HCG is human chorionic gonadotropin. Low values of human chorionic gonadotropin after frozen embryo transfer may be caused by biochemical pregnancy, ectopic pregnancy and late embryo implantation. 1. Biochemical pregnancy: refers to pregnancy that does not implant in the uterus after frozen embryo transfer. Patients may find that the value of human chorionic gonadotropin is low and also decreases gradually. 2. Ectopic pregnancy: the fertilized egg is deposited outside the uterine cavity due to inflammation or dysplasia of the reproductive system. In this case, the human chorionic gonadotropin cannot increase exponentially, i.e. the doubling is unsatisfactory, and low human chorionic gonadotropin values may occur. 3. Late implantation: when the transferred embryo implanted late, the amount of human chorionic gonadotropin secreted by placental trophoblast cells during the examination is small, and low human chorionic gonadotropin value may appear, and the value will rise significantly afterward if the embryo develops normally. It is recommended that patients with low human chorionic gonadotropin values after frozen embryo transfer should go to the hospital in time and follow the doctor’s instructions for treatment, so as to avoid delaying their condition.