Day 6 vitrified blastocyst transfer on day 5 vs. day 6 of progesterone shows similar outcomes in HRT-FET cycles: a cohort study

Studies have suggested transferring Day 6 (D6) vitrified blastocyst in a frozen embryo transfer (FET) cycle. This study investigates whether D6 blastocysts should be transferred on the 5th or 6th day of progesterone administration during a hormonal replacement therapy (HRT) FET cycle. A cohort of 746 vitrified D6 single blastocyst HRT-FET cycles was examined in a public Fertility Clinic. Of these, 576 blastocysts were transferred on the 6th day of progesterone (2013-2021), and 170 on the 5th day (2021-2022). The primary outcome was live birth rate (LBR), secondary outcomes were biochemical pregnancy rate (PR), clinical pregnancy rate (CPR), and early pregnancy loss rate (EPLR). No significant differences in LBR, PR, CPR and EPLR were observed, between transfer on the 5th vs. the 6th day of progesterone (23.5% vs. 25.7%, p = 0.76, 38.8% vs. 41.2%, p = 0.61, 26.0% vs. 29.5%, p = 0.40 and 33.3% vs. 28.3%, p = 0.45, respectively). There was no significant difference in basic characteristics and top-quality blastocyst transfers. The timing of D6 vitrified blastocyst transferred on the 5th or the 6th day of progesterone administration in HRT-FET cycles did not significantly impact reproductive outcomes.