in a process called hatching.

Expansion is categorised into six stages:

  • Stage 1: Early blastocyst development, where a small fluid cavity is just beginning to form
  • Stage 2: Early expanding blastocyst, with a growing cavity but still relatively small within the shell
  • Stage 3: Expanded blastocyst, where the cavity is larger and the embryo occupies more space, but hatching has not yet begun
  • Stage 4: Fully expanded blastocyst, filling most of the shell, which has thinned significantly
  • Stage 5: Beginning to hatch, with part of the embryo emerging through the shell
  • Stage 6: Fully hatched, where the embryo has completely escaped the zona pellucida and is ready for implantation.

Inner cell mass (ICM) characteristics (A–D)

The inner cell mass is the group of cells that will go on to form the baby. It is graded from A to D based on appearance:

  • A: Many healthy cells that are tightly packed
  • B: Several cells with a slightly looser arrangement
  • C: Few cells that are scattered and less cohesive
  • D: Very few or degenerating cells (considered non-viable).

Trophectoderm characteristics (A–D)

The trophectoderm forms the placenta and is also graded from A to D:

  • A: Many cells are present, forming a strong and cohesive layer
  • B: A moderate number of cells with less uniformity
  • C: Few cells with an irregular appearance
  • D: Very few or degenerating cells (considered non-viable).

Putting it all together

When you see an embryo grade, it is simply a shorthand way of combining these three observations into a single description.

For example, a grade of 5AB means:

  • 5: The blastocyst is beginning to hatch from its outer shell
  • A: The inner cell mass has many tightly arranged cells
  • B: The trophectoderm has a reasonable number of cells forming a mostly cohesive layer (although the layer may not be uniform).

While this grading helps embryologists make informed decisions in the laboratory regarding embryo quality, it’s important to remember that no single grade can determine the outcome of an embryo.

Embryo grades are not fixed and can change over time

Embryo grading provides a snapshot in time. Embryos are dynamic and continue to grow, divide and change as they develop.

For example, an embryo graded early on day five as 2CB may look quite different later the same day, potentially developing into a 4BB embryo. This progression is entirely normal and reflects ongoing development in a viable embryo.

The same principle applies when an embryo is transferred, while others continue developing in the laboratory. An embryo transferred on day five may be graded 3BB, while its siblings reassessed later could receive a seemingly higher grade, like 5AA. This difference is often due simply to timing and additional hours of development – not because the transferred embryo was a poorer option.

At the time of transfer, the chosen embryo was assessed as the best option based on its developmental stage, appearance and timing. Because grading is so closely linked to when an embryo is observed, it is not a reliable way of comparing embryos with one another. Instead, grading is just one part of a broader decision-making process.

You can learn more about this approach in our blog, How we select embryos for transfer.

What grading can (and can’t) tell us

In general, higher embryo grades are associated with higher pregnancy rates, which is why grading remains a useful tool when deciding which embryos to transfer or freeze.

However, embryo grading is not an absolute predictor of outcome. A high-grade embryo does not guarantee a pregnancy, and a lower-grade embryo does not mean a pregnancy will not occur. In practice, we regularly see examples where a high-grade embryo, such as 6AA, does not implant, while a sibling embryo with a lower grade goes on to result in a healthy baby.

Once a pregnancy is established, the embryo’s grade is no longer relevant. It does not predict whether a pregnancy will continue, nor does it reflect the future health of the baby.

If an embryo has been transferred or frozen, it has been deemed viable and has potential. Every viable embryo, regardless of grade, has the capacity to become a baby. For patients with embryos of varying grades in storage, or those who have had a lower-grade embryo transferred, there is every reason to remain hopeful.

Looking beyond letters and numbers

Embryo grading is only one part of how we assess embryo development. In addition to visual grading, we use time-lapse imaging, key developmental milestones, embryologist expertise and AI-powered tools to observe how embryos grow and behave over time.

Together, these approaches provide a better understanding of embryo health and potential, helping us make the most informed decisions for your treatment.

Our guidance for patients

We encourage Newlife IVF patients to focus on the number of viable blastocysts rather than becoming fixated on letters and numbers. An embryo’s potential is far greater than its grade alone.

If an embryologist has transferred or frozen your embryo, the embryo has the capacity to continue developing, and this potential matters far more than a single snapshot assessment.

Interested to learn more about embryo grading or IVF?

If you would like to understand more about embryo grading or IVF treatment, be sure to listen to the embryo optimisation podcast episode. Our team at Newlife IVF is also here to support and guide you throughout your fertility journey – call us on (03) 8080 8933.

How embryos develop – from egg retrieval to blastocyst

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After retrieval, the egg and sperm are combined, and if fertilisation is successful, your embryo spends the first few days growing in the lab under the expert care of our team. This blog will help guide you through these early embryo development steps before transfer or freezing.

Factors affecting embryo development

Embryo development is a complex process that requires a combination of genetic, environmental and physiological factors to progress successfully. To achieve good embryo development, we require:

Healthy egg and sperm

The egg and sperm provide the genetic blueprint for development. Each embryo needs a complete set of 46 chromosomes – 23 from the egg and 23 from the sperm. Some embryos inherit incorrect genetic instructions that can impact embryo development and make it harder for them to divide and grow as expected.

Mitochondrial energy

Embryos need energy to divide and grow, which is provided by mitochondria (tiny energy-producing structures in cells). Poor mitochondrial function can slow or stop embryo development.
Efficient metabolic function:

An embryo’s metabolic function provides both energy and the building blocks needed for development. This includes the creation of protein and fats, and the removal of waste products that can be toxic to the embryo. Together, these provide what the embryo needs for growth, cell division and viability.

Timely cell division

Embryos should ideally divide at a regular pace (2-cell, 4-cell, 8-cell, etc.). Uneven and/or delayed division can mean the embryo is of suboptimal quality and may not develop as expected.

Embryonic genome activation

Around Day 3 (see diagram below), embryos start using their own DNA instead of relying on maternal DNA (genetic code from the egg). If this DNA transfer is inadequate, interrupted or missing, this can slow or stop the development of an embryo.

Stable conditions

Embryos need the right temperature, oxygen and pH balance to grow. These factors are carefully assessed and monitored continuously in the laboratory environment.

Difficulties or inaccuracies in any of these processes can affect the way an embryo grows and can impact whether an embryo will reach the blastocyst stage (where it has divided into many cells), making it suitable for transfer or freezing.

Safeguarding your embryos is our highest priority

We understand how important every embryo is to your journey. That’s why we use the most advanced technology and scientifically proven methods to create the ideal environment for embryo development. From carefully performing every procedure to closely monitoring each embryo’s progress, our highly trained embryologists work tirelessly to give each embryo the best possible chance to grow and thrive.

We maintain strict laboratory conditions, including precise temperature, humidity and air quality control to mimic the natural environment as closely as possible. Our team carefully observes each embryo’s development at every stage. Even though not all embryos will reach the blastocyst stage, please know that we do everything in our power to maximise their potential. Your dream of building a family is at the heart of everything we do, and we are committed to providing the best possible care every step of the way.

If you have any questions about embryo development or your fertility journey, please reach out to Newlife IVF. In the meantime, let’s walk through the different stages of embryo development to help you better understand the process.

 

Comparing the merits of fresh versus frozen embryo transfer: is fresh really best?

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Once here, it must ‘implant’ in the wall of the womb and grow before we can say that IVF has resulted in a successful pregnancy. The timing of embryo transfer can vary, depending on whether the embryos being transferred are ‘fresh’ or ‘frozen’. Fresh embryo transfer refers to embryos that are transferred to the uterus 3–5 days after a woman’s eggs have been collected and fertilised by sperm. Frozen embryo transfer refers to embryos that have first been frozen before being thawed at a later date for transfer into the womb.

In the early days of IVF, fresh embryo transfer was the favoured approach. However, the techniques used for freezing and thawing of embryos have since improved to a point where more than 90% of embryos will survive the process. Consequently, a ‘freeze-all’ strategy has become more common, whereby all embryos are frozen following successful growth. They generally remain frozen for at least a month before the best embryos are thawed and transferred into the womb.

Understandably, people undergoing IVF are often eager to get pregnant as quickly as possible – and may assume that fresh embryo transfer is both faster and more effective. But is one approach better than the other?

To appreciate the pros and cons of fresh versus frozen embryo transfer, you first need to understand the so-called ‘window of uterine receptivity’.

The window of uterine receptivity

The success of embryo transfer depends on a number of factors, one of which is uterine receptivity – that is, how ready the uterus is to ‘receive’ the embryo. Outside this window of receptivity, the embryo may fail to implant in the wall of the uterus.

To receive the embryo successfully, the uterus must be ‘primed’ by the hormones oestrogen and progesterone. Under natural conditions, the priming of the uterus is perfectly timed with a woman’s monthly cycle, such that if an egg is released from the ovary and fertilised by sperm, the uterus is ready to receive the embryo. In the IVF setting, however, this timing may be less than perfect.

During IVF, the ovaries are stimulated via self-injectable medication so that the highest possible number of eggs can be collected. By artificially driving the release of so many eggs, the levels of oestrogen and progesterone skyrocket – they can rise to 10 times higher than normal peak levels. This may cause the uterus to prematurely prepare itself for embryo implantation, bringing forward the time frame in which the uterus is receptive. The problem with this is that by the time a fresh embryo is grown and ready for transfer, the window of uterine receptivity may have passed.

Frozen embryo transfer overcomes this problem by delaying the transfer process. This gives the hormone levels time to return to normal and the embryo is then transferred at a later date, when the uterus is receptive again.

When is frozen transfer best?

There are some situations where frozen embryo transfer may be considered the best option, including:

  • High levels of progesterone: When progesterone levels are high at the time of egg retrieval, there is a higher chance that the window of uterine receptivity will shift forward. In these cases, it is generally better to freeze the embryos and transfer them later when the uterus is receptive again.
  • Polycystic ovary syndrome (PCOS): Studies have found that women with PCOS tend to have better results from frozen versus fresh transfer. Frozen transfer reduces the risk of ovarian hyperstimulation syndrome (a complication of egg retrieval), and is also associated with a higher chance of ongoing pregnancy.
  • Embryos that require genetic testing: The genetic testing of embryos takes time. Consequently, by the time a tested embryo is ready for transfer, the window of uterine receptivity is likely to be over. In this scenario, frozen embryo transfer is usually more appropriate.

What are the arguments for fresh transfer?

On the other side of the coin, fresh transfer avoids the need for the freeze-thaw process. While current technology enables a greater than 90% survival rate for frozen embryos, this level of risk may not be acceptable for some patients – especially if they already have a low number of embryos. Fresh transfer potentially also results in a shorter time to pregnancy.

Which is the best option for you?

When choosing between fresh and frozen embryo transfer, there is no one-size-fits-all approach. As with all aspects of fertility care, the decisions need to be personalised to your individual circumstances. Our fertility specialists will assess the specifics of your situation and tailor their advice accordingly.

If you would like professional advice about the next steps to take on your fertility journey, you can make an appointment with one of our fertility specialists by calling Newlife IVF on (03) 8080 8933. Alternatively, you can book online via our appointments page.