A beautiful-looking embryo can still fail, and a slower one can still become the right choice — a reality that shapes how embryologists like Sergi Rovira Fontanals, Lab director at Fertilab and Fertilabank in Barcelona, Fertilab, assess embryos during IVF.
Hosted by Eleanor Wharf, Clinical Embryologist & Accredited Fertility Coach, UK.
Patients often receive an embryo grade as if it were a final verdict. In practice, embryo grading is better understood as a tool for comparison. It helps a lab choose among available embryos, but it does not answer the question every patient cares about most: will this embryo implant and become a pregnancy?
The current gold standard for assessing embryo quality is morphology — the embryo’s appearance as it develops in culture. Most embryos are graded on day 5 or day 6, which is when embryos are commonly transferred now. That grade usually combines a number and two letters to describe what the embryo looks like at the blastocyst stage.
That information is useful, but limited. A good-looking embryo has a better correlation with implantation potential than a poor-looking one. Still, appearance does not reveal everything about the embryo’s genetic quality. A morphologically strong embryo may not be chromosomally normal, and a very poor-quality embryo can still be euploid. Those two facts are why embryo selection cannot rely on appearance alone, but also cannot rely only on genetics.
For patients, the practical message is simple: a grade is a snapshot, not a guarantee. It helps rank embryos within one cycle. It does not reliably explain why one embryo implanted and another did not, and it should not be used to compare embryos across different clinics.
Embryo assessment starts much earlier than the final grade. It begins at fertilization, then follows the embryo’s progress day by day. That matters because embryo development is dynamic, and the path an embryo takes can be as informative as the way it looks at the end.
The first check is whether fertilization appears normal. Embryologists look for 2 nuclei, 1 from the egg and 1 from the spermatozoa. After that, the embryo is expected to keep dividing. By day 3, a normal embryo is expected to have at least 8 cells. As development continues, the embryo should reach around the 16-cell stage on day 3 or day 4, then undergo compaction and form a morula.
By day 5 or day 6, the goal is a blastocyst. At that stage, the embryo develops a cavity and separates into two cell lines. The trophectoderm is the outer layer that will implant and become the placenta and other membranes. The inner cell mass is the cluster of cells that will become the baby. A good blastocyst is expected to have many trophectoderm cells and a compact inner cell mass.
This is why embryo quality cannot be reduced to a single image. The final grade reflects one moment, but the embryo’s earlier behavior also matters. A slow embryo may simply need more time. An embryo that looks unready in the morning may look very different a short time later.
One of the biggest misconceptions in IVF is that embryo quality is fixed and obvious. In reality, embryos change constantly. A grade assigned at one point may not reflect what happens next.
That is where time-lapse incubators can add context. These systems allow embryos to be monitored during culture without repeated removal from the incubator. In some labs, that means the embryo can remain in a stable environment from day 0 — the day of microinjection or insemination — until the end of culture. The advantage is not just convenience. It allows the lab to see how the embryo divides over time, rather than relying only on occasional checks.
That timing can affect decisions. An embryo may not be ready for biopsy at 09:00 but be ready by 11:00. A slow embryo on day 5 may become an excellent blastocyst on day 6. Some embryos even continue to day 7, although the chances of pregnancy are lower than with day 5 or day 6 biopsy.
For patients, this is a reminder that embryo grading is partly a judgment made at a specific moment. It is useful, but it is not destiny.
The hardest part of embryo grading is that appearance and chromosomes do not always match. Morphology and chromosomal integrity are complementary, not interchangeable.
In young women, around 20% of good-quality embryos are not normal. The proportion of top-quality embryos that are chromosomally normal decreases with age, and in patients over 37 years, fewer of the best-looking embryos are normal when tested by PGT-A.
That helps explain why the “best” embryo on paper may not be the right embryo for transfer. A high grade may reflect strong development and good structure, but it does not tell the lab whether the embryo is aneuploid or chromosomally normal. At the same time, a normal PGT-A result does not automatically make a poor-quality embryo a good transfer candidate. If the embryo’s overall quality is very poor, normal chromosomes may not be enough.
This is why many IVF decisions are not binary. Morphology can guide selection. PGT-A can add information. Neither one replaces the other.
For patients with only one or two embryos, the decision about PGT-A can feel especially fraught. Testing may offer useful information, but it also depends on whether the embryo is strong enough for biopsy.
PGT-A is ideally performed on day 5 or day 6. Day 3 testing misses a lot of information. Day 7 biopsy is possible in some cases, but the chances of pregnancy are lower than with day 5 or day 6, so that tradeoff needs to be understood before moving forward.
Sometimes the embryo is not good enough for biopsy in the first place. In that situation, the embryo may already be poor quality, and transfer without PGT-A may be a reasonable option if the patient understands that the chances are low. Transfer without PGT-A may also be acceptable in some cases when the patient accepts the possibility of abortion, implantation failure, or Down syndrome.
There are also gray zones. A PGT-A result may come back unclear. A second biopsy may then be considered, but only if the embryo is strong enough to tolerate another biopsy, another vitrification, and another warming. If it is not, transferring the embryo as it is may be the better choice.
These are not purely technical decisions. They are value-based decisions shaped by embryo number, embryo quality, age, and what risks a patient is willing to accept.
When a strong-looking embryo does not implant, patients often assume something must have been missed in the grading. Sometimes the explanation lies elsewhere.
The laboratory environment has to be stable throughout culture. The right culture media must be used, and they must work properly. Carbon dioxide and oxygen concentrations need to be adjusted so embryos remain at the correct pH. Surface temperature also matters. Even a good culture system can fail if the dish is not kept hot enough.
But the lab is only part of the picture. The uterus has to be receptive. The embryo needs the right uterine environment in order to implant. In some cases, microbiome studies may be considered, and the uterine implantation window can be studied to see whether progesterone should start earlier or later than expected.
Then there is sperm. sperm DNA fragmentation cannot be seen by simply looking at sperm, yet it may contribute to implantation failure or miscarriage. That makes it one of the more frustrating hidden factors in IVF, especially when embryo appearance looks reassuring.
IVF remains, in many ways, an experimental field. Some interventions are not supported by evidence-based medicine, which is why patients can hear about many add-ons without always getting a clear answer about what truly helps.
Not every cause of IVF failure is controllable, but some are. That distinction matters because patients often feel pressure to “do everything,” even when much of the outcome is outside their control.
The lifestyle factors mentioned most consistently are:
Smoking has a real impact on fertility results, and stopping smoking during treatment can still be beneficial. Overweight is also a detrimental factor for implantation. These changes are worth making because they are within a patient’s control and may improve the overall conditions for treatment.
Sperm-related interventions may also be considered in selected cases. If DNA fragmentation is raised, lifestyle changes are one option. oral antioxidants may help in some cases, but they should be discussed with an andrologist. In the lab, techniques such as microfluidics or annexin columns may help select spermatozoa with less fragmentation. hyaluron-binding sperm selection may also help in some settings, especially in women over 35, based on the theory that older eggs may not repair sperm damage as well.
None of this means patients are to blame if treatment fails. Lifestyle improvements can be useful and may help patients feel better during treatment, but they do not create certainty.
If there is one variable that outweighs almost everything else in IVF, it is age. Embryo grading can help prioritize embryos, but it cannot reverse the effect of ovarian aging.
The peak of fertility is around 32 years. By 41 or 42 years, getting pregnant is already very difficult. At 46 years, pregnancy with a woman’s own eggs is almost impossible. That is why age remains the most predictive factor in IVF, even when embryos look good under the microscope.
This also explains why there is no reliable way to “improve egg quality.” The eggs available later in life are generally not the best quality, and there is no evidence that treatments marketed to improve egg quality actually work.
For some patients, egg donation changes the equation entirely. Once donor eggs are used, the patient no longer needs to worry about her own egg quality, because the eggs come from a donor and should be of very good quality. In that setting, age is no longer the main limitation in the same way.
Embryo grades are most helpful when they are used for what they were designed to do: compare embryos within the same cycle. They are far less helpful when patients try to use them as a universal scorecard.
A grade does not tell you whether an embryo can implant and make a pregnancy. It does not tell you whether the embryo is chromosomally normal. It does not account for sperm DNA fragmentation, uterine receptivity, or the many laboratory and biological variables that shape IVF outcomes. And it does not translate neatly from one clinic to another.
That is why raw embryo information can be more confusing than reassuring. Videos, AI tools, and time-lapse data may generate more detail, but detail alone does not help unless it is interpreted in context. Patients need enough explanation to participate in decisions, especially when choices involve PGT-A, transfer timing, or whether a slower embryo is still worth waiting for.
The most reassuring truth about embryo grading is also the most humbling one: IVF is not a beauty contest. The embryo that looks best at one moment may not be the embryo that leads to a baby, and a slower embryo may still become a strong candidate a day later.
That uncertainty can be hard to live with, but it also means a disappointing grade is not the whole story. Embryo appearance matters. So do chromosomes, age, sperm quality, the uterine environment, and the conditions inside the lab. The goal is not to find a perfect embryo score. It is to make the best decision possible with the information available, while remembering that even a carefully managed cycle still has limits no one can fully control.
Embryo quality is assessed primarily by morphology, which is the gold standard. Embryologists also look at how the embryo divides during culture, how many cells it has each day, and how many nuclei are visible. Assessment is based on the whole process from fertilisation to embryo selection, not just the final-day appearance.
That depends on the technology in the laboratory. With time-lapse incubators that have built-in cameras, embryos do not need to be removed from the incubator. They are placed on day zero, which is the day of microinjection or insemination, and are not removed until the end of culture because a continuous culture system is used.
The day after insemination, the first assessment is fertilization. Normal fertilization is confirmed by seeing two nuclei, one from the egg and one from the spermatozoon. After that, the embryo is expected to divide into two cells with each division until it reaches the 16-cell stage around day three or day four, and then it should form a morula.
By day three, an embryo is expected to have at least eight cells. If it has fewer than eight cells, it is not considered ideal, although it may simply need another day of culture. In that situation, day five or day six may still be good options for transfer or vitrification.
By day 5 or day 6, the embryo should be a blastocyst. At that stage, a cavity has formed inside the morula, and two cell lines can be distinguished for the first time: the trophectoderm, which forms the surface and later contributes to the placenta and membranes, and the inner cell mass, which will become the baby. A good blastocyst has many trophectoderm cells and a large, compact inner cell mass.
Embryo grading correlates with implantation potential, but a morphologically good embryo is not always chromosomally normal. Morphology, chromosomal status, and expansion are complementary pieces of information. An embryo can look good and still not be suitable for transfer if it is not chromosomally normal. In young women, around 20% of good quality embryos are not normal.
No. Embryo grade alone does not tell us whether an embryo will implant or make a pregnancy. It is only one snapshot in time and does not always reflect what will happen next. Grades are useful for comparing embryos within the same cycle, but they should not be overinterpreted or compared directly between patients or clinics. The grade needs to be explained in clinical context by a clinician or biologist.
Embryo quality is not everything. In the laboratory, the correct culture media must be used, and the media must work properly. Carbon dioxide and oxygen levels in incubators must be adjusted to maintain the correct pH. Temperature and gas mix must remain stable throughout culture, and surface temperature is also important.
Outside the laboratory, sperm DNA fragmentation, PGTA performed correctly, smoking, overweight, and uterine factors may all affect outcomes. Microbiome studies and assessment of the uterine implantation window may also be relevant in some cases.
There are 3 main approaches. First, lifestyle changes are important, especially reducing smoking and alcohol. Second, oral antioxidants may help in some cases and should be discussed with an andrologist. Third, laboratory sperm selection techniques such as microfluidics or annexin columns may help select non-fragmented spermatozoa and reduce DNA fragmentation.
Good embryo grades do not guarantee success. If sperm number was insufficient for Zymot, Sperm slow is a good option. But other factors still matter, including whether the embryos are euploid, whether the uterus is okay, and whether immunology is normal. Some clinics also use immunologic treatments. A transfer without PGT-A may be reasonable in selected cases, but the chances are lower when embryo quality is poor.
A second biopsy may be needed if the PGTA result is unclear, but this depends on embryo quality. Some embryos cannot tolerate a second biopsy, a second vitrification, and a second warming. In those cases, it may be better to transfer the embryo as it is rather than rebiopsy it.
If an embryo develops more slowly, a day 7 biopsy may be performed, and pregnancies can still occur from these embryos. However, the chances of pregnancy are a little less than with day 5 or day 6 biopsy, and that should be explained clearly before proceeding.
The best-looking embryo may not be the right one, especially in older patients. In patients over 37 years old, fewer top-quality embryos are normal when tested by PGT-A, and that number decreases with age. Embryo grading helps with selection, but it does not tell us about genetic quality, so age and other treatment factors must also be considered.
Lifestyle changes and a better diet can help you feel better and more in control, even though you cannot control the genetic quality of your eggs or embryo development after fertilisation. If treatment does not work after making lifestyle changes, it is not your fault. These measures can also support mental health during fertility treatment.
