The number of frozen embryo transfers (FET) cycles has increased significantly, largely due to the advancements in cryopreservation techniques and reassuring long-term perinatal outcomes. Synchronization between a receptive endometrium and an implantation-competent embryo is essential for successful outcomes in FET cycles. Endometrial preparation can be achieved through the natural cycle (true or modified), mild ovarian stimulation, or hormone replacement cycles. Natural cycles retain the physiological benefits of the corpus luteum, with improved obstetric outcomes, but require monitoring to identify ovulation. Hormone replacement cycles allow scheduling flexibility but are associated with an increased risk of hypertensive disorders of pregnancy and adverse perinatal outcomes. The timing of embryo transfer in relation to ovulation or progesterone exposure is critical and must align with the window of implantation. Luteal phase support is mandatory in hormone replacement cycles. Appropriate protocol selection based on patient characteristics and clinical context is essential to optimize reproductive and perinatal outcomes.
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