Impact of estradiol-to-progesterone ratio before progesterone initiation on pregnancy outcomes in frozen embryo transfer cycles with hormone replacement therapy: an analysis of over 25,000 cycles

Background: Endometrial receptivity in hormone replacement therapy-frozen embryo transfer (HRT-FET) cycles depends on a precisely coordinated estradiol-progesterone environment. An imbalance between estradiol (E2) and progesterone (P) before progesterone exposure may impair the endometrial transition from proliferation to secretory transformation. This study investigated whether the estradiol-to-progesterone (E2/P) ratio prior to the initiation of progesterone is associated with pregnancy outcomes. Methods: This retrospective cohort study analyzed HRT-FET cycles performed between 2017 and 2022 at a single reproductive medicine center. Serum E2 and P levels were measured prior to the initiation of progesterone, and the E2/P ratio was calculated to reflect their relative concentrations. Patients were categorized into quartiles according to their E2/P ratios. Multivariable and multi-model logistic regression analyses were conducted to evaluate the association between E2/P ratios and clinical pregnancy rate (CPR) as well as live birth rate (LBR). Results: Both CPR and LBR declined progressively with increasing E2/P ratios across quartiles (CPR: 60.6%, 57.7%, 52.7%, and 47.7%; LBR: 50.6%, 47.1%, 42.6%, and 36.7% from the lowest to highest quartile, respectively; P < 0.001). After adjusting for confounders including female age, body mass index, infertility diagnosis, infertility indication, embryo transfer stage, number of embryos transferred, and endometrial thickness, higher E2/P ratios remained independently associated with reduced CPR and LBR. The trend remained significant in all regression models (P for trend < 0.001). Conclusions: In HRT-FET cycles, higher pre-progesterone E2/P ratios were associated with lower clinical pregnancy and live birth rates. The E2/P ratio may reflect the hormonal status of the endometrium, but its clinical utility requires confirmation through prospective studies.