Zona Pellucida Thickness, Reproductive Aging, and Oocyte Competence in Women with Diminished Ovarian Reserve...

Background/Objectives: The zona pellucida (ZP) is an extracellular matrix surrounding the oocyte. It is unclear whether mean ZP thickness is associated with reproductive age or developmental competence in women with diminished ovarian reserve (DOR). We examined pre-ICSI ZP thickness in relation to age, ovarian response, and early reproductive outcomes. Methods: This prospective observational study included 255 women aged 22-46 years with clinically diagnosed DOR who underwent intracytoplasmic sperm injection. ZP thickness was measured at three equidistant points before injection, independently verified by two observers, and summarized as the patient-level mean across the available metaphase II oocytes. Women aged <35 and ≥35 years were compared. The association between ZP thickness and chronological age was also evaluated using Spearman correlation. A transfer-adjusted logistic regression model evaluated beta-hCG positivity using age group, mean ZP thickness, laser-assisted hatching, day-5 transfer, and metaphase II oocyte count. Results: Median patient-level mean ZP thickness was similar in women aged <35 and ≥35 years (36.99 [34.90-40.61] vs. 36.91 [34.04-40.87] µm; p = 0.875). Mean ZP thickness was not correlated with chronological age (Spearman's ρ = -0.059; p = 0.347). Younger women had higher AMH levels, antral follicle counts, retrieved oocyte counts, and metaphase II oocyte counts. Beta-hCG positivity was also higher in younger women (31.8% vs. 14.6%; p = 0.006). In the adjusted model, ZP thickness was not independently associated with beta-hCG positivity (OR 1.03 per µm, 95% CI 0.95-1.11; p = 0.487), whereas age ≥ 35 years was associated with lower odds of beta-hCG positivity. Conclusions: In this DOR cohort, mean pre-ICSI ZP thickness was not associated with chronological age, either as a continuous variable or after categorization at 35 years. It was also not independently associated with beta-hCG positivity. These findings do not exclude age-related changes in other structural, mechanical, or dynamic properties of the ZP that were not measured in this study.