Embryo transfer (ET) is performed according to the embryonic stage after ovulation is identified in both protocols. To overcome this problem, mNC is the preferred protocol in which human chorionic gonadotropin (hCG) is administered to trigger ovulation when the leading follicle is measured at ≥18 mm. However, it might be difficult even in the frequent follow-up and monitoring. Luteinizing hormone (LH) surge and ovulation are determined by serial ultrasonographic examination of follicular growth, serum LH level measurement, or urinary LH kits. The tNC occurs by preparing the endometrium for implantation with endogenous hormones in patients with regular menstrual cycles. Numerous protocols are available however, the most commonly used are natural cycle (NC) and artificial cycle (AC).
With the increased use of FET cycles and knowledge on the importance of endometrial thickness, maturation, and receptivity in achieving pregnancy, clinicians have more closely focused on optimal endometrial preparation. Favorable results have been attributed to better synchronization between the endometrium and transferred embryo in FET cycles ( 5, 6). Moreover, some studies revealed higher pregnancy outcomes in FET cycles than fresh ones ( 5, 6, 7). Frozen-thawed cycles are considered as an option for clinicians to use the freeze-all strategy, prevent ovarian stimulation syndrome, and perform preimplantation genetic testing (PGT) in recent years ( 3, 4).
Survival rates, embryo morphology, and pregnancy outcomes have improved in FET cycles after the widespread use of the vitrification method relative to slow freezing for cryopreservation ( 1, 2). Frozen-thawed embryo transfer (FET) has become a promising approach in assisted reproductive technique cycles in recent years due to cryopreservation technique improvements.
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