The aim of this case series study was to investigate the effect of adjusting the length of progesterone exposure on clinical pregnancy rates in cryopreserved embryo transfer cycles of patients with out-of-phase classic endometrial dating. Eighty infertile women with previous implantation failure and good-quality embryos underwent endometrial biopsy before cryopreserved embryo transfer and were included in this study. The main outcome measures were clinical pregnancy rate and histologic endometrial dating.