What is embryo grading?

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After successful fertilization of retrieved eggs, the embryo or embryos will start to develop through different stages in a petri dish in an IVF lab. The quality of their development is graded to estimate whether the progress is normal, i.e., proceeding according to scientific standards. Embryo grading allows for a standardized measurement of an embryo's quality as being good, fair, or poor. The grading can then be used to assess the likelihood of a transfer being successful and to prioritize the highest quality embryo for transfer first.ⁱ  

Overall, embryo grading scores the quality of an embryo based on its morphology (structure and appearance). However, the fact that there are many variations in criteria for embryo grading as well as the way grades are assigned can make comparisons between clinics or between studies extremely difficult.ⁱⁱ

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At what stage are embryos graded?

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Embryo development is assessed at different stages by the embryology team, and the timing of assessments can vary depending on the protocols used by the clinic. Most often, cleavage-stage (day 3) embryos are assessed for whether and how the cells have divided, and then blastocyst (day 5-7) grading is performed to further assess the quality of the embryo.

Embryos are first assessed on the third day after egg retrieval. At this point, the embryologist is looking at cell number and embryo morphology under a high-powered microscope. A day 3 embryo has cells that are actively dividing but the embryo itself is not growing in size/volume. Day 3 embryos ideally should have 6-10 cells, with 8 cells being optimal to develop into viable blastocyst embryos.ⁱⁱⁱ

Embryos are also almost always graded at the blastocyst stage (if they reach this point), though rarely they may be graded at the morula stage (day 4) when a day 4 transfer is planned. The significance of examining an embryo between days 5-7 is the ability to assess the quality of its development after embryonic genes are activated. Examining at the blastocyst stage also gives the benefit of assessing both the inner cell mass and trophectoderm. The grading of the trophectoderm (TE) will reflect the embryo’s potential to attach and implant in the uterine lining, as it will later become a large part of the placenta. On the other hand, the grading of the inner cell mass (ICM) will assess the potential development of the embryo (and later fetus) itself.^iv      

Some IVF clinics perform the embryo-grading process at other stages (i.e., zygote and morula), but grading at the 8-cell cleavage stage and blastocyst stage are the most common.  

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What do the different embryo grades mean?

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The early blastocyst grading system was first introduced by Gardner and Schoolcraft in 1999 and was quickly adopted by most IVF laboratories. Although this system did not cover all features of embryo morphology, it was useful to classify the quality of blastocyst development, assessing the fluid-filled cavity (the blastocoel), the ICM, and TE under a microscope. Grading the expansion was important, since formation of the fluid-filled cavity requires the embryo to produce a lot of energy and therefore reflects embryo quality. The Gardner-Schoolcraft system also assigns grades of A, B, or C separately for the structural appearance of the ICM and TE.^v  

The Society for Assisted Reproductive Technology (SART) in the USA then developed a simple and widely used embryo grading guideline in 2006. Using this methodology, embryos are graded into three broad categories of good, fair, or poor using morphologic features. By identifying the embryos as good, fair, or poor, the SART system helps patients and embryologists select which embryo(s) to transfer.  However, it does not further narrow down or rank “good” embryos.^vi  

In 2011, experts from the European Society of Human Reproduction and Embryology (ESHRE) and Alpha Scientists in Reproductive Medicine issued updated guidelines to standardize the embryo grading system. This classifies embryo quality based on cell number, the degree of fragmentation (uneven division of cells), the symmetry of the cells (even cell size), the presence of multiple cell nuclei, and compaction status.^vii

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Embryo Grading Chart – Blastocysts

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Grading systems vary among embryology labs, but many use some type of adaptation of the Gardner and Schoolcraft grading system.^viii

Below is a table of a typical grading system used to score a blastocyst, but it should be noted that many clinics have variations in their scoring systems.^ix

Table 1. Sample blastocyst scoring system*

^{"The Istanbul consensus workshop on embryo assessment: proceedings of an expert meeting." Human reproduction 26, no. 6 (2011): 1270-1283. https://academic.oup.com/humrep/article/26/6/1270/2913882}

How is the table read?  

The grading includes a number corresponding to the stage of blastocyst expansion, and two letters for grades of the ICM and TE. These are assessed by an embryologist under a microscope.  A hatching blastocyst refers to the embryo emerging, or hatching, out of the zona pellucida shell. AA is the highest quality score an embryo can have at each stage of development.  

Example: 5AA

This refers to a hatching embryo (5) with a tightly-packed inner cell mass (A) and a good quality trophectoderm (A) forming a cohesive covering layer. If a clinic uses a numerical scale instead, this would be a 511 instead of 5AA.  

Embryo Grading Chart - Cleavage stage

Cleavage stage embryos are assessed under the microscope to determine the number of cells (7-9 is optimal), the cell size and symmetry, the presence of one nucleus (structure that contains DNA), low vacuolization (fluid-filled sacs), as well as degree of fragmentation.^x  

Fragmentation measures the number of small membrane-bound cell pieces that have broken off. These are usually less than 40 μm in size. Less fragmentation is associated with better outcomes. Experts agree that large vacuoles (>14 μm in diameter) can be associated with fertilization failure, but that a few smaller vacuoles generally have little effect. There should be only one nucleus present and multi-nucleation is often associated with poor outcomes.^xi  

Below is a table of a typical grading system used to score a day 3 cleavage stage embryo. However, it should be noted again that many clinics have variations in their scoring systems.^xii

Table 2. Sample cleavage-stage scoring system*

"The Istanbul consensus workshop on embryo assessment: proceedings of an expert meeting." Human reproduction 26, no. 6 (2011): 1270-1283. https://academic.oup.com/humrep/article/26/6/1270/2913882 Table adapted from: https://doi.org/10.1093/humrep/der037

How is the table read?

The typical scoring format for Day 3 embryos includes the grade and number of cells. The grade is assigned as explained in the table, and this is followed by the number of cells that the embryo has when examined under the microscope. For example, a 2/7 is graded fair (2) and has 7 cells. Some embryology labs may report other details, such as “15 percent fragmentation” to the patient.  

Ideally, a day 3 embryo (~68 hours post-insemination) would have 8 equally-sized cells (called blastomeres) with <10 percent fragmentation, only one nucleus, and no large vacuoles visible.

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How does embryo grade impact success rates?

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The number of well-graded embryos that result from an IVF patient’s oocyte retrieval may be predictive of how many embryos continue on to the implantation stage, but this can vary according to the stage at which embryos are assessed by the embryology team.

For blastocyst grading, many studies show a correlation between better-graded blastocysts and success rates. In one example, a 2021 study showed a significant difference in implantation rates for good-quality blastocysts (79.8 percent) versus poor-quality blasts (48.1 percent).^xiii However, not all studies observe this relationship of good-quality day 5 blastocysts providing better outcomes than poorly-graded embryos.  A 2019 study of over 600 frozen embryo transfers observed similar live birth rates between good quality day 5 embryos (48.65 percent) and poor-quality day 5 embryos (47.50 percent).^xiv  

Despite reported correlations, it is important to note that even poorly-graded blastocysts still commonly result in live births, while the highest graded embryos may not implant.^xv  

With day 3 embryos, grading appears to be much less consistent in predicting implantation and pregnancy rates – some studies show that day 3 grading does not correlate with success, while others show it is useful in predicting implantation. For example, a 2018 study showed that ongoing pregnancy rates were not significantly different between grade I, II, and III embryos, with clinical pregnancy rates of 58.0 percent, 57.9 percent and 55.6 percent, respectively.^xvi  

In contrast, other studies have reported that when day 3 embryos are transferred, live birth rates improved with increasing cell number (up to the 8-cell stage); 16.2 percent when >8 cells vs. 2.9 percent when <6 cells.^xvii And a study of 426 day 3 fresh embryo transfers showed that Grade I embryos had a higher implantation rate (41.8 percent) than Grade III embryos (18.4 percent).^xviii  

While embryo grading may help to predict implantation, its main purpose is to be a useful tool to decide which embryo(s) to transfer first. There remain inconsistencies in the predictive value of embryo grading, particularly at the cleavage stage (i.e., day 3).

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What is the difference between grading and PGT?

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Embryo grading and preimplantation genetic testing (PGT), previously called preimplantation genetic diagnosis/screening (PGD/PGS), are both commonly used to select embryos for transfer. While embryo grading visually evaluates the quality of an embryo’s morphology, it does not provide information about whether the embryo is genetically competent (euploid). In contrast, PGT assesses an embryo for genetic abnormalities (such as aneuploidy) by analyzing the embryo chromosomes obtained through embryo biopsy. Embryos with an abnormal number of chromosomes are more likely to stop growing and fail to implant.^xix  

A 2019 study showed that embryos with higher grades were more likely to be euploid than those with lower grades.^xx Nonetheless, morphological grading should not be used to make assumptions about whether or not an embryo is euploid.

Overall PGT is better able to predict likelihood of clinical pregnancy than embryo grading. Morphological grading can still be used in selecting embryos for transfer, but this only applies when there are multiple euploid embryos to choose from.^xxi In other words, once an embryo is determined to be euploid by PGT-A, the embryo grading is less important in the embryo selection process than the fact that the embryo is euploid.^xxii,xxiii

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Conclusion

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Grading embryos at different stages gives the embryology team the opportunity to determine which embryos may be most likely to implant after embryo transfer. While embryos are graded based on their quality in relation to cell division and appearance, even a poor embryo with a lesser letter grade may implant and result in pregnancy and a live birth. It is also important to remember that embryo grading does not predict the health of an embryo or of a future child.  

Nevertheless, the process gives doctors one more tool to help achieve better outcomes with IVF treatment.