Frozen embryo-transfer: a review on the optimal endometrial preparation

Frozen-thawed embryo transfer (FET) has become a widely adopted method in assisted reproductive technology (ART), thanks to the advancements in vitrification techniques. FET now accounts for a significant proportion of ART cycles, but there is still no clear consensus on the optimal protocol for endometrial preparation. This review aims to evaluate the reproductive outcomes of three commonly used FET protocols: natural cycle (NC), modified natural cycle (mNC), and artificial cycle (AC), with a focus on implantation rates, clinical pregnancy rates (CPR), live birth rates (LBR), and obstetric complications such as hypertensive disorders of pregnancy (HDP) and preeclampsia (PE). Studies published between 2014 and 2024 were reviewed and compared across these protocols. The results show that NC and mNC protocols are associated with higher CPR and LBR compared to AC. AC, on the other hand, is linked to an increased risk of HDP and PE, likely due to the absence of a functional corpus luteum (CL), which plays a vital role in early pregnancy support. The presence of a CL appears to be crucial for minimizing obstetric complications, making NC and mNC preferable options. Future research is needed to refine FET protocols, focusing on personalized approaches to endometrial preparation that optimize both pregnancy outcomes and maternal health.