Background: The estradiol-to-oocyte ratio (E2/O), measured in pmol/L per oocyte, has been associated with oocyte yield and embryo quality in IVF cycles, yet few studies have examined its relationship to oocyte competence and embryo developmental potential. This study evaluated the impact of E2/O on oocyte competence, blastocyst development, and specifically blastocyst euploidy. Methods: A retrospective cohort study was conducted including 298 patients undergoing autologous IVF/ICSI with PGT-A at a single university-affiliated fertility clinic (2021–2025). Patients were stratified into four E2/O groups (<750, 750–1250, 1250–1750, >1750 pmol/L). Primary outcomes included fertilization rate, blastocyst development, blastocyst utilization rate (BUR), and euploidy rate. Statistical analysis was performed using ANOVA with Tukey HSD post hoc testing and multivariable models adjusted for age, oocyte number, and infertility diagnosis were completed. Results: Among 2425 zygotes, 1265 blastocysts were biopsied (52.2%). Increasing E2/O was associated with progressively fewer retrieved oocytes (p < 0.00001), while blastocyst number per patient also differed significantly across groups (p = 0.0009). Fertilization rate, blastocyst rate, BUR, blastulation timing, and euploidy did not differ significantly across E2/O groups. Multivariable analyses similarly demonstrated no independent association between E2/O and fertilization, BUR, blastocyst development, or euploidy. Discussion: E2/O ratio was associated with different oocyte yield and absolute blastocyst yield, but not with proportional measures of downstream embryologic competence. The E2/O ratio was not independently associated with embryologic outcomes or euploidy status. Larger prospective studies are needed to determine whether E2/O provides prognostic information beyond established predictors of IVF outcomes.
