Elective freeze-all embryos: What is the scientific evidence?

BackgroundWith the advancement in cryopreservation techniques, studies on freezing all survived embryos and frozen-thawed embryo transfer (FTET) in the next cycle have been conducted in assisted reproduction. AimTo review the theoretical backgrounds and clinical outcomes of freeze all embryos, followed by FTET, compared with conventional fresh embryo transfer (FET), and suggest future research direction for policy on this issue.MethodsA through literature search comparing clinical outcomes of FTET and FET was conducted in PubMed. The following keywords were used: ‘frozen-thawed embryo transfer’, ‘fresh embryo transfer’, ‘cryopreserved-thawed embryo transfer’, ‘freeze all policy’ from their inceptions through July 2017.ResultsClinical researches conducted before 2013 indicated better in vitro fertilization (IVF) results as well as obstetric and perinatal outcomes with FTET than with FET. However, the 2017 Cochrane systemic review including more recent studies showed no difference in the cumulative live birth rate per woman between the two methods. FTET was associated with fewer miscarriages and a higher birth weight of singleton babies, but also with an increased rate of pregnancy complications such as preeclampsia.Conclusions While theoretical evidence of FTET after electively freezing all embryos has been suggested to have good outcomes through basic and clinical research, more high-quality randomized clinical trial results with the same primary outcomes under the well-controlled conditions are needed in the future for the generalization of the elective freeze-all method