How many times the lining should be tested prior to frozen embryo transfer depends on the method of testing and the patient’s condition. Generally, ERT technique requires 1 test, and vaginal ultrasound or sex hormone testing combined with ultrasound requires anywhere from 3-5 tests. The ERT technique can be used to determine the window period for transfer before transferring the blastocysts, and 1 endometrial check is sufficient. If the window period for transfer is determined by vaginal ultrasound, or sex hormone testing combined with ultrasound, it will be based on the growth of the patient’s endometrium. It is sufficient to check that the endometrium is about 10 mm thick and has a volume of no less than 2 mL. Some patients may need to be tested 3 times and some may need to be tested about 5 times. The window period for endometrial transfer is simply the period during which the lining allows the embryo to implant. Embryo transfer is performed during the window period in order to achieve synchronization between the embryo and the endometrium, and if the embryo does not develop in sync with the endometrium it will result in a failed transfer. For frozen embryo transfer, it is recommended to go to the reproduction department of a regular hospital.