Cryopreservation of embryos and oocytes: an update.

Widespread incorporation of human embryo cryopreservation into IVF programs may reduce the risk of multiple gestation and severe ovarian hyperstimulation syndrome as well as contributing to an overall increase in pregnancy rates. Slow cooling techniques appropriate to the stage of embryo development are most commonly employed. Growing experience with ultrarapid freezing suggests that this technique may offer similar success rates and has the advantages of simplicity and economy. The developmental stage at cryopreservation has not been conclusively shown to influence successful pregnancy outcome, although several retrospective studies suggest improved embryo survival and pregnancy rate following freezing at the pronuclear stage. Endometrial preparation using a number of different regimens before thawed-embryo transfer appears to confer no advantage over the natural cycle, but may be necessary with ovulatory dysfunction. The use of gonadotropin-releasing hormone (GnRH) analogs during ovarian stimulation for IVF has been suggested to decrease subsequent embryo freeze-thaw survival rates. However, increased pregnancy rates have been reported following transfer of previously cryopreserved embryos originating in cycles using GnRH analog (GnRHa), possibly reflecting increased numbers of embryos available for freezing. Cryopreservation of embryos originating from fertile donor oocytes has been very successful and may be used to facilitate synchronization between a donor and one or more recipients. Recent work in oocyte cryopreservation has addressed the problems of meiotic spindle disruption, the risks of aneuploidy, and decreased fertilization associated with zona hardening and polyspermy. Further refinements in technique will be required before widespread oocyte banking becomes feasible.