Oocyte quality remains the primary limiting factor for success in ART programs. The updated ESHRE/ALPHA Istanbul Consensus (2025) has, for the first time, systematically identified 24 morphological features of clinical significance in oocyte selection; among these, abnormalities of shape and the outer layers hold a central position (Alpha Scientists in Reproductive Medicine & ESHRE, 2025).
The Consensus recommends combining static assessment (observation at ×400 magnification) with time-lapse recordings, documenting the oocyte’s shape, the state of the zona pellucida (ZP), the perivitelline space (PVS), and the cytoplasm at standard time points (MII + 0 h; +4 h; +16 h).
Classification of Dysmorphisms
| Group | Examples | Potential Causes | Risks for Embryogenesis* |
| Extracytoplasmic | • Irregular shape (oval, pear-shaped, “teardrop-shaped”) • Indented/thickened ZP • Large granules in the PVS |
Disruption of the cortical cytoskeleton; excessive stress during oocyte retrieval | Low maturity, aneuploidy, delayed cleavage |
| Intracytoplasmic | • SER aggregations • Granular/dark cytoplasm • Vacuoles |
ER stress dysfunction, mitochondrial defects | Reduced blastocyst quality, increased rate of discarded embryos |
according to the systematic review by Ahmad et al. (2024)
Oocyte Shape: Between Variation and Pathology
Oocytes naturally exhibit some morphological diversity, from moderate ovoid shapes to slight invaginations of the outer layers. Current data indicate that these variations do not have a critical impact on fertilization capacity or blastocyst development potential (Sauerbrun-Cutler et al., 2015; Yang et al., 2022).
Oval Shape Although older studies suggested that elongated oocytes might be of lower quality (Fjeldstad et al., 2024), these findings have been refuted by recent meta-analyses. Currently, an oval shape is not considered clinically significant (Alpha Scientists in Reproductive Medicine & ESHRE, 2025).
Zona Pellucida (ZP) Moderate variability in the thickness or texture of the ZP is a common finding in healthy oocytes. Its impact on embryonic outcomes is minimal unless combined with other dysmorphisms, such as SER aggregations or large vacuoles (Balaban et al., 2011; Morimoto et al., 2025).
Perivitelline Space (PVS) The presence of small granules or moderate expansion of the PVS does not significantly affect fertilization or early embryo development, provided the oocyte nucleus has normal morphology and maturity (Rienzi et al., 2011).
Collectively, these data support the conclusion of the Istanbul Consensus (2025): the morphological shape of the oocyte, in the absence of severe pathologies, should not be a basis for exclusion from a treatment cycle (Alpha Scientists in Reproductive Medicine & ESHRE, 2025).
Cytoplasmic Changes
- SER Aggregations. A meta-analysis of 22 studies showed a significant decrease in the rate of normal 2PN zygotes (OR 0.61) and an increase in early aneuploidy; the 2025 Consensus recommends “cautious use” with patient consent (Ebner et al., 2008; Istanbul 2025).
- Granular Cytoplasm. By itself, it is not an absolute criterion for deselection, but it requires extended culture to D5-D6 for the selection of viable embryos (Balaban et al., 2011).
2025 Istanbul Consensus Recommendations Regarding Shape
| Feature | Category | Recommended Action |
| Giant or severely deformed oocyte | Severe | Discard |
| Oval / indented ZP without other defects | Moderate | Use with extended monitoring ± PGT-A |
| Mild granularity, thin ZP | Mild | Use; standard protocol |
Decision-Making Algorithm in the Laboratory
- Degree of dysmorphism (severe / moderate / mild).
- Availability of supplementary methods: time-lapse, PGT-A.
- Reproductive history: patient’s age, number of oocytes retrieved.
- Discussion of risk-benefit with the patient.
Anomalies in the shape and outer layers of the oocyte have a clear but graded impact on subsequent embryonic development. According to the current 2025 Consensus, only severely deformed or giant oocytes are subject to unconditional exclusion; most “moderate” dysmorphisms are permissible for use, contingent upon extended culture, molecular screening, and informed consent. Standardized morphometry and AI-assisted shape assessment are opening new avenues for personalized ART strategies.
References (APA 7)
Alpha Scientists in Reproductive Medicine & European Society of Human Reproduction and Embryology (ESHRE) Special Interest Group of Embryology. (2025). The Istanbul consensus update: a revised ESHRE/ALPHA consensus on oocyte and embryo static and dynamic morphological assessment. Human Reproduction, 40(6), 989-1035. https://doi.org/10.1093/humrep/deaf021 pmc.ncbi.nlm.nih.gov
Ahmad, M. F., Elias, M. H., Mat Jin, N., et al. (2024). Oocytes quality assessment—The current insight: A systematic review. Biology, 13(12), 978. https://doi.org/10.3390/biology13120978 pmc.ncbi.nlm.nih.gov
Balaban, B., Brison, D., Calderón, G., et al. (2011). Istanbul consensus workshop on embryo assessment: Proceedings of an expert meeting. Reproductive BioMedicine Online, 22(6), 632-646.
Ebner, T., Moser, M., Sommergruber, M., et al. (2008). Cytoplasmic strings between the oocyte and surrounding cumulus cells: Morphology and clinical relevance. Human Reproduction, 23(11), 2581-2587.
Fjeldstad, J., Qi, W., Siddique, N., et al. (2024). Segmentation of mature human oocytes provides interpretable and improved blastocyst outcome predictions by a machine learning model. Scientific Reports, 14, 10569. https://doi.org/10.1038/s41598-024-60901-1 nature.com
Lin, J., Fang, L., Yao, L., et al. (2024). The embryological characteristics and clinical outcomes of oocytes with indented zona pellucida. Journal of Obstetrics & Gynaecology, 44(1), 2428944. https://doi.org/10.1080/01443615.2024.2428944 pubmed.ncbi.nlm.nih.gov
Morimoto, K., Wang, H., et al. (2025). Ultrastructure of coarse granules in the perivitelline space and its impact on embryo development. Frontiers in Endocrinology, 16, e1425578. pmc.ncbi.nlm.nih.gov
Sauerbrun-Cutler, M. T., Vega, M., Breborowicz, A., et al. (2015). Oocyte zona pellucida dysmorphology is associated with diminished in-vitro fertilization success. Journal of Ovarian Research, 8, 5. cambridge.org
Yang, Y., Pan, Y., & Zhang, L. (2022). Zona pellucida phenotypes as predictors of IVF outcomes: A systematic review and meta-analysis. Reproductive Biology, 22(4), 100740. pmc.ncbi.nlm.nih.gov
