A healthy embryo changes dramatically in appearance over just five days of development, moving from a simple cluster of cells to a complex, fluid-filled structure called a blastocyst. If you’re going through IVF, your clinic will evaluate your embryos at specific checkpoints, and understanding what they’re looking for can help you make sense of the grading reports you receive.
Day 3: The Cleavage Stage
By day 3 after fertilization, a healthy embryo contains 6 to 8 cells, each called a blastomere. These cells sit inside a protective outer shell called the zona pellucida, which looks like a clear, round halo surrounding the embryo under a microscope. At this stage, embryologists are looking at three things: cell count, cell symmetry, and fragmentation.
The ideal day 3 embryo has 8 evenly sized, symmetrical cells. Cells that vary widely in shape or size suggest the division process isn’t going smoothly. Fragmentation, which appears as tiny irregular pieces broken off from the cells, is the other major concern. Some fragmentation is completely normal in human embryos, but the amount matters. Less than 10% fragmentation is considered excellent. Embryos with more than 25% fragmentation have significantly lower chances of implanting.
A top-grade day 3 embryo (often labeled “8A” on your report) has 7 to 8 evenly sized cells, minimal fragmentation, and no cells with multiple nuclei. Grade B embryos have the right cell count but more fragmentation or less symmetry. Lower grades typically mean fewer cells, heavier fragmentation, or both.
Day 5: The Blastocyst Stage
The transformation between day 3 and day 5 is striking. Instead of a loose collection of individual cells, a healthy day 5 embryo has developed into a blastocyst: a hollow sphere with a fluid-filled cavity, a distinct inner cell mass (the clump of cells that will become the fetus), and an outer layer of cells called the trophectoderm (which will become the placenta). Under a microscope, a good blastocyst looks like a round ball with a clearly visible dark cluster pressed against one side of the interior wall.
Most IVF clinics use the Gardner grading system to score blastocysts, which produces a grade like “4AA” or “3BB.” The number (1 through 6) describes how expanded the embryo is. A score of 1 means the fluid-filled cavity takes up less than half the embryo’s volume. By score 3, the cavity fills the entire embryo. At 4, the embryo has expanded beyond its original size and the zona pellucida is visibly thinning. Scores of 5 and 6 indicate the embryo is hatching out of or has completely escaped its shell.
The two letters grade the inner cell mass and the outer layer, respectively. An “A” for the inner cell mass means a tightly packed cluster with many cells. A “B” means a looser grouping with fewer cells. A “C” means very few cells. The same scale applies to the trophectoderm: “A” is a smooth, cohesive layer of many cells, “B” is fewer cells forming a loose layer, and “C” is very few, large cells. A grade of 3BB or higher is generally considered good quality.
How Grades Translate to Success Rates
Embryo appearance does correlate with pregnancy outcomes, though it’s not the whole picture. In studies of genetically normal (euploid) embryos, good-quality blastocysts (graded 3BB or above) produced live birth rates around 57%, compared to roughly 40% for lower-quality embryos. That’s a meaningful difference, but it also means lower-graded embryos still result in healthy pregnancies a significant portion of the time.
Timing also matters. Embryos that reach the blastocyst stage on day 5 tend to outperform those that take until day 6. Day 5 blastocysts have clinical pregnancy rates around 55% compared to about 44% for day 6 blastocysts, and live birth rates of roughly 49% versus 39%. Interestingly, even a lower-quality day 5 blastocyst may have similar potential to a high-quality day 6 blastocyst, suggesting that developmental speed is itself a marker of embryo health.
What Unhealthy Embryos Look Like
Beyond fragmentation and slow growth, embryologists watch for a few specific red flags. Vacuoles, which look like small, round, bubble-like inclusions in the cells, are one of the more concerning findings. They appear as circular, highly refractive spots within the cell’s interior. Embryos with vacuoles form blastocysts at significantly lower rates, and as either the size or the number of affected cells increases, the chance of reaching a high-quality blastocyst drops sharply, from roughly 29% down to about 15%. They’re also associated with higher rates of chromosomal mosaicism, where some cells in the embryo have an abnormal number of chromosomes.
Other warning signs include uneven or asymmetric cell division, cells with more than one nucleus (multinucleation), extremely heavy fragmentation that makes it impossible to count individual cells, and failure to progress through developmental stages on schedule.
The Hatching Process
Before a blastocyst can implant in the uterus, it needs to break free from the zona pellucida. A healthy hatching embryo (grade 5 on the expansion scale) will show cells pushing through a breach in the shell, with the trophectoderm starting to protrude outward. About two-thirds of embryos hatch through a single breach point, while roughly one-third actually begin breaking through at multiple sites simultaneously. A fully hatched embryo (grade 6) has escaped the shell entirely and appears as a free-floating structure without its surrounding halo.
Beyond What the Eye Can See
Traditional embryo grading is done by an embryologist looking through a microscope, which makes it inherently subjective. Two embryologists can assign different grades to the same embryo. Time-lapse imaging systems now photograph embryos every few minutes throughout development, tracking dozens of timing milestones: when pronuclei form, when each cell division occurs, how long each cell cycle takes, when compaction begins, and when the blastocyst cavity first appears. Embryos that hit these milestones within expected windows are more likely to be chromosomally normal.
Artificial intelligence systems are pushing this further by analyzing features invisible to the human eye. These tools evaluate subtle cytoplasmic movements in the earliest hours of development, the precise diameter and rate of blastocyst expansion, and patterns in the timing data that predict whether an embryo is chromosomally normal. Heat maps generated by these systems highlight regions of the embryo that distinguish viable from nonviable embryos, sometimes differentiating between embryos that look identical to an experienced embryologist. These tools are increasingly used alongside, not in place of, traditional grading to select the most promising embryo for transfer.