Whether you’re deep into your fertility journey or just starting to explore your options, a term you may often hear is ’embryo grading.’
But what does it actually mean, and how important is it for you on your fertility journey?
In this article, I’ll help you understand how embryos are graded and why it matters. I often tell my patients that while IVF is a treatment, it also serves as a valuable diagnostic tool. It allows us to observe the egg, the sperm, how they interact, and how the embryo develops. When pregnancy isn’t occurring, we can examine the embryology to gain insights into where the challenges lie and how we might improve the chances of success and feel a little more in control of the process.
First, let’s break down embryo grading and its stages in simple terms.
What Is Embryo Grading?
Embryo grading is a systematic assessment used in in-vitro fertilisation (IVF) to evaluate the development and potential of human embryos. Technically, embryologists observe embryos under a microscope and assign grades based on structural features, cell symmetry, and the presence of specific cell types.
Embryos are graded at two key developmental stages: the cleavage stage and the blastocyst stage and assigned points, or ‘graded,’ based on
- cell number,
- symmetry,
- degree of fragmentation at the cleavage stage,
- structural features such as the inner cell mass (ICM) and trophectoderm epithelium (TE) at the blastocyst stage.
This grading helps provide important insight into the embryo’s potential for implantation and healthy development.
Embryo Development
Embryo development follows a well-defined sequence, and the process begins when an unfertilised egg is successfully fertilised in the IVF lab. This results in a single-cell zygote, which then divides repeatedly, progressing through the cleavage stage and eventually developing into a blastocyst by day five or six.
Embryo Quality
The term ’embryo quality’ refers to the rate of development, the appearance and organisation of the cells, and the embryo’s overall potential to implant successfully in the uterus.
Stages of Embryo Development in IVF
An embryo is formed when a sperm successfully fertilises an egg. And while eggs themselves are not graded in terms of quality, they are graded in terms of genetic maturity. And it’s only those genetically mature eggs, also known as ‘metaphase two eggs,’, that are suitable for fertilisation with sperm.
Day Zero: The day of the egg collection and the day that egg and sperm come together is called ‘Day Zero.’
Stage 1: Cleavage Stage Embryo
Embryos should have 6–8 equal-sized cells with minimal fragmentation by Day 3
Embryo Grading Cleavage Stage:
At the cleavage stage, embryos are assessed based on
- Cell number (ideally 6–8 cells by Day 3)
- Symmetry of the cells
- Degree of fragmentation (fewer fragments mean better quality)
Our scientists grade the embryos ‘1, 2, and 3.’ Other units grade them A, B, and C, and essentially, these reflect ‘Good,’ ‘Fair,’ or ‘Poor. ‘ Embryo development on Day 3 largely reflects the quality of the eggs. High-quality cleavage-stage embryos have evenly sized cells and minimal fragmentation, reflecting healthy and timely cell division.
Stage 2: Blastocyst Development
By Day 5 or 6, the embryo is more than 100 cells. It’s divided into two groups that form a blastocyst, characterised by a fluid-filled cavity, a clearly defined inner cell mass (ICM) that will become the baby, and an outer layer called the trophectoderm (TE) that will develop into the placenta and other pregnancy-related tissue.
Embryo Grading Blastocyst Stage
At the blastocyst stage, grading becomes more detailed. Embryos are evaluated based on three main criteria:
- Blastocyst expansion (scored 1 to 6), which indicates how fully the fluid-filled cavity has developed.
- Inner cell mass (ICM), graded from 1 – 3 based on cell compactness and organisation; the ICM becomes the fetus.
- Trophectoderm (TE), also graded from 1 – 3, which assesses the outer cell layer that will form the placenta and pregnancy-supporting tissues.
How Embryos Are Graded in IVF Clinics
Embryo grading is performed at these two main developmental stages, and the process of grading uses established systems, most commonly the Gardner grading system, to evaluate the morphological features of each embryo and guide decisions regarding transfer, freezing, or further testing.
Blastocyst Grading and Structure (Gardner System)
The Gardner system remains the most widely used blastocyst grading scale in clinical IVF practice, helping embryologists identify embryos with the best structural integrity for transfer or cryopreservation.
This grading is based solely on visual characteristics. Some embryos that appear morphologically ideal may still carry genetic abnormalities, while lower-grade embryos can sometimes result in healthy pregnancies.
For a more comprehensive assessment, preimplantation genetic testing (PGT) may be recommended in conjunction with grading to evaluate chromosomal normalcy, particularly in women with recurrent implantation failure, advanced maternal age, or known genetic conditions.
High-Grade Embryo Blastocyst
A high-grade blastocyst or high-quality embryo is often classified as 4AA or higher. It typically shows:
- Many tightly packed cells in the ICM and TE
- Proper blastocyst expansion
- Strong structural organisation, like a cohesive, well-organised trophectoderm layer.
It’s critical to note that while these features are associated with higher implantation potential, a higher grading does not guarantee pregnancy.
Low Cell Mass (I)CM Quality
At first glance, receiving a 3 grade for the quality of the inner cell mass (ICM) might seem discouraging, but it doesn’t automatically mean the embryo lacks potential.
In many cases, a 3-grade ICM may simply indicate that the embryo is still in an earlier phase of development. Given another day in culture, that same ICM can compact, organise, and improve to a 2 or even an 1 grade, reflecting enhanced developmental potential.
The same principle applies to blastocoel expansion. An embryo graded as a 1 on the expansion scale, meaning it’s an early blastocyst, may continue to grow and expand into a fully expanded blastocyst (grade 5 or 6) within 24 to 48 hours.
Embryos develop at slightly different rates, and grading provides a snapshot in time. That’s why many clinics perform grading across multiple days and consider overall progression, not just static scores, before making final decisions about transfer or freezing.
1. Factors That Affect Embryo Quality
Embryo quality is influenced by multiple factors. These include:
- Maternal age and egg quality
- Sperm quality, including DNA fragmentation
- Embryo culture conditions in IVF labs
- Underlying genetic health and other factors
It’s important to note that embryo grading reflects but does not guarantee implantation or a successful pregnancy. Lower-grade embryos may still result in a healthy baby, while some high-grade embryos may not implant.
While embryo grading provides valuable guidance, it does not define every outcome; every couple’s journey is unique, and hope remains even when the grading is less than ideal.
2. Why Embryo Grading Matters in Early Pregnancy Testing
Understanding embryo grade plays a crucial role in guiding clinical decisions related to early pregnancy testing. Here’s why:
Offer Insights
It helps clinicians determine the optimal timing for testing following embryo transfer, as embryos of higher quality may implant sooner and produce detectable hCG levels earlier.
Additionally, embryo grading provides insight into expected hCG patterns, enabling better interpretation of test results. In cases of implantation failure, grading can help differentiate whether the cause lies with embryo development or other factors such as uterine lining conditions or immune responses.
Helps Predict IVF Outcomes
Higher-grade embryos are typically linked to better chances of implantation and live birth, offering hopeful prospects for many patients.
Guiding Clinical Decisions
Embryo grading is a critical tool that helps fertility specialists determine which embryos are best suited for transfer or freezing and make decisions like whether to proceed with a single embryo transfer or to transfer multiple embryos, balancing effectiveness and safety.
For patients, understanding the embryo grade helps set realistic expectations about the chances of a positive pregnancy test and directs suitable steps if the initial tests are negative.
Thus, embryo grading supports
- Embryo transfer decisions, especially in fresh embryo transfer or blastocyst stage embryo transfer
- Prediction of successful pregnancy rates
- Determining whether to perform single vs. multiple embryo transfers
Limitations of Embryo Grading
Despite its usefulness, embryo grading is based solely on visual assessment of morphology and does not provide information about the embryo’s genetic health.
This means that some embryos appearing lower grade may be genetically normal and capable of resulting in a healthy pregnancy, while some high-grade embryos might carry genetic abnormalities.
Therefore, grading should be considered as one component of a comprehensive evaluation and should often be supplemented by genetic testing and clinical judgement to guide personalised treatment decisions.
Emerging Role of Artificial Intelligence (AI)
AI tools are increasingly used to assess blastocyst development and cell structure with greater objectivity. These systems analyse features like tightly packed cells, irregular cell division, and early signs of degeneration, thus enhancing embryo viability predictions.
Similarly, in the second stage, accurate grading relies on evaluating blastocyst organisation, cell number, and outer cell structure. Integrating AI with genetic testing can offer a more complete picture of embryo health, supporting better embryo selection and increasing the chances of a healthy pregnancy.
What Patients Should Know About Embryo Grades
Understanding embryo grading can help set realistic expectations during IVF. While it can offer insight into an embryo’s potential for a healthy pregnancy, it’s important to remember that it does not predict outcomes with certainty.
As a fertility specialist, I encourage patients to have open discussions about embryo grades, the possible role of genetic testing, and how these factors relate to individual fertility history. These conversations, in turn, can help prospective parents opt for treatment choices that are aligned with personal and reproductive goals.
