A 5AA grade does not guarantee pregnancy, just as a lower grade does not rule it out. That distinction lies at the heart of embryo assessment, the speciality of Vasilis Geladaris, clinical embryologist at Newlife IVF Greece.
For patients considering IVF, an embryo’s combination of numbers and letters can seem like the clearest answer in a process filled with uncertainty. Better-looking embryos do have higher chances of implantation and live birth. But appearance cannot tell the whole story, and a grade should not be read as a verdict on whether an embryo can become a healthy baby.
Grading primarily guides the laboratory’s transfer order. By ranking embryos according to their apparent potential, clinicians aim to achieve pregnancy as soon as possible. That makes grading a selection tool rather than a guarantee: a high-grade embryo may not result in pregnancy, while a low-grade embryo still can.
The distinction matters because morphology, the appearance assessed during grading, leaves out information that can change an embryo’s prognosis. It does not account for maternal age or whether an embryo is euploid or aneuploid. Two embryos that look equally good may therefore have very different chances of producing a pregnancy.
Understanding a grade means looking beyond whether it is “good” to what it measures and what other information is available. Genetic testing, when available, can change which embryo takes priority.
Most clinics use the Gardner grading system to assess embryos at the blastocyst stage, usually reached at 5 or 6 days. A blastocyst includes the inner cell mass, which forms the fetus, and the trophectoderm, which forms the placenta. It also has an outer covering called the zona pellucida.
The number describes how far the blastocyst has expanded and progressed toward leaving that covering. Grades 1 and 2 describe an early blastocyst and a blastocyst, respectively. Grade 3 is a full blastocyst, and grade 4 is an expanded blastocyst. At grade 5, the blastocyst is hatching; at grade 6, it has hatched.
The letters describe the two groups of cells. The first letter assesses the inner cell mass: A indicates many tightly compacted cells, B indicates fewer, loosely grouped cells, and C indicates very few, loosely bound cells. The second letter assesses the trophectoderm, with A describing many cohesive cells, B fewer, looser cells, and C very few loose cells.
A 5AA embryo is therefore a hatching blastocyst with an A assessment for both cell groups. A 3BC embryo is a full blastocyst with a B-rated inner cell mass and a C-rated trophectoderm. These codes describe what the embryologist sees; neither code, by itself, establishes whether pregnancy will occur.
Good expansion combined with A or B ratings generally identifies a good-quality blastocyst. Even so, the system is more useful for comparing embryos than for dividing them into those that “will work” and those that “will not.” A lower-grade blastocyst may still produce a healthy baby, although its chances are reduced.
The grade is only one part of the laboratory’s assessment. Most clinics grow embryos for five or six days to see which reach the blastocyst stage, gaining more information before choosing an embryo for transfer. When morphology is the only assessment available, embryologists consider both the grade and the day of development.
An embryo that reaches the blastocyst stage on day 6 has lower potential than one that reaches it on day 5. That does not mean a day 6 embryo has no chance. It means that developmental timing adds context that the letters and numbers alone cannot provide.
The same caution applies when comparing grades between patients. Maternal age is the most important factor when estimating pregnancy chances, and two embryos with the same morphology can have different prospects depending on the woman’s age. An embryo from a woman aged 40 years has much lower potential than one from a younger woman, even if their appearance is similar.
Another patient’s pregnancy outcome therefore cannot define the prospects of your embryo. A matching code does not necessarily represent a matching prognosis. An individual assessment needs to bring together the embryo’s appearance, its developmental timing, maternal age and any genetic testing results.
Growing embryos to the blastocyst stage also helps clinics select embryos for single embryo transfer. This approach is often recommended after blastocyst culture to support the chance of success while reducing the risk of twins. Selecting the embryo with the best apparent potential is therefore part of planning the transfer, not simply awarding a laboratory score.
Embryo development is continuous, whereas a grade records an assessment at a particular point. An early blastocyst can develop into a full blastocyst, changing the number used to describe it. A different grade at a later assessment may therefore reflect progression rather than a contradiction.
Freezing and thawing can also alter what an embryo looks like. After vitrification, it may appear smaller or more shrunken and need time for re-expansion. That appearance can be normal after the freezing-and-thawing process and should not, on its own, be treated as evidence that the embryo has lost its potential.
There can also be variation between embryologists’ assessments. When embryos move between clinics, a different embryologist may assign a different rating even though the embryo itself is the same. A useful discussion is therefore not just whether the code has changed, but when it was recorded and what the laboratory believes the change represents.
A morphologically perfect embryo can be euploid or aneuploid. Conversely, an embryo graded BC or CC can be euploid and result in pregnancy. Appearance and genetic status are different pieces of information, which is why a beautiful-looking embryo is not necessarily the first choice when genetic results are available.
When testing has identified euploid embryos, euploid status takes priority over morphology. The laboratory then uses appearance to help choose among those embryos. In other words, grading still has a role, but it operates within the additional information supplied by testing.
A euploid result does not eliminate every possible reason for an unsuccessful transfer. An embryo may stop developing because of factors involving the embryo itself, the uterine environment or other influences. If several euploid embryos have been transferred without success, the doctor may need to investigate the uterus and consider further examinations or testing.
Testing also involves decisions about laboratory procedures. For an embryo that has already been frozen, thawing it for a PGT-A biopsy and freezing it again can affect implantation rates. The decision therefore needs to weigh the expected benefit of obtaining genetic information against the possible effect of those additional procedures.
A grade and a genetic result still may not support a precise percentage for an individual embryo. A day 6, 3BB euploid blastocyst, for example, retains potential even though its morphology is not among the strongest. The useful conversation is about its realistic prospects and place in the transfer plan, rather than a certainty that the available information cannot provide.
A time-lapse incubator adds another perspective by taking pictures of an embryo every few minutes. The resulting video shows how development unfolded rather than only how the embryo looked at a particular assessment. Developmental timing and abnormal events visible in that record may help embryologists choose which embryo to transfer.
That extra information should not be confused with proof that the incubator makes more embryos reach the blastocyst stage. Its value in selection is distinct from a claim that it improves development itself.
The same distinction matters when considering treatments marketed as ways to improve embryo quality. There is not enough evidence to know for sure whether add-ons such as PRP help. Laboratory conditions, well-trained embryologists and starting treatment earlier are the central factors identified in the notes, with age having the greatest influence on embryo quality.
Supplements also call for realistic expectations. Patients should ask their doctor before taking them and should not expect a drastic change in egg or embryo quality. A disappointing grade can make additional treatment appealing, but an available intervention is not necessarily one with an established benefit.
An embryo grade is useful information, but it is not a complete forecast. Its strongest role is helping a clinic decide which embryo to transfer first, alongside developmental timing and, when available, genetic results. Maternal age remains essential to understanding what those findings mean.
For patients, the most helpful discussion goes beyond whether a grade looks impressive. Ask how the embryo compares with the others available, why it has been placed in a particular transfer order and whether freezing, developmental timing or testing changes the interpretation. A high grade offers reason for optimism without certainty, while a lower grade can still leave a meaningful possibility of a healthy baby.
Yes. Low-grade blastocysts can produce healthy babies, which is why they are still transferred. However, studies show that the chance is reduced.
Yes. Embryo development is ongoing, so an early blastocyst can develop into a full blastocyst. Laboratory procedures can also affect its appearance.
After thawing, an embryo may look smaller and more stringy. This is normal because of the vitrification protocol, and the embryo may need time to re-expand.
When only morphology is available, embryo grade and developmental timing are used. If genetic testing is available, euploid status is checked first because it is more important than morphology. Morphology is then used to choose among euploid embryos.
Time-lapse data can also help by showing developmental patterns and abnormalities.
Yes. A 4AA embryo from a 40-year-old and one from a 30-year-old have the same morphology-based grade, but their chances of pregnancy are completely different. The embryo from the 40-year-old has much lower potential.
Maternal age is the most important factor when assessing chances. Morphology-based grading does not account for age or euploid status.
Many factors can be involved, including the embryo itself, the uterine environment or other uterine factors. If multiple euploid embryos are transferred without a result, the doctor may need to check the uterus and perform additional examinations or testing.
A specific percentage cannot be given. A day 6 3BB embryo is of medium to poor quality, but it still has a chance because it is a blastocyst.
An embryo with a truly aneuploid result has almost zero chance of pregnancy and should not be transferred unless the laboratory made a mistake.
Genetic counselling should be undertaken before transferring a mosaic embryo, and the decision should be based on the genetic report. Studies suggest that both high-level and low-level mosaic embryos seem to be safe.
Yes. Studies suggest that low-level mosaic embryos have a very high chance of producing a pregnancy, almost similar to a euploid embryo.
They are rarer than euploid embryos. An approximate estimate is one per month, although this is uncertain.
A time-lapse incubator is a normal incubator with a camera that takes pictures of the embryo every few minutes. These images create a video of development from one cell to eight cells and then to a blastocyst.
It provides more information and is especially useful for PGT-A because the embryo can be monitored without being removed from the incubator.
There is no big statistical difference between a normal incubator and a time-lapse incubator in outcomes such as blastulation rate. Nevertheless, time-lapse is a useful tool, especially when PGT-A is performed.
There are many add-ons being tried, including PRP, but there is not enough certainty yet to know whether they help.
The main measures are ensuring perfect laboratory conditions, training embryologists well and coming for treatment earlier. Age is the most important factor affecting embryo quality.
Yes. This usually becomes apparent when patients change clinics or embryos are received from another clinic. Different embryologists may give different ratings, but the embryo itself is the same. Patients should trust the embryos in their clinic unless proven otherwise.
New studies suggest that freezing a day 5 blastocyst, thawing it for PGT-A biopsy and freezing it again can affect implantation rates. Every procedure can affect embryo quality and success rates.
The benefits may still be greater, which is why the procedure is chosen. Extra testing can sometimes be worthwhile after unsuccessful transfers.
CHLOE appears to refer to an AI system that classifies embryos using time-lapse incubator data. It gathers developmental information, including timing to the four-cell stage and patterns such as reversed cleavage, and gives a score that can help determine which embryo to transfer first.
It is too early to evaluate oocytes in that way. Morphology is not the most important factor for oocytes. For now, the main focus should be maturity, such as whether the oocyte is mature or at the M1 or M2 stage.
Noninvasive tests exist, but they are not widely used yet and do not replace normal PGT. PGT is invasive because cells must be taken from the embryo, and the embryo must be frozen and thawed. Noninvasive PGT may be the future, but it is not there yet.
