Hyperstimulation: the need for cryopreservation of embryos.

Successful application of in-vitro fertilization (IVF), zygote intra-Fallopian transfer (ZIFT) and gamete intra-Fallopian transfer (GIFT) requires ovarian hyperstimulation for the maturation of multiple follicles. To control the risk of multiple pregnancies, the number of gametes (GIFT) or embryos (IVF, ZIFT) replaced is limited to three. For the supernumerary embryos resulting from IVF, ZIFT or GIFT, the strategy is cryopreservation for a later transfer. Cryopreservation was performed using either dimethylsulphoxide or 1,2-propanediol as a cryoprotective agent. Embryos were frozen either in the pronucleate stage with 1,2-propanediol or in the multicellular stage with dimethylsulphoxide or 1,2-propanediol. Survival after thawing was scored for both cryoprotective agents as a function of the developmental stage of the embryo and the embryonic quality. Evaluation of survival after thawing was performed on the basis of morphological intactness of the 1-cell pronucleate embryo or of the blastomeres of multicellular embryos. For pronucleate stage embryos, the use of 1,2-propanediol resulted in a 60% survival after thawing. For 2-cell stage embryos the survival was similar for dimethylsulphoxide and 1,2-propanediol. Later stage embryos survived better when dimethylsulphoxide was the cryoprotectant. For all stages, embryo quality before freezing was a crucial factor in survival after thawing. The pregnancy rate (12.2%) was similar for the two cryopreservation protocols. In conclusion, the choice of an appropriate cryoprotective agent can increase the survival after thawing when embryos are of good quality before freezing.