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Background: During hormone replacement therapy (HRT) to optimize endometrial growth for thawed-frozen embryo transfer (FET), patients normally undergo estrogen administration for 14 days (day 14 group). However, if optimal endometrial thickness is not achieved in 14 days, estrogen treatment is continued for seven more days at a higher dose (day 21 group). This study aimed to determine the effects of this extended estrogen administration regimen on pregnancy outcomes. Methods: A retrospective analysis was performed using our patient database from November 2016 to March 2018. We analyzed 303 cycles of blastocyst-FET and compared patient background characteristics and pregnancy outcomes between day 14 and day 21 groups. Results: In day 21 group, the pregnancy rate tended to be lower while the miscarriage rate and ectopic pregnancy rate were higher than those in day 14 group. However, the differences between the groups were not statistically significant. Conclusions: Delayed endometrial growth resulted in poorer pregnancy outcomes in FET, but the differences were not statistically significant. Thus, it could be meaningful to add estrogen administration for 7 days to avoid the poor outcomes caused by a thin endometrium.