The relationship between serum oestrogen levels and clinical outcomes of hormone replacement therapy-frozen embryo transfer: a retrospective clinical study.

<bold>Background: </bold>This study aimed to explore the relationship between serum oestrogen (E2) levels before endometrial transformation and pregnancy outcomes of hormone replacement therapy-frozen embryo transfer (HRT-FET) cycles, which has been investigated for years without any consensus.<bold>Methods: </bold>A retrospective cohort study of 10,209 cycles HRT-FET cycles was conducted at the Reproductive Medicine Center of Nanjing Drum Tower Hospital from March 2017 to December 2020. A smooth fitting curve was constructed to identify the relationship between serum E2 levels before endometrial transformation and the clinical pregnancy rate. Then, threshold and saturation effect analysis was employed to explore the cut-off value of serum E2 levels. In addition, patients were divided into 2 groups based on their levels of serum E2 measured before progesterone-induced endometrial transformation: Group 1, < 300 pg/mL (n = 6251) and Group 2, ≥ 300 pg/mL (n = 3958). The clinical pregnancy and miscarriage rates of all groups were compared. Further smooth fitting curve analysis was employed by different subgroups segmented according to different endometrial thicknesses.<bold>Results: </bold>When the serum E2 level was greater than 300 pg/mL, the clinical pregnancy rate decreased significantly (62.9% vs. 59.8%, p < 0.01), but the miscarriage rates were similar (13.5% vs. 15.6%, p = 0.14). While serum E2 level reached or exceeded 1400 pg/mL, there was no significant correlation between the clinical pregnancy rate and E2 level. The clinical pregnancy rate reached its higher level at lower E2 levels, regardless of the different endometrail thicknesses.<bold>Conclusions: </bold>Patients with a lower pretransformation serum E2 level (less than 300 pg/mL) have a higher clinical pregnancy rate and there was no correlation between the clinical pregnancy rate and a higher serum E2 level (greater than 1400 pg/mL) in HRT-FET cycles. [ABSTRACT FROM AUTHOR]