An embryo can look excellent under the microscope and still carry genetic problems, a limit patients should understand before they pin too much hope on appearance alone.
In this fascinating Inside the IVF Laboratory webinar, Tomáš Rieger, PhD, Head Embryologist at GYNEM Fertility Clinic, takes us behind the scenes of the IVF laboratory to explain how embryologists assess reproductive cells and developing embryos — and where the limits of that assessment lie.
Hosted by: Barbara Scott, Chair of the Association of Reproductive Reflexologists & Founder of Seren Natural Fertility.
Throughout an IVF cycle, the lab is making quality assessments. Embryologists assess oocytes, sperm, and embryos under microscopes, using both standard magnification and higher-powered systems when needed. Eggs and embryos can be evaluated at lower magnification, while sperm morphology often requires much closer inspection, including software magnification up to 6,000. Some labs also use computer analysis and AI to make these assessments more consistent and reduce human error.
Those observations guide decisions at every stage, from identifying mature eggs to selecting sperm for fertilisation and prioritising embryos for transfer. But appearance has a ceiling. The most important factor for treatment success is the genetic information inside the cell, and that cannot be seen just by looking down a microscope.
This gap between appearance and genetics explains one of the most frustrating parts of IVF. A beautiful embryo may still fail, while a lower-graded embryo may still become a healthy child. The lab can estimate potential. It cannot promise an outcome.
The first assessment happens right after oocyte pickup, when eggs arrive in the lab in different stages of maturity. The key visual sign of maturity is the first polar body. If that marker is visible, the egg is considered mature and ready for fertilisation. If it is not visible, the egg is considered immature.
Not every immature egg is immediately ruled out. Some are kept in culture during the day to see whether they mature, and if no mature eggs are available, GV oocyte or MI oocyte stages may even be cultured overnight. If the egg extrudes the first polar body the next day, rescue ICSI can be performed. These eggs are usually less effective than mature eggs, but they can still occasionally produce blastocyst embryos and healthy babies.
Egg quality also depends on how the egg looks internally. Some show abnormal aggregates or irregular cytoplasmic quality, features associated with a lower chance of forming a good blastocyst and a successful outcome. Even so, an abnormal-looking egg is not automatically discarded. Some still develop into usable embryos, which is why many labs give them a chance while being honest with patients that the odds may be lower.
The number of eggs retrieved matters too, but not in the way many patients assume. More is not always better. The optimal number at pickup is around 10 to 15. When fewer than 10 are collected, there may not be enough material to work with. When more than 15 are collected, overall egg quality may decline, and final outcomes may be worse.
Male-factor issues are common in fertility treatment, so sperm selection is a major part of lab work. Embryologists use microscopic selection methods, including the IMSI procedure, to choose the best-looking sperm for fertilisation.
Even here, the numbers can be surprising. That can sound alarming to patients reading a semen analysis, but it also shows why raw percentages do not tell the whole story.
Fertilisation still isn’t guaranteed, even when the lab selects the best sperm available, and the eggs appear mature. About 15% of oocytes may remain unfertilized for reasons that are not fully understood. If repeated abnormal development appears across many eggs in the same cycle, that can raise concern about sperm DNA quality, but it is often difficult to know whether a problem began in the sperm, the egg, or both.
Timing can offer clues. The first three days of embryo development depend on materials already present in the egg. If embryos look abnormal from the beginning, the problem may be more likely related to the egg or to low quality in both gametes. If development looks normal for the first three days and then problems appear during compaction and blastulation, sperm may be contributing more to the issue.
Once fertilisation happens, the embryo enters a series of visible developmental stages. A normally fertilised egg should show two pronuclei, one from the mother and one from the father. That is the classic sign of accurate fertilisation.
Not every embryo follows that textbook pattern. Irregular pronuclear number or size can occur. Embryos derived from one pronucleus or no visible pronuclei may still produce healthy children in some cases, and if there is no better embryo available, they may be considered for transfer if the patient understands the risks. Embryos with three pronuclei, however, are considered to have a very high chance of genetic problems and are not allowed for transfer without PGT testing.
After that, embryologists track the embryo through the cleavage stage. Day 0 is egg retrieval, and by day 5 the embryo should ideally reach the blastocyst stage if development is going well. Along the way, the lab watches for cell number, symmetry, and fragmentation.
Fragmentation is common in human embryos. These are membrane-covered cell fragments without genetic information. A higher level of fragmentation is associated with a lower chance of a healthy outcome, but it does not mean there is no chance. One grading example described a grade 3 embryo as having more than 25% of its volume made up of fragments.
IVF is full of grey zones. Lower quality does not mean hopeless. It means lower probability.
Traditional embryo culture allows only brief checks because embryos should not be disturbed too often. Time-lapse incubation changes that by photographing embryos every 10 minutes over 5 days, creating a movie of development rather than a few isolated snapshots.
That extra detail can reveal how and when an embryo divides, when compaction begins, and how it progresses toward blastocyst formation. Morphokinetics — the timing of these developmental events — can help identify embryos with a better chance of becoming a healthy child and can help decide which embryo should be transferred first.
This is especially useful because many IVF cycles narrow dramatically as they progress. A patient may start with 10 or 15 oocytes, develop up to 10 embryos in culture, and end up with only 2 blastocysts. When there are only a few embryos left, choosing the transfer order matters.
Still, time-lapse data is not used as a reason to discard lower-scoring embryos early. Embryos are cultured for the full 5 to 6 days, and lower-scoring embryos may still be used later if the first transfer is unsuccessful.
By the blastocyst stage, grading becomes more sophisticated. Embryologists assess expansion and the quality of the inner cell mass and trophectoderm. Expansion is graded from 1 to 6, reflecting how far the embryo has progressed from early blastocyst to hatching and fully hatched stages.
The letters matter too. The first letter refers to the inner cell mass, which becomes the future child. The second refers to the trophectoderm, which contributes to implantation-related structures. An A grade means the structure is clearly visible with many well-arranged cells. B is less optimal. C means it is barely seen.
On paper, this can make embryo ranking look precise. In reality, grading is only a probability tool. A 4 AA or 5 AA embryo can have up to 150 cells, while a 4 CC embryo may have only a few cells in the inner cell mass and up to 10 or 20 cells in the trophectoderm. Better-looking embryos generally perform better. But they are far from guaranteed.
Even excellent-looking AA embryos may have around 50% DNA fragmentation or DNA mistakes. One figure given for AA embryos was pregnancy rates of up to 50% to 55%. Another estimate put even a beautiful embryo’s chance of success at only about 65% chance for success, not 70%, 90%, or 100%.
The flip side is just as important for patients who have only lower-graded embryos. In a large Australian study, CC embryos still produced healthy children in 14% of cycles. A poor grade can lower expectations, but it does not automatically end the conversation.
Because visual grading cannot reveal chromosome status, preimplantation genetic testing can add another layer of information. In PGT-A, the lab removes 5 to 10 cells from the trophectoderm rather than the inner cell mass, because the inner cell mass is the future child. The embryo is then frozen while the lab waits for results, which take approximately 2 to 4 weeks or about 30 days.
If the embryo shows 23 pairs of chromosomes, it is considered genetically normal for transfer in a PGT-A cycle. That can be especially useful when there are multiple blastocysts and the goal is to decide which should be transferred first and which should not be transferred because of major genetic problems.
But PGT-A is not perfect. Mosaicism is common in human embryos, and a biopsy samples only a small part of the embryo. Some results may reflect the sampled cells rather than the whole embryo, and the procedure itself is invasive. For patients with only one embryo available, that uncertainty matters more. The information may be less than 100% accurate, which can make testing less straightforward than it sounds.
In practice, PGT-A is most useful when there is a real choice to make among several blastocysts. It is less decisive when there is only one embryo and no alternative.
When embryos repeatedly fragment heavily or stop developing around day 3 or 4, patients naturally want a simple explanation. IVF rarely offers one. Fragmentation is multifactorial, but age was identified as the most important factor affecting DNA quality in eggs.
At 39 or 40 years of age, the chance of having healthy oocytes was stated to be only about 10% to 15%. In another discussion, the chance of a healthy oocyte at 39 years of age was described as around 20% to 25%. Either way, the message is the same: age-related genetic problems in eggs become a major limiting factor, and donor sperm cannot correct damage that is already present in the oocyte.
Some developmental patterns may be less ominous than they appear. Partial compaction, for example, may reflect an embryo’s self-assessing capacity or self-repairing quality, with the embryo discarding some cells rather than failing outright. That is one reason labs do not automatically discard embryos that look imperfect at a single moment in time.
Patients often assume success rates will tell them exactly where to go. They help, but they do not tell the whole story. Publicly available numbers such as cycle counts and outcomes can offer some insight into lab quality, yet marketing claims may not match real-world results.
Claims of an 80% chance after 1 transfer should be viewed sceptically. Even strong embryos can fail, and success depends on more than embryo appearance alone. Uterine factors and immunology may also matter, especially after repeated failed transfers.
For patients trying to judge a clinic, numbers are only one part of the decision. Meeting the team, asking how the lab works, and getting a sense of whether the clinic is transparent may be just as important.
IVF labs can learn a great deal from how eggs, sperm, and embryos look and behave, especially with tools like time-lapse incubation and PGT-A. But none of these methods can turn reproduction into a certainty. A top-grade embryo may still fail. A lower-grade embryo may still work. And sometimes the most honest answer is that biology remains unpredictable.
That uncertainty is difficult, but it is not the same as hopelessness. The practical goal of embryo assessment is not to find a perfect embryo. It is to make the best possible decision with the information available, preserve embryos that still have potential, and give each cycle its fairest chance.
