Cryopreservation in human assisted reproduction is now routine for embryos but remains a research procedure for oocytes.

Human embryo cryopreservation represents an indispensable extension of in-vitro fertilization (IVF) programmes as long as they are based upon the recovery of a large number of oocytes. The most widely used procedures include the cryopreservation of human zygotes or embryos in early cleavage, using 1,2-propanediol and sucrose as cryoprotectants. Our results over a 10 year period (1986-1995) on 5032 thawed cycles involving 14 222 stored embryos make it possible to appraise the results and the contribution of embryo freezing to assisted reproduction. Embryos survived the freeze-thaw process in 73% of cases leading to 4590 transfers of 2.2 embryos (91% of thawed cycles). The clinical pregnancy rate per transfer was 16%, the live birth rate 12%, and the rate of babies born alive per transferred embryo was 6%. Embryo freezing monitored 10 years later produced an average of 8% of additional births. By then, 86% of stored embryos had been thawed for transfer to patients. Destruction or donation were required for only 8% of all frozen embryos and there was no news from the parental couple in relation to almost 6% of embryos. The fate of the vast majority of embryos was decided during the first 5 years of storage. Blastocyst cryopreservation is making new strides, thanks to co-culture systems and embryo selection. Micromanipulation procedures seem to have little impact on the outcome of embryo freezing. Human oocyte freezing is again clinically applied. Indeed, much of the concern about injuries to the oocyte structures through the freeze-thaw process do not seem to be justified, and the problems with frozen-thawed oocyte fertilization has been overcome using intracytoplasmic sperm injection (ICSI). As long as oocyte in-vitro maturation is not well controlled, better results will probably be obtained with mature oocyte cryopreservation. Emerging methods include the freezing of immature oocytes, follicles and ovarian tissue.