Endometrial preparation for frozen embryo transfer: natural cycle vs artificial cycle
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The use of frozen-thawed embryo transfer (FET) has increased over the past decade with improvements in technology and increasing live birth rates. FET facilitates elective single-embryo transfer, reduces ovarian hyperstimulation syndrome, optimizes endometrial receptivity, allows time for preimplantation genetic testing, and facilitates fertility preservation. Endometrium preparation schemes designed for the thawed embryo generally include natural cycles (NCFET) and artificial cycles (AC-FET), in which the endometrium is artificially prepared with exogenous steroid hormone therapy. For NC- FET, the time of ovulation can be estimated based on the detection of the luteinizing hormone (LH) surge in either blood or urine (constituting ‘true’ NC-FET) or after triggering ovulation of the dominant follicle using human chorionic gonadotrophin (hCG) (‘modified’ NC-FET) or with support of the luteal phase with progesterone (natural cycle with luteal phase support (NC-LPS)). An AC is advantageous for patients with irregular cycles as it reduces the frequency of clinic visits, helps to schedule the date of FET, and decreases ET cancellation rates. For these reasons, many eumenorrheic women undergo AC-FET. FET cycles have been associated, however, with an increased risk of hypertensive disorders of pregnancy for reasons that are not clear. Recent evidence suggests that absence of the corpus luteum (CL) could be at least partly responsible for this increased risk. Despite the widespread use of FET, the optimal protocol with respect to live birth rate, maternal health, and perinatal outcomes has yet to be determined. Future practice regarding FET should be based on high-quality evidence, including rigorous controlled trials.
DRA. NATALIA POSADA VILLA Médico especialista en Medicina Reproductiva, In Ser Colombia. Directora de la Red Latinoamericana de Reproducción Asistida Región Noroeste (REDLARA).
