Fresh versus frozen embryo transfer: Backing clinical decisions with scientific and clinical evidence

Abstract

Background: Improvements in vitrification now make frozen embryo transfers (FETs) a viable alternative to fresh embryo transfer, with reports fromobservational studies and randomized controlled trials suggesting that: (i) the endometrium in stimulated cycles is not optimally prepared for implantation; (ii) pregnancy rates are increased following FET and (iii) perinatal outcomes are less affected after FET. methods: This review integrates and discusses the available clinical and scientific evidence supporting embryo transfer in a natural cycle. results: Laboratory-based studies demonstrate morphological and molecular changes to the endometrium and reduced responsiveness of the endometriumtohCG, resulting fromcontrolledovarian stimulation.Theliteraturedemonstrates reducedendometrial receptivity incontrolledovarian stimulation cycles and supports the clinical observations that FET reduces the risk of ovarian hyperstimulation syndrome and improves outcomes for both the mother and baby. conclusions: This review provides the basis for an evidence-based approach towards changes in routine IVF, which may ultimately result in higher delivery rates of healthier term babies.