Human embryo viability related to freezing and thawing procedures.

Freezing and thawing procedures did not affect the chances of successful transfer of human embryos. The postimplantation viability of frozen-thawed embryos was equivalent to that of fresh, immediately transferred embryos. Selection of the embryos that were more suitable for freezing did influence the pregnancy rate after transfer of the remaining fresh embryos. The therapeutic efficacy of in vitro fertilization and embryo transfer programs will be greatly increased if only one embryo is transferred in the in vitro fertilization cycle, the others being separately frozen for further embryo transfer in subsequent cycles.