ABSTRACT
Ovarian stimulation commonly results in the generation of more embryos than are necessary for the fresh embryo transfer. Therefore, cryopreservation and subsequent replacement of frozen–thawed embryos is an integral part of assisted reproductive technology (ART) programs. Frozen embryo replacement (FER) cycles contribute to around 25% of all ART births (1). FER clinical pregnancy rates (CPRs) vary widely. This is at least in part because clinics have varying protocols as to the quality of embryos suitable for cryopreservation, the day of development at which the embryo is frozen, and the technique (slow freezing or vitrification) used. Blastocyst vitrification is now being used more extensively since its widespread introduction over a decade ago (2). As much of the evidence used to guide practice currently is derived from studies using slow freezing, practice will change with increasing evidence from vitrification studies.
