Impact of Transfer Timing on Day-6 Blastocyst Pregnancy Outcomes in HRT Cycles of 2021-2024.

Background: The achievement of a healthy live birth from a singleton pregnancy has become the primary objective of modern Assisted Reproductive Technology (ART). Consequently, the effective utilization of a single blastocyst is of paramount importance. However, the optimal approach to improving clinical pregnancy rates for Day-6 (D6) blastocysts in the hormonal replacement therapy frozen embryo transfer (HRT-FET) cycle remains unclear. Methods: This study retrospectively analyzed 990 D6 single blastocyst HRT-FET cycles conducted at the Reproductive Medicine Center of Nanjing Drum tower Hospital from January 2021 to April 2024. Patients were categorized based on the timing of embryo transfer (on the 6th or 7th day of progesterone administration), and both univariate and multivariate regression analyses were employed to assess the impact of transfer timing on clinical pregnancy outcomes. Results: The results revealed no significant differences in baseline characteristics or treatment cycle parameters between the two groups based on transfer timing. Univariate analysis identified several factors, including the age of both partners, infertility etiology, anti-Müllerian hormone (AMH) levels, antral follicle count (AFC), and the number of prior ART cycles, which may influence clinical pregnancy outcomes. After adjusting for these variables, multivariate regression analysis indicated that the timing of endometrial preparation for D6 single blastocyst transfer did not significantly affect clinical pregnancy rates. Conclusion: In conclusion, our retrospective data suggest that transferring single D6 blastocysts on either the sixth or seventh day of progesterone administration in HRT-FET cycles yields comparable clinical pregnancy outcomes. Until further prospective evidence emerges, our findings do not support routine delay of D6 blastocyst transfers in HRT cycles.