Cryopreservation of human embryos and oocytes.

Human embryo cryopreservation is now a fully established adjunct to assisted reproduction, with thawed embryos implanting at a rate approaching that of fresh embryos. As with fresh embryos, the quality of frozen-thawed embryos is affected by patient age at the time of oocyte retrieval, and by culture conditions. Ovarian stimulation incorporating gonadotropin-releasing hormone analogs does not appear to be detrimental to cryopreservation success, and natural or artificial thaw cycles seem equally effective. Successful cryopreservation seems feasible for one- to eight-cell embryos and blastocysts using a variety of protocols. Selection criteria for embryos to be frozen can vary, and embryo quality has the most significant impact on post-cryopreservation viability. Unfertilized oocyte cryopreservation is coming closer to routine clinical application.