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 technique (ART) programs. Frozen embryo replacement (FER) cycles contribute to around 25% of all births achieved by ART (1). The clinical pregnancy rate associated with FER varies widely. This is at least in part because clinics have varying protocols as to the quality of the embryos that are cryopreserved after a fresh cycle, the day of development at which the embryo is frozen and the technique (slow freezing or vitrifi cation) that is used for cryopreservation.
