JBRA Assisted Reproduction 2025;29(Suppl.2 SBRA 2025):126
Poster Presentation
29^th Annual Congress of the SBRA. São Paulo/SP - Brazil, 2025
doi: 10.5935/1518-0557.20263760

P-114. Good up to a point: the association between higher estradiol levels and improved embryo morphokinetic development and implantation potential in frozen-thawed embryo transfer cycles

Edson Borges Jr.¹, Amanda Setti², Daniela Paes Almeida Braga³, Maite Del Collado³, Assumpto Iaconelli Jr.¹

¹ Fertility Medical Group / FertGroup - São Paulo – SP- Brasil
² Associação Instituto Sapientiae -São Paulo – SP – Brasil
³ Science For Everymind - São Paulo – SP- Brasil

Objective: This study aimed to comprehensively evaluate the association between serum estradiol (E2) levels and embryo morphokinetic development, blastocyst formation, and clinical outcomes in frozen-thawed embryo transfer (FET) cycles following intracytoplasmic sperm injection (ICSI). Additionally, it investigated whether adjusting E2 levels by oocyte yield, through the ratios of E2 to total oocytes and E2 to metaphase II (MII) oocytes, enhances the predictive accuracy for embryo quality and reproductive success.
Methods: A retrospective cohort study was conducted including 16,760 embryos from 2,584 ICSI cycles performed between March 2019 and December 2022 at a single IVF center. All embryos were cultured individually in time-lapse incubators, allowing continuous and detailed monitoring of morphokinetic parameters. Embryo quality was assessed using the KIDScore algorithm, which integrates developmental timing and morphology to estimate implantation potential. Patients were stratified into four groups based on serum E2 levels measured on the day of ovulation trigger: Q1 (<1511 pg/mL), Q2 (1511–2360 pg/mL), Q3 (2360–3008 pg/mL), and Q4 (>3008 pg/mL). This stratification permitted comparison of ovarian response, embryo development kinetics, blastocyst formation rates, and clinical outcomes across varying E2 exposures. To account for hormonal exposure per oocyte, E2/total oocyte and E2/MII oocyte ratios were calculated. These ratios serve as individualized measures of stimulation intensity normalized by the number of oocytes retrieved. Statistical analyses included generalized linear and mixed-effects models adjusted for confounding variables such as maternal age, body mass index (BMI), antral follicle count, and follicle-stimulating hormone (FSH) per follicle. Restricted cubic spline regressions were applied to capture potential nonlinear dose-response relationships between E2 levels and outcomes.
Results: Higher serum E2 quartiles were associated with significantly improved ovarian response and embryonic development. Compared with Q1, patients in Q4 had nearly double the number of mature oocytes (12.9 vs. 6.7) and blastocysts formed (6.8 vs. 3.5). Embryo development was faster in Q4, with time to 5 cells (t5) averaging 49.2 hours versus 52.4 hours in Q1. The mean KIDScore increased from 6.6 in Q1 to 7.3 in Q4 (p<0.001), indicating higher embryo quality. Clinically, implantation rates improved from 31.5% in Q1 to 39.8% in Q4, and clinical pregnancy rates rose from 28.6% to 37.1%. However, miscarriage rates were modestly higher in the highest E2 quartile (22.3% vs. 17.9%). Interestingly, when E2 was normalized by oocyte count, the association reversed. Elevated E2/MII and E2/total oocyte ratios correlated negatively with blastocyst formation (β = –0.013, p<0.001), KIDScore (β = –0.012, p<0.001), implantation rates (β = –0.016, p=0.004), and odds of clinical pregnancy (OR = 0.893, 95% CI: 0.860–0.924). Fertilization rates were not significantly affected. Spline regression analysis revealed a nonlinear relationship between E2 and outcomes. KIDScore improved at moderate estradiol levels (β = +2.02, p<0.001) but declined significantly at higher levels (β = –3.24, p<0.001), indicating a threshold beyond which increasing E2 adversely affects embryo quality. Clinical pregnancy probability followed a similar pattern, peaking in mid-range E2 and declining at the highest levels. Miscarriage risk increased progressively with rising E2 (β = +512.5, p=0.033).
Conclusion: Serum estradiol exerts a nonlinear, dose-dependent effect on embryo development and reproductive outcomes in ICSI-FET cycles. Moderate E2 levels optimize embryo morphokinetics, quality, and pregnancy rates. Conversely, excessive estradiol exposure, particularly when considering hormone levels relative to oocyte yield, is linked to impaired blastocyst formation, delayed development, reduced implantation, and lower clinical pregnancy rates. These findings highlight the importance of individualized ovarian stimulation protocols that balance estradiol exposure to maximize embryonic competence and assisted reproduction success, even in hormonally regulated frozen embryo transfer cycles.