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.