Impact of Follicle Size at Trigger on Reproductive Outcomes in Letrozole-Modified Natural Frozen Embryo Transfer...

To evaluate whether follicle size at hCG trigger influences reproductive outcomes in letrozole-modified natural frozen embryo transfer (let-mNC-FET) cycles among high-responder patients.

This observational cohort included 170 let-mNC-FET cycles. Patients were stratified by follicle-size percentiles at trigger: 0-25th (15-17 mm; n = 43), 25-75th (18-20 mm; n = 90), and > 75th (21-24 mm; n = 37). Oral dydrogesterone provided luteal support. Serum progesterone (P4) on embryo-transfer (ET) day was measured with an assay that does not detect dydrogesterone (reflecting endogenous luteal production). The primary outcome was the ongoing pregnancy rate (OPR). Group comparisons used ANOVA/Kruskal-Wallis and χ² tests; predictors of OPR were evaluated with logistic regression.

Positive hCG and OPR did not differ across percentile groups (51.2%, 52.2%, 55.6%; p = 0.920 and 48.8%, 50.0%, 52.7%; p = 0.833, respectively). Endometrial thickness at trigger differed by group (medians 8.0, 9.0, 7.8 mm; p < 0.001), while ET-day P4 increased with larger follicles (medians 19.74, 21.00, 26.50 ng/mL; p = 0.001; post-hoc 0-25th vs > 75th p = 0.0009). In multivariable analysis, younger age (aOR 0.834; 95% CI 0.762-0.914; p = 0.0001), higher BMI (aOR 1.169; 1.015-1.346; p = 0.0303), fewer stimulation days (aOR 0.798; 0.647-0.983; p = 0.0343), larger leading follicle size (aOR 1.343; 1.059-1.703; p = 0.0151), and higher ET-day P4 (aOR 1.067; 1.027-1.108; p = 0.0007) independently predicted OPR; EMT and AMH were not associated (p ≥ 0.08 and p = 0.25).

Although OPR did not differ across follicle-size strata, larger follicle size at trigger and higher endogenous luteal P4 were independent predictors of OPR in highresponders. Confirmation in adequately powered prospective studies is warranted.