P-164 The impact of oocyte morphological abnormalities on fertilization rate and blastocyst quality
Authors: Chukwudi O , Ifenatuoha W  C , Okewale B 
Published: July 2024

Study question

Do distinct morphological abnormalities in oocytes have an adverse effect on both fertilization rates and the quality of resulting blastocyst?

Summary answer

While most oocyte morphological abnormalities did not significantly affect outcomes, dark cytoplasmic color correlated with lower fertilization rates and the absence of good quality blastocysts

What is known already

Variability exists in the fertilization and developmental potential of mature (metaphase II - MII) human oocytes. A high-quality MII oocyte is characterized by a spherical shape, homogeneous cytoplasm, smooth appearance, a uniform zona pellucida (ZP), a normal-sized perivitelline space (PVS), and a single unfragmented polar body (PB). Dysmorphisms, such as variations in cytoplasmic features and anomalies in PVS size and PB regularity may occur. There are conflicting reports on the relationship between oocyte morphology and IVF outcomes. A recent meta-analysis highlighted associations between poor fertilization potential and specific dysmorphisms, including large/irregular PB, a large PVS, refractile bodies, and vacuoles.

Study design, size, duration

This study analyzed 367 MII oocytes obtained from 40 young patients. The fertilization rate (FR) and blastocyst quality (BQ) were compared among 132 dysmorphic and 235 normal MII oocytes (from October 2023 to January 2024). Dysmorphic oocytes were categorized as having a non-spherical shape, large PVS, PVS debris, abnormal ZP, fragmented PB, excess cytoplasmic granules, vacuoles, and dark cytoplasmic color.

Participants/materials, setting, methods

All MII oocyte denuded were examined under the microscope at x-400 magnification. Dysmorphic and normal oocytes were separated, injected, and cultured individually until day 5 under conditions of 37oC and 6% CO2. Fertilization was assessed 14 to 18 hours post-injection, and subsequent embryonic development was evaluated on day 2 and day 5. Blastocysts graded 4AA to 4BC are considered good quality, while those graded 4CC to 3CC are classified as fair quality.

Main results and the role of chance

The mean age of patients was 24.14 ± 3.77. In total, 36% of all oocytes exhibit at least one dysmorphism (n = 132). From the overall dataset analyzed, there was no significant difference (p &gt; 0.9999) in FR between the normal 77.87% (183/235) and dysmorphic group of oocytes 78.03 (103/132). The odd ratio and 95% confidence interval are 0.9909 and 0.5957 to 1.633 respectively. However, this was not the same for oocytes with dark color cytoplasm. The presence of this dysmorphism significantly lowered FR (p = 0.0176) with an odd ratio of 5.544 (95% CI: 1.419 to 21.67).

Similarly, the result showed no statistically significant difference (p = 0.1073) in the likelihood of obtaining a fair quality blastocyst between the normal oocytes 23.40% (55/235) and dysmorphic group of oocytes 15.79 (21/133). This was also the same for the likelihood of obtaining good quality blastocyst. There was no statistically significant difference (p = 0.1536) between the normal group 20.00% (47/235) and the dysmorphic group 26.52 (35/132). Oocyte with dark color cytoplasm gave no good quality blastocyst.

Limitations, reasons for caution

The exclusive use of oocytes from young patients may limit the generalizability of findings to a broader population of IVF patients. Also, the sample size may limit the study’s ability to detect subtle associations. A reason for caution when interpreting this study is its focus on morphological assessments alone.

Wider implications of the findings

The knowledge from our findings may contribute to optimizing oocyte selection strategies for intracytoplasmic sperm injection. Also, the knowledge could potentially be integrated into AI modules for oocyte morphological assessment. However, more studies would be required to develop a robust and scalable system

Trial registration number

Not applicable