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Thanks to the current level of assisted reproductive technology (ART), we now have a unique opportunity to closely monitor the very first hours and days of human embryo development — from fertilization during IVF to the formation of the blastocyst — all under the microscope. Each stage the embryo passes during this time plays a critical role in proper development.
Sometimes, everything seems to be progressing normally — but then the process unexpectedly stops. Even with ideal fertilization, not all eggs complete the full developmental cycle. Some arrest at very early stages, never reaching the morula or blastocyst stage (which typically forms between days 5 and 7 post-fertilization, containing around 156–200 cells). Why does this happen? Let’s take a closer look.
What does successful fertilization mean? Why doesn’t it guarantee further embryo development?
Just 16–18 hours after fertilization, embryologists assess the presence of two pronuclei (2PN) — one from the sperm and one from the egg. This 2PN stage confirms the proper fusion of genetic material, meaning the resulting zygote carries a full diploid set of chromosomes (46,XX or 46,XY) and is theoretically capable of further development.
However, even if 2PN is observed, not all fertilized eggs go on to form viable embryos. Fertilization is only the first — albeit essential — step. What follows is a complex series of molecular and cellular processes: activation of the embryonic genome, a sequence of accurate mitotic divisions, and the formation of symmetrical blastomeres. Even small disruptions during these early events can result in developmental arrest.
This is not an anomaly but rather a common phenomenon — and it occurs both in natural conception and in IVF. According to current estimates, approximately 40–60% of fertilized eggs never develop into blastocysts [1].
The biological system is highly selective — it naturally “filters out” cells with low developmental potential as part of evolutionary selection. Understanding these mechanisms allows embryologists to better assess each embryo’s chances for continued development.
A zygote 18.5 hours after ICSI fertilization showing two pronuclei (2PN). Magnification: 400x
An oocyte fertilized via ICSI showing one pronucleus (1PN), indicating an abnormal (aneuploid) chromosomal set
Following ICSI, one large pronucleus (bottom) and two smaller ones are visible — a total of three pronuclei (3PN), suggesting an aneuploid chromosomal set resulting from fertilization of a giant oocyte. Magnification: 400x
Why can embryo development stop during the first cell divisions?
Early embryonic development is an incredibly complex and sensitive process. For the embryo to successfully complete its initial cell divisions, multiple systems — genetic, cellular, and metabolic — must work in perfect coordination. Even minor disruptions can lead to developmental arrest.
- Aneuploidy and mitotic errors. If chromosomes do not divide evenly during mitosis, the embryo receives an abnormal set of chromosomes and is likely to stop developing. Such errors can be caused by age-related changes in the oocyte or defects in the sperm.
- Failure of embryonic genome activation (EGA). In the first few days after fertilization, the embryo relies on maternal RNA and proteins stored in the egg. But by day 2–4, it must begin to activate its own genome — a process known as embryonic genome activation (EGA, also called zygotic genome activation). If this switch fails or is faulty, the embryo will stop dividing. Contributing factors may include epigenetic disruptions, DNA instability, or chromatin structure defects. According to a systematic review (Vera-Rodriguez et al., 2015), EGA is one of the critical checkpoints where many embryos with low developmental potential fail to progress.
- Damaged gametes. Even if sperm and egg cells appear morphologically normal, they may carry hidden issues such as DNA fragmentation, centrosome abnormalities, cytoskeletal defects, or irregular epigenetic marks. These are often undetectable through routine assessment but significantly reduce developmental potential.
- Stressful culture conditions. Fluctuations in temperature, pH, humidity, light exposure, or mechanical disturbance (e.g., frequent opening of the incubator) can negatively impact the embryo. Even brief exposure to suboptimal conditions may critically impair its ability to develop further.
These are considered the most common reasons for early developmental arrest, but in many cases, there is no clear explanation. Negative factors can act independently or together, and they are not always predictable or preventable. Embryo arrest is a multifactorial phenomenon — shaped by both intrinsic (genetic) and extrinsic (laboratory) factors. Even with the most advanced equipment and controlled conditions, a degree of biological unpredictability remains.
What changes can be observed under the microscope?
One of the embryologist’s main responsibilities is the daily assessment of embryo quality to determine which ones have the highest potential for continued development and successful transfer to the uterus. This evaluation is based on visual characteristics that can be observed under a microscope. Some of these features may indicate reduced developmental potential.
- Fragmentation — the presence of small cytoplasmic fragments between blastomeres. A small amount of fragmentation is considered normal, but if it exceeds 25–30%, it may be a sign of cellular stress, and is often associated with lower implantation potential.
Day 2 embryo at the 4-cell stage with symmetrical blastomeres and mild fragmentation (10–15%).
4-cell embryo showing 15–20% fragmentation. A single large fragment is present along with dispersed smaller ones. Blastomeres are equal in size.
4-cell embryo with 40% fragmentation and uneven blastomeres of varying sizes.
- Blastomere asymmetry — cells of unequal size, indicating uneven cell divisions. Ideally, early-stage blastomeres should be symmetrical in both shape and size
Day 2, 4-cell embryo with blastomeres of unequal size
6-cell embryo showing two large and four smaller blastomeres.
- Vacuoles and Cytoplasmic Granularity. The presence of vacuoles or a granular cytoplasmic structure — such as small internal cavities or uneven texture — may indicate metabolic stress, cellular dysfunction, or problems with organelle activity.
Fertilized oocyte at 20 hours post-ICSI with two visible pronuclei (2PN) and signs of vacuolization (small round cytoplasmic structures).
Fertilized oocyte with granular cytoplasmic texture visible at the periphery.
- Delayed Cell Division. If an embryo has only 2–4 cells on day 3 instead of the expected 6–8, this may indicate delayed or abnormal development.
These features are not strict indicators of success or failure — some embryos with non-ideal characteristics may still result in pregnancy. However, they help embryologists rank embryos by developmental potential and guide clinical decisions.
What Does the Embryologist Do?
The embryologist closely monitors each fertilized oocyte every day, carefully documenting all observations. Their main goal is to determine which embryos have the highest chance of successful implantation and pregnancy.
To make this decision, they evaluate several key factors:
- Developmental timing (morphokinetics): The speed and synchrony of cell divisions. This is often assessed using modern time-lapse imaging systems. When division timing falls within expected ranges, it suggests healthy development.
- Morphological features: Similar to morphokinetics, this includes cell shape, size, and uniformity, as well as the presence of fragmentation or vacuoles.
- Culture conditions: These include the composition of the culture medium, temperature stability, pH balance, and efforts to minimize stress (such as limiting incubator openings or light exposure).
- Individual patient characteristics: These may include age, medical history, and results from genetic screening (e.g., PGT). Such context helps in selecting the most suitable embryo for transfer.
By day 5 or 6–7, embryos that have reached the blastocyst stage are thoroughly evaluated as candidates for transfer or cryopreservation. The embryologist selects those with the highest implantation potential and likelihood of leading to a healthy pregnancy.
Embryos that do not meet these criteria or have stopped developing may be kept under observation for a short time or discarded if further development ceases entirely.
This process requires not only technical expertise but also experience and attention to detail — because the success of a pregnancy often depends on selecting the right embryo.
How do modern technologies help predict embryo development?
In contemporary embryology laboratories, incubators with continuous time-lapse video monitoring systems are increasingly used. These allow the creation of complete “films” showing how each embryo developed. Such observation is a valuable auxiliary tool, as it enables monitoring the pace of embryo development and forming a so-called morphokinetic profile — the embryo’s own “biography” of life during the first days.
A large amount of information about optimal development dynamics has already been collected. By comparing the established morphokinetic profiles with clinical databases, it is possible to create a well-founded prediction and assess the chances of successful implantation.
Artificial intelligence (AI) also plays an important role in assessing embryo quality. It can detect the smallest changes in development that may be imperceptible to the human eye. In a 2025 study, AI models demonstrated an average accuracy of 75.5%, whereas embryologists achieved on average 65.4%. Thanks to integration with clinical databases, the accuracy of AI predictions increased to a record 81.5% [3].
However, it is very important to remember that evaluation based solely on morphokinetic parameters cannot fully replace preimplantation genetic testing for aneuploidy (PGT-A), nor does it substitute a specialist’s consultation, since it cannot account for possible genetic alterations. Therefore, it serves only as a useful tool that helps make more accurate and objective decisions.
The incubator with a time-lapse system in the “Reprolife” embryology laboratory provides continuous and thorough monitoring of embryo development.
Conclusions
Embryo developmental arrest is not a rare mistake but rather a part of natural selection that occurs both in natural and laboratory fertilization. Unfortunately, even under ideal fertilization conditions, about half of embryos do not reach the blastocyst stage.
The causes can be various: genetic abnormalities, invisible cellular defects, or simply the factor of biological unpredictability. It is a complex and delicate system where every detail matters.
It is important to remember: in embryology, there is no “perfect scenario.” There is experience, attentiveness, and technology that allow selecting the best embryos for each patient. Therefore, daily monitoring is not a formality but a tool that can truly change the outcome.
Unfortunately, it is impossible to eliminate all risks completely, but everything can be done to increase the embryo’s chances for successful implantation. We understand the complexity of this journey — and that is why we make every effort to help you achieve the joy of your long-awaited parenthood.
References:
- Orvieto R, Jonish-Grossman A, Maydan SA, Noach-Hirsh M, Dratviman-Storobinsky O, Aizer A. Cleavage-stage human embryo arrest, is it embryo genetic composition or others? Reproductive Biology and Endocrinology [Internet]. 2022 Mar 17;20(1). Available from: https://doi.org/10.1186/s12958-022-00925-2
- Filipova EP, Uzunova KH, Vekov TY. Comparative analysis of therapeutic efficiency and costs (experience in Bulgaria) of oral antidiabetic therapies based on glitazones and gliptins. Diabetology & Metabolic Syndrome [Internet]. 2015 Jul 15;7(1). Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4539691/
- Moysis, Lazaros, Lazaros Alexios Iliadis, George Vergos, Sotirios P. Sotiroudis, Achilles D. Boursianis, Achilleas Papatheodorou, Konstantinos-Iraklis D. Kokkinidis, Mohammad Abdul Matin, Panagiotis Sarigiannidis, Ilias Siniosoglou, and et al. 2025. “Artificial Intelligence-Empowered Embryo Selection for IVF Applications: A Methodological Review” Machine Learning and Knowledge Extraction 7, no. 2: 56. https://doi.org/10.3390/make7020056
- European Society of Human Reproduction and Embryology. Atlas of Human Embryology: From Oocytes to Preimplantation Embryos. Translated by L.V. Melnyk. Access mode:http://treatment.kiev.ua/wp-content/uploads/Atlas-Embriologii-liudyny.pdf
