Abstract: Objective：To explore the related factors of biochemical pregnancy during freeze-thaw embryo transfer (FET) in assisted reproductive technology. Methods：A total of 120 cases of biochemical pregnancy undergoing FET treatment in our reproductive center were included in this retrospective study, from January 2012 to October 2016. The control group included 110 cases of clinical pregnancy and successful delivery in the same period. The general situation, infertility factors, endometrial preparation, fertilization methods and embryo transfer were compared between the two groups. The factors with statistical and clinical significance were selected for multivariate logistic regression. Results：In patients with history of spontaneous abortion, the risk of biochemical pregnancy was significantly increased(OR=2.182，95%CI：1.058-4.501, P＜0.05). The risk of biochemical pregnancy in the cases with ≥2 embryos transferred was lower than that in those cases with ＜2 embryos transferred(OR=0.081，95%CI：0.010-0.674, P＜0.05). The risk in the cases with at least one high-quality embryo transferred was also significantly lower(OR=0.473，95%CI：0.268-0.834, P＜0.05). Conclusions：The history of spontaneous abortion is a risk factor of biochemical pregnancy. The number of  ≥2 transferred embryos and the transfer of high-quality embryo are two protective factors of biochemical pregnancy.

Key words: Fertilization in vitro, Embryo transfer, Cryopreservation, Pregnancy, Logistic models