This strategy facilitates elective single-embryo transfer, reduces the risk of ovarian hyperstimulation syndrome, and allows time for results from preimplantation genetic testing (PGT) to return. Moreover, the use of the freeze-only strategy, with cryopreservation of all potentially viable embryos, has steadily increased in recent years ( 6, 7). In the United States, embryo cryopreservation with subsequent FET has increased from 7.9% of cycles in 2004 to 40.7% in 2013 ( 2), with similar increases globally ( 3– 5).
Future practice regarding FET should be based on high-quality evidence, including rigorous controlled trials.įrozen-thawed embryo transfer (FET) has increased dramatically over the past decade as the indications for the procedure have expanded, in part owing to improvements associated with vitrification compared with older slow-freeze methods( 1). 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. Emerging evidence has also revealed other adverse obstetrical and perinatal outcomes, including postpartum hemorrhage, macrosomia, and post-term birth specifically in programmed FET cycles compared with natural FET cycles.
The CL produces not only E 2 and P, but also vasoactive products, such as relaxin and vascular endothelial growth factor, which are not replaced in a programmed FET cycle and which are hypothesized to be important for initial placentation. In these cycles, ovulation is suppressed and therefore the CL is absent. FET cycles are commonly performed in the context of a programmed cycle in which the endometrium is prepared with the use of exogenous E 2 and P. In a recent prospective cohort study, programmed FET cycles (no CL) were associated with higher rates of preeclampsia and preeclampsia with severe features compared with modified natural FET cycles. Recent evidence suggests that absence of the corpus luteum (CL) could be at least partly responsible for this increased risk. FET cycles have been associated, however, with an increased risk of hypertensive disorders of pregnancy for reasons that are not clear. FET facilitates elective single-embryo transfer, reduces ovarian hyperstimulation syndrome, optimizes endometrial receptivity, allows time for preimplantation genetics testing, and facilitates fertility preservation. The use of frozen-thawed embryo transfer (FET) has increased over the past decade with improvements in technology and increasing live birth rates.