Elevated serum estradiol levels on embryo transfer day predict higher pregnancy success in HRT-FET cycles

To investigate the association between serum estradiol (E2) levels on the day of embryo transfer and clinical pregnancy outcomes in hormonally prepared frozen–thawed embryo transfer (HRT-FET) cycles with optimal progesterone (P4) levels. In this retrospective study, 175 HRT-FET cycles conducted between January and May 2024 were analyzed. Serum E2 and P4 levels were measured on the first and fourth days of progesterone administration. A decision tree analysis identified the optimal E2 threshold for embryo transfer day, dividing patients into two groups based on this cut-off. Clinical pregnancy and early pregnancy loss rates were compared between the groups. Binary logistic regression was performed to evaluate the relationship between E2 levels and pregnancy outcomes. A serum E2 threshold of 201 pg/mL on embryo transfer day was identified. Patients with E2 > 201 pg/mL had significantly higher clinical pregnancy rates (62.6% vs. 2.6%, p < 0.001) and significantly lower early pregnancy loss rates (7.25% vs. 87.5%, p < 0.001). Logistic regression confirmed a significant positive association between higher serum E2 and clinical pregnancy rates. Serum estradiol levels above 201 pg/mL on the day of embryo transfer are independently associated with significantly improved pregnancy outcomes, including higher clinical pregnancy rates and lower early pregnancy loss, in HRT-FET cycles. These findings highlight the potential value of E2 monitoring as a complementary tool to optimize embryo transfer timing and enhance the success of hormonally prepared frozen embryo transfer protocols.