When you’re staring at your embryo report, especially the section with PGT results, it can feel like you need a decoder ring. Letters. Numbers. Day 5 vs. Day 6. Grades like 4AA or 3BB. Words like “euploid” or “mosaic.” It’s easy to assume your embryology PGT results tell a clear, ranked story, or that there’s a “best” embryo and a “worst” one. But that’s not how embryologists actually think about it. An embryo report isn’t a verdict on your future pregnancy. It’s a snapshot in time, layered with probabilities, lab context, and clinical judgment. And understanding that nuance can make the whole process feel a little less mysterious.
How embryologists evaluate PGT results first
What does it really mean when an embryo is labeled Day 5, Day 6, or Day 7 after PGT testing? And how do embryo grading and genetic testing results actually factor into those labels?
Traditionally, embryos that reach the blastocyst stage by Day 5, meaning they develop and expand more quickly, have been considered superior to those who reach a similar stage by Day 6. But once genetic testing enters the picture, the decision becomes more complex. Here’s why: timing a biopsy isn’t just about the calendar. It’s about cell number, structure, and resilience.
Day 5 vs. Day 6 embryos: why timing affects PGT results
For example, is it better to biopsy a smaller Day 5 embryo with fewer cells, one that may struggle to re-expand after thaw, or to wait an extra day so the embryo can grow, expand further, and develop more trophectoderm cells? With more cells present, the biopsy removes a smaller percentage of the embryo overall. Increasingly, many labs are choosing this second approach. They’re seeing higher rates of conclusive PGT results, improved survival after thaw, and in some cases, stronger pregnancy outcomes.
When embryologists decide whether an embryo is ready for biopsy, they’re looking beyond just “Day 5” or “Day 6.” They carefully assess how each part of the embryo is developing, especially the inner cell mass (which becomes the fetus) and the trophectoderm (the outer layer of cells that becomes the placenta and is used for biopsy).
Sometimes the trophectoderm looks robust enough to safely remove cells for PGT testing, while the inner cell mass is still catching up. In that scenario, an embryologist may recommend waiting another day. Because once an embryo is biopsied, it’s frozen in its current state. And thoughtful timing can help preserve its future potential.
This is why embryo grading, developmental timing (Day 5 vs. Day 6), and PGT results aren’t competing data points. They’re interconnected pieces of a much bigger picture — one that skilled embryologists interpret with far more nuance than a simple ranking sheet suggests.
Embryo grading vs. genetic testing
Embryo grading and genetic testing measure two very different things, and understanding that distinction can make your report much less confusing.
Simply put, embryo grading is a visual assessment. It’s based on what an embryologist sees under the microscope. And while labs follow standardized systems, grading is still somewhat subjective. Ask a room full of embryologists to evaluate the same embryo, and you may hear slightly different opinions.
The most commonly used system is the Gardner grading scale, which evaluates three components of a blastocyst.
How the Gardner grading scale works
The Gardner system looks at:
- Expansion (graded 1–6)
How expanded the blastocyst is, and whether it has started hatching from its shell. - Inner cell mass (graded A–D)
The cluster of cells that will become the fetus. - Trophectoderm (graded A–D)
The outer layer of cells that will become the placenta — and the part biopsied for PGT testing.
Inner cell mass (ICM) grading
The inner cell mass becomes the baby. Its grade reflects how many cells are present and how organized they appear.
- Grade A: Many tightly packed cells
- Grade B: Several cells, more loosely grouped
- Grade C: Very few cells
- Grade D: Poorly defined or difficult to identify
Embryologists pay close attention here, even though this portion isn’t biopsied, because it reflects developmental potential.
Trophectoderm grading
The trophectoderm becomes the placenta and is the portion sampled during PGT testing.
- Grade A: Many cells forming a cohesive layer
- Grade B: Fewer cells forming a looser layer
- Grade C: Very few, larger cells
- Grade D: Sparse or uneven cell structure
Because this is the layer biopsied, its quality can influence how safely and effectively genetic testing is performed.
Where genetic testing fits in
Genetic testing (PGT) evaluates something entirely different: chromosomal makeup. An embryo can look beautiful under the microscope and still be aneuploid. And an embryo with average visual grading can come back euploid.
Most labs assign an embryo a grade at the time of biopsy. Some will reassess and update that grade later, especially at the time of transfer, because embryos can continue to expand and change after warming. This variation highlights two important truths:
- Grading reflects a moment in time.
- Genetic testing reflects chromosomal status.
How PGT results are interpreted in context
PGT results are an important piece of the puzzle, but they are never interpreted on their own. Embryologists look at genetic findings alongside key factors like age, egg source (using your own eggs versus donor eggs), sperm source, and an individual’s treatment history. For example, age plays a role in how common chromosomal abnormalities are, while both the egg and sperm sources can influence expectations around embryo development and success rates. Prior cycles, fertilization patterns, and embryo progression also help provide valuable context for what the results may mean.
This is why two embryos with the same PGT outcome may be viewed differently depending on the person or couple pursuing treatment. The results are part of a broader clinical picture, not a standalone verdict.
Embryologists also think in terms of relative potential, not guarantees. A chromosomally normal embryo may have strong potential, but it is never a promise of success. Similarly, an embryo with lower relative potential is not automatically ruled out. Lab teams use PGT results to help prioritize embryos and guide decision-making, understanding that embryo development exists on a spectrum rather than as a simple pass-or-fail outcome.
Common Misconceptions Embryologists See
“One euploid embryo guarantees success.”
A euploid result is meaningful and encouraging, but it is not a guarantee. PGT tells us about chromosome copy number in a small biopsy of trophectoderm cells, not whether an embryo will implant or result in a live birth. Many other factors still matter, including the uterine environment, the possibility that the biopsy may not fully represent the entire embryo, timing, and aspects of biology we cannot yet measure.
“Lower grades mean low chances.”
Morphology grades describe how an embryo looks under the microscope at a single point in time to an embryologist. They do not measure genetic health or developmental potential, and to some degree, are subjective. Lower-graded embryos can be euploid and capable of healthy pregnancies, while higher-graded embryos can be aneuploid and/or not successfully implant. Grading provides context, not a verdict.
“Mosaic means unusable.”
Mosaic results are often misunderstood. Mosaicism means the biopsy contained a mix of chromosomally normal and abnormal cells, not that the entire embryo is abnormal. A small but meaningful percentage of embryos fall into this category, though reported rates vary by lab and testing platform. Outcomes can vary widely depending on the level and type of mosaicism. Importantly, different genetics labs use different thresholds to define what counts as euploid, low-level mosaic, or high-level mosaic, which means the same embryo could be classified differently depending on where it is tested.
“More testing means more certainty.”
PGT adds valuable information, but it does not eliminate uncertainty. Results are based on a small sample of cells, technology has limits, and biology is dynamic. More testing does not always mean clearer answers, it often means more data to interpret thoughtfully.
From an embryologist’s perspective, these misconceptions are completely understandable. They come from a natural desire for clarity and reassurance in a process that is inherently probabilistic. PGT is a powerful and helpful tool, but it is most effective when viewed as one piece of a much larger picture.
What matters less than patients think in PGT results
When it comes to the letter grades in the Gardner scale, it can help to think of them like grades on a school assignment. Two teachers might read the same essay and walk away with slightly different opinions. One might see it as a clear A, while another might reserve A’s for only the very best work and grade the same essay a B.
Embryo grading works in a similar way. While the A–D scale is standardized, assigning those letters still involves human judgment. Because embryologists are evaluating visual features under a microscope, some variability is inevitable. That does not mean the grading is wrong, just that it reflects professional interpretation rather than an exact measurement.
This is why letter grades are best viewed as helpful context, not absolute predictors, especially when PGT results come into play. A visually beautiful embryo that earns an “A” grade under the microscope can still come back as aneuploid. At this point, we simply do not have the tools to determine genetic normality based on appearance alone. Morphology tells us how an embryo looks, not what its chromosomes look like, which is why genetic testing provides an entirely different layer of information.
How embryologists think about embryo selection
Embryo selection is about prioritizing embryos based on the information available, not identifying a single “perfect” option. Transfer order is usually guided first by genetic status, with euploid embryos prioritized when available. Developmental timing and morphology are then used to help fine-tune that order.
When multiple euploid embryos exist, differences in day of development or appearance may guide decisions, but these embryos often have more similarities than differences in potential.
When there are few or no euploid embryos, selection becomes more individualized. Mosaic or inconclusive results may be considered alongside treatment history and clinical goals, recognizing that no two situations are the same.
Read More: Should I Do PGT Testing with Donor Eggs?
What embryologists want intended parents to understand about embryo grading and PGT
As a former embryologist, I can tell you that embryos are biologically complex and often unpredictable. Even in the lab, where conditions are carefully controlled and every detail is monitored, development does not always follow a straight or expected path. Variability is part of biology, not a failure of the process.
PGT is an incredibly powerful tool. It allows us to see chromosomal information that morphology alone cannot reveal and helps guide decisions around transfer and treatment. That said, it is not a crystal ball. A euploid result does not guarantee success, and an aneuploid result can be difficult to process, even when it accurately reflects the embryo’s genetic makeup.
The anxiety surrounding PGT results is real and valid. These are not abstract data points. They represent hope, time, and deeply personal goals. Feeling nervous, overwhelmed, or unsure about what the results mean is completely understandable.
If the results feel unclear or leave you with more questions than answers, please know this: you are not missing something. Even as embryologists, we interpret layers of information and probabilities rather than absolutes. Uncertainty reflects both the complexity of early human development and a field that continues to evolve, guided by careful science, experience, and thoughtful decision-making at every step.
