Modern rules for assessing the quality of oocytes and embryos that patients should know

At the beginning of 2025, ESHRE and ALPHA, two leading organizations in the field of reproductive medicine, updated their recommendations regarding the morphological (visual) evaluation of oocytes and embryos. These standards have become essential for clinical practice, as they allow embryologists worldwide to use a unified approach and to predict treatment outcomes more accurately.

The new recommendations replaced the 2011 version. Over the past few years, a significant amount of scientific data has been accumulated and new technologies have emerged, which have allowed for clarification of which morphological signs are truly related to the embryo's developmental potential and which have limited clinical significance. These clarifications served as the basis for the updated criteria.

Our blog on key events

In our blog, we covered in detail the event ESHRE 2025 in Paris, where key innovations in infertility treatment were presented.

New approaches to oocyte assessment

Oocytes are extremely sensitive cells. Their potential cannot be fully assessed, but morphological features provide the doctor with landmarks to predict their quality. The updated standards propose a clear and structured description system.

Main parameters:

• Maturity of the oocyte. MII-stage oocytes with the presence of a polar body are considered the most valuable. They are ready to be fertilized. Immature oocytes (GV and MI) cannot be used, but they can mature in vitro (RESCUE-IVM technology). In 2025, ESHRE/ALPHA clearly indicated for the first time that these oocytes could be used in clinical practice in cases of poor prognosis.
• Cytoplasm. Normally, it is transparent or slightly grayish. Previously, a dark or yellowish tint was considered a sign of cellular stress, but new data show that there is no clear link between cytoplasm color and embryo development outcomes.
• Shape. The ideal shape is round. However, oval oocytes can also develop normally.
• Bright zona pellucida (zona pellucida). Previously, some modifications were considered negative, but it is now emphasized that not all anomalies of the envelope impact subsequent development.
• Oocyte-cumulus complex (COC). Its state helps assess oocyte maturity. Even atypical complexes are now acceptable.

Thus, the new recommendations reduce subjectivity and allow a broader use of cells that were previously considered "doubtful".

How is fertilization success determined?

16 to 18 hours after fertilization, the embryologist evaluates zygote formation. Main signs:

• presence of two pronuclei (2PN) — correct fusion of genetic material;
• two polar bodies — proof of meiosis completion;
• symmetry and placement of the PN;
• cytoplasm structure without pathological inclusions.

In 2025, emphasis is placed not only on the course of fertilization but also on the morphology of the zygote, which allows for more precise prediction of its development.

Days 2-3 embryo division

The embryo begins to divide actively: first into two cells, then into four and eight. Embryologists evaluate:

• the number of blastomeres: 3-4 on the second day, 7-10 on the third day;
• size and symmetry of the cells;
• degree of fragmentation (particles between cells);
• presence of multinucleation (multiple nuclei within a cell);
• start of compaction.

The 2025 recommendations strengthened attention to the fragmentation percentage, cell aggregation, and dynamic development using time-lapse technologies.

Day 4 morula stage

The embryo transforms into a dense mass of cells, and compaction processes occur. Assessed are:

• connection density;
• uniformity of structure;
• early signs of cavity formation.

Embryos with complete compaction or early cavitation signs are more likely to develop into blastocysts.

Day 5 blastocyst stage

This stage is crucial for selecting embryos to transfer:

• embryos reaching the blastocyst stage on day 5 have the greatest potential;
• expansion degree, quality of the trophectoderm (TE), and inner cell mass (ICM) are evaluated;
• For the first time, the spontaneous collapse of the blastocyst is systematically considered as a prognostic factor.

The new standards highlight flexibility: even embryos with some anomalies can be viable.

Importance of updated recommendations

• More precise assessment of oocytes and embryos.
• Introduction of criteria for the morula stage (day 4).
• Prioritization of the morphology of the trophectoderm and blastocysts.
• More detailed explanations to patients about embryo selection.

At the NatuVitro Barcelona IVF clinic, we:

• strictly follow the updated protocols of ESHRE/ALPHA and ASRM;
• use evidence-based data and modern technologies;
• support patients at every step until the child's birth.

The updated standards make IVF more predictable, safer, and more effective, thereby increasing the chances of pregnancy success.

Sources list

1. Ghevaria, H., SenGupta, S., Naja, R., Odia, R., Exeter, H., Serhal, P., Viñals Gonzalez, X., Sun, X., & Delhanty, J. (2022). Next Generation Sequencing Detects Premeiotic Errors in Human Oocytes. International Journal of Molecular Sciences, 23(2), 665. https://doi.org/10.3390/ijms23020665
2. Hu, J., Ma, X., Bao, J. C., Li, W., Cheng, D., Gao, Z., Lei, A., Yang, C., & Wang, H. (2011). Insulin–transferrin–selenium (ITS) improves maturation of porcine oocytes in vitro. Zygote, 19(3), 191–197. https://doi.org/10.1017/s0967199410000663
3. Rienzi, L., Vajta, G., & Ubaldi, F. (2010). Predictive value of oocyte morphology in human IVF: a systematic review. Human Reproduction Update, 17(1), 34–45. https://doi.org/10.1093/humupd/dmq029

Coticchio, G., Ahlström, A., Arroyo, G., Balaban, B., Campbell, A., De Los Santos, M. J., Ebner, T., Gardner, D. K., Kovačič, B., Lundin, K., Magli, M. C., Mcheik, S., Morbeck, D. E., Rienzi, L., Sfontouris, I., Vermeulen, N., & Alikani, M. (2025). Updated Istanbul consensus: Revised ESHRE/ALPHA consensus on static and dynamic morphological assessment of oocytes and embryos. Human Reproduction. https://doi.org/10.1093/humrep/deaf021