The possible window of implantation for embryos in the first frozen embryo transplantation cycle: A retrospective analysis.

OBJECTIVE: This study aimed to investigate the duration of progesterone (P) therapy on clinical pregnancy rates as measured by the window of implantation (WOI) in the first cycle of frozen embryo transplantation. METHODS: The study compared the pregnancy rates between 345 cleavage stage transfers and 348 blastocyte transfers of frozen embryos with modified natural cycles in patients from July 1, 2020, to November 30, 2020. Four different P durations were analyzed in the cleavage stage embryo transfer group, i.e., two, three, four, and five days. Five different P durations were analyzed in the blastocyst transfer group, i.e., three, four, five, six, and seven days. RESULTS: The baseline demographics and clinical characteristics of the cleavage stage embryos and blastocyst transfer groups were not comparable. The clinical pregnancy rates following the cleavage stage embryo transfer after two, three, four, and five-day P administration were 45.71%, 44.60%, 38.40%, and 30.43%, respectively (the difference among the subgroups was not significant). Following the blastocyst transfer, the clinical pregnancy rates after three, four, five, six, and seven-day P administration were 50.65%, 63.51%, 60.00%, 54.55%, and 61.54%, respectively (the difference among the subgroups was not significant). In contrast, these two transfer groups showed significantly different clinical pregnancy rates following four and five-day P exposure (P < 0.05). CONCLUSION: For cleavage-stage embryo transfer, the most effective WOI was found between days two and five of P administration. The effective WOI for blastocyst transfer was observed between days three and seven of P administrations.