Background:Assisted reproductive therapy (ART) has been developed remarkably in these decades; however, the rate of unsuccessful embryo implantation especially in the frozen-thawed embryo transfer (FET) cycles remains high and is reported up to 70%. The current study was designed to compare the effect of intramuscular injection of hCG on endometrium preparation and embryo implantation, in women undergoing FET compared to the control group.Methods:This clinical trial was done on 140 infertile women that underwent FET. The study sample was randomly allocated to the intervention group (two 5000 unit ampoules of hCG were injected intramuscularly before the first dose of progesterone administration) and the control group (without hCG injection). In both groups, 4 days after progesterone administration, the cleavage stage embryos were transferred. The study outcomes were biochemical pregnancy, clinical pregnancy and abortion rate.Results:The average age of intervention and control group was 32.65±6.05 and 33.11±5.36 years, respectively. The basic information between two study groups did not differ significantly. The chemical (30% vs. 17.1%, P=0.073, relative risk (RR)=0.57) and clinical (28.6% vs. 14.3%, P=0.039, RR=0.50) pregnancy rates were higher in the intervention group compared to the control group; these higher ratios were only significant in clinical pregnancy rate. Abortion rate was not significantly (P=0.620) different between the intervention and control groups (4.3% vs. 1.4%, respectively).Conclusion:This study showed that intramuscular injection of 10000 IU hCG before the endometrial secretory transformation phase in cleavage-stage embryo, improves IVF cycle outcomes.