Background: Cryopreservation's success rate varies depending on woman age, as low as 14.8% (if eggs were extracted from 40-year-old women), and as high as 31.5% with 25-year-old women. The goal of the research is to improve laboratory methods for freezing-thawing embryos, leading to elevated embryo survival rate. Yet, in hormonal replacement therapy frozen-thawed embryo transfer (HRT-FET) cycles, effective endometrial preparation before embryo transfer attracted less focus. The present research’s aim is to see whether there's a link between blood progesterone levels and pregnancy rates the day before frozen-thawed embryo transfer. Methods: This prospective observational research has investigated 120 patients of frozen-thawed embryo transfer cycle treatment, only 100 individuals subdivided to 2 groups for serum level of progesterone one day before frozen-thawed embryo transfer. The subjects visited both the Obstetrical and Gynecological Department of Tanta University as well as private centers between May 2020 and January 2021. Results: We discovered no correlation between maternal age with pregnancy test results. Yet, our study discovered highly significant variation among both groups regarding endometrial thickness one day preceding embryo transfer, and regarding pregnancy rate (p<0.05). Conclusion: The serum progesterone hormone preceding frozen embryo transfer has significant and direct relation and impact upon pregnancy rates. The present research detected low serum progesterone less than 10 nanograms/ml in the day before frozen-thawed embryo transfer in HRT-FET cycles significantly decreased probability of ongoing pregnancy post frozen-thawed embryo transfer.