Objective To investigate the role of complement C3 in early predicting pregnancy outcomes of frozen-thawed embryo transfer(F-ET).Methods A total of 378 F-ET cycles were prospectively collected and divided into group A(complement C3≤1.05,120 cycles)and group B(complement C3>1.05,258 cycles)based on the best cutoff value of complement C3 for predicting F-ET pregnancy outcomes.The outcomes of the two groups were compared,and the best cutoff value of complement C3 for predicting F-ET spontaneous abortion was analyzed in group B.Results Age was a risk factor for successful F-ET pregnancy(P<0.05),and complement C3 and embryo type were protective factors for successful F-ET pregnancy(P<0.05).The area under the receiver-operating characteristic curve(ROC)of complement C3 for predicting F-ET pregnancy outcome was 0.702,and the best cutoff value was 1.05 g/L,with a clinical pregnancy sensitivity of 87.60%and a specificity of 52.00%.The clinical pregnancy rate and embryo implantation rate in group B were both significantly higher than those in group A(67.05%vs.52.75%,P<0.05).The best cutoff value of complement C3 for predicting spontaneous abortion after F-ET was 1.32 g/L,with an area under the ROC curve of 0.760,a sensitivity of 69.00%,and a specificity of 81.20%.Conclusions Complement C3 is of significance in the early prediction of F-ET pregnancy outcome.When complement C3 exceeds the level of 1.32 g/L,it may lead to an increase in the rate of spontaneous abortion.