Discussion
ABNORMAL MENSTRUAL CYCLE, EARLY PREGNANCY
HRT for FET cycles includes the sequential administration of exogenous estrogen and progesterone to imitate the physiological hormonal exposure of the endometrium in a normal menstrual cycle and to accurately time the transfer of the thawed embryo to the receptive endometrium (However, the hypothesis that prolonging the duration of progesterone administration could increase the pregnancy rates has not yet been confirmed (The hypothesis that prolonging the duration of progesterone administration could diminish the early pregnancy loss rate has not yet been confirmed (Based on these results, it may be concluded that enhanced flexibility would be possible when programming vitrified-warmed embryos transfer after progesterone administration, because no differences were observed between the duration of progesterone administration ≥1 day of embryonic age as far as clinical pregnancy, implantation, live birth, and early pregnancy loss rates are concerned (In addition, this study discovered that blastocysts might have higher pregnancy outcomes in FET cycles compared with that of cleavage-stage embryos (The data also displayed that the pregnancy outcome of embryos at the cleavage stage might be related to the woman’s age, while the blastocyst pregnancy outcome might be uncorrelated with it (This study had similar or different results from those of other studies, possibly due to epidemiological variables, limitations of its design, a high level of heterogeneity in the study populations, and insufficient sample size. Although this study adjusted for multiple potential confounding factors, caution should be taken when considering the results as a basis for definitive policy due to the prospective design of a single-center study with a limited sample size, as well as possible differences in research protocol and clinical performance. Consequently, confirmation of these findings in a multi-center study with a large sample size and a more rigorous research design is warranted.