As one of the earliest markers for predicting pregnancy outcomes,human chorionic gonadotropin (hCG) values have been inconclusive on reliability of the prediction after frozen and fresh embryo transfer (ET).In this retrospective study,patients with positive hCG (day 12 after transfer) were included to examine the hCG levels and their predictive value for pregnancy outcomes following 214 fresh and 1513 vitrified-warmed single-blastocyst transfer cycles.For patients who got clinical pregnancy,the mean initial hCG value was significantly higher after frozen cycles than fresh cycles,and the similar result was demonstrated for patients with live births (LB).The difference in hCG value existed even after adjusting for the potential covariates.The area under curves (AUC) and threshold values calculated by receiver operator characteristic curves were 0.944 and 213.05 mIU/mL for clinical pregnancy after fresh ET,0.894 and 399.50 mIU/mL for clinical pregnancy after frozen ET,0.812 and 222.86 mIU/mL for LB after fresh ET,and 0.808 and 410.80 mIU/mL for LB after frozen ET with acceptable sensitivity and specificity,respectively.In conclusion,single frozen blastocyst transfer leads to higher initial hCG values than single fresh blastocyst transfer,and the initial hCG level is a reliable predictive factor for predicting IVF outcomes.