There are several currently employed replacement protocols for FET. The choice of protocol depends on the individual woman's ovarian function and convenience of the method, as well as on the experience gained with the method by the physicians. Whatever protocol is used, the success requires optimal synchronization between the embryonic stage at thawing and date of the endometrium within the endometrial preparation cycle. While there is a consensus regarding the duration, route of delivery and dosage of estrogen supplementation, and the optimal ultrasonographic endometrial appearance and thickness, the effect of the different modes of luteal support remains unclear.