Randomization
We selected women who met the inclusion criteria and started daily endometrial preparation for FET. The details of the trial were explained to the women by a member of the project, and eligible women who signed the consent form were randomized on the day of progesterone administration. Women were randomly allocated to the hCG treatment or the control group according to a randomization list generated by a computer. The specific process was as follows: 300 random numbers were produced by a computer and divided into A and B groups, with 150 in each group. Then a random group table was made and blinded on computer. The 300 random numbers obtained above were randomly distributed to 300 sequence numbers. For every patient included, a random number was obtained according to the order of inclusion. Then the number was unblinded by the computer.The randomization process was completed by a member of the project. Therefore, the staff who conduct the randomization process and the participants were not blinded. The physician who performed the endometrial preparation protocol and determined the number and grade of embryos transferred was blinded to the grouping. Laboratory staff and staff who conducted the data analysis and follow-up were blinded to the allocation.