Endometrial Preparation for Frozen Embryo Transfer: Protocols and Pregnancy Outcomes

Frozen embryo transfer (FET) has become widely used over the past decade, largely because of advances in vitrification techniques. To optimize pregnancy rates in FET cycles, accurate synchronization between embryo development and endometrial receptivity is essential. However, the optimal individualized approach to endo-metrial preparation remains a matter of ongoing debate. This review summarizes the current literature on endo-metrial preparation protocols and their impact on preg-nancy and perinatal outcomes, with a particular focus on artificial, natural, and stimulated cycles. Although many studies have methodological limitations, some well-de-signed randomized controlled trials suggest improved pregnancy rates and better perinatal outcomes with FET compared with fresh embryo transfer, including a low-er risk of preterm delivery and ovarian hyperstimulation syndrome (OHSS). Nonetheless, artificial endometrial preparation-characterized by the absence of a corpus luteum-has been associated with a higher incidence of hypertensive disorders of pregnancy and large-for-ges-tational-age (LGA) infants. This has renewed interest in whether transferring frozen embryos into a more physio-logic endometrium, as occurs in natural or modified nat-ural cycles, may not only enhance implantation but also reduce maternal and neonatal morbidity. The emergence of personalized luteal phase support across different protocols is another promising area that warrants further investigation. This narrative review provides a compre-hensive overview of endometrial preparation strategies for FET and their effects on clinical outcomes based on the most recent available evidence.