Introduction: Nowadays infertility is an important worldwide issue. Frozen embryo transfer (FET) is currently used widely but the
influential factors in the success of implantation are still a question. So, the aim of this study was the role of endometrial thickness
and echo pattern on the day of embryo transfer in frozen embryo transfer (FET) cycle outcome. Material and Methods: This is a
census cross-sectional study conducted on 140 patients in Isfahan fertility and infertility center. The patients were divided into two
groups using random even/odds numbers. Groups were divided based on endometrial preparation for endogenous and exogenous
hormone cycles. Demographics including; age, duration of infertility, BMI, number of transfer cycles, endometrial thickness,
endometrial echo pattern, method of endometrial preparation and cycle outcome were collected and recorded in a checklist and
variables were analyzed via SPSS 20 version. Results: In this study, 140 females were studied. The mean age of the patients was 32.9
± 5.1 years. Patients with successful implantation were statistically different from those with unsuccessful cycles in terms of age,
infertility duration, number of previous unsuccessful cycles and endometrium thickness prior to HCG therapy and on the day of
implantation (P-value < 0.05). Considering 9.15 millimeter of endometrial thickness prior to HCG therapy had 79.1% sensitivity and
78.9% specificity while cut-off of 9.45-millimeter endometrial thickness had 79.1% sensitivity and 80% specificity for successful
implantation. Conclusion: Based on findings of current study, endometrium thickness of 9.45 millimeter can be accompanied with
high rate of FET success with sensitivity of 79.1% and specificity of 80%. In addition, other factors such as patients' age, infertility
duration, number of previous unsuccessful cycles and endometrium thickness prior to HCG therapy can affect results of FET.
Keywords: Endometrial thickness, frozen embryo transfer, Infertility.